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Astronomical Glossary


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Astronomical Glossary


Aberration. A defect in an optical instrument or system. There are several types of abberation, descriptions of a few follow: Chromatic aberration occurs in lenses and is caused by different wavelengths of light focusing at different points and results in coloured fringes around objects; Spherical aberration is caused when the inner and outer parts of a lens or mirror have different focal lengths and results in blurred images; Astigmatism is where an image is focused into a cross or ellipse and is caused by optics that are not a uniform, spherical shape; Coma causes elongated or fanned images toward the edge of the field of view.

Aberration (of starlight). An effect of the Earth's motion around the Sun which slightly alters the true positions of the the stars. Light moves at a speed of almost 300 000km (186 000 miles) per second and the Earth orbits the Sun at an average speed of 28km (17 miles) per second. As a result the stars appear to be shifted from their true position. Aberration may alter a star's position by up to 20.5 seconds of arc.

Absolute magnitude. The apparent magnitude or brightness that a star or other celestial object would have if it was viewed from a standard distance of 10 parsecs, (32.6 light years). Absolute magnitude is therefore the true or intrinsic brightness of an object. (See also Apparent magnitude and Magnitude.)


Absolute Zero.  The coldest theoretical temperature, equal to 0 kelvin (-459.67 F or
-273.15 C)

Absorption line. A break of depression in a continuous spectrum caused by the absorption of photons within narrow wavelengths by some types of atom, ion, or molecule. Any atom, ion or molecule has its own set of characteristic absorption lines which appear when electrons associated with the atom, ion, or molecule absorb radiation and jump to higher energy levels.


Accretion. The accumulation of dust and gas onto larger bodies such as stars, planets, and moons.

Accretion Disk. A disk surrounding a black hole or star in which matter gravitationally falls onto the central object.


Achromatic. A term referring to a lens that has been manufactured to eliminate the worst effects of chromatic aberration. Such a lens is in fact made up of two separate lenses, known as elements, that together correct the worst effects of chromatic aberration.

Active Galactic Nuclei. The exceptionally bright cores of some galaxies, thought to be fuelled by matter falling into supermassive black holes. A class of galaxies that expell massive amounts of energy from their centres, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the centre of these galaxies and power their explosive energy output.

Active Galaxy. A galaxy emitting unusually large amounts of energy from a compact central source.


Adaptive Optics. A system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring.


Aerolite. A meteorite whose main composition is stony.

Airglow. The faint background glow in the night sky caused by gas in the ionosphere. Because of airglow the night sky is never completely dark as seen from Earth's surface.


Airy disk. The apparent size of a star's disk produced even by a perfect optical system. Since the star can never be focused perfectly, 84 per cent of the light will concentrate into a single disk, and 16 per cent into a system of surrounding rings.

Albedo. The reflecting power of a planet or other non-luminous object. For example, the Moon is a poor reflector of light and its albedo is just 0.12. Albedo is typically expressed by a simple numerical factor, 1.0 means a perfect reflector, 0.0 means no reflection at all.

Almanac. A book of tables detailing the positions, times and other data about celestial objects, usually produced annually.

Altazimuth mount. A type of telescope (or other instrument) mounting where the telescope can move freely in any direction - generally, up and down with respect of the horizon (altitude); and left and right along the horizon (azimuth).

Altitude (Alt). The measurement of an object's angle, in degrees, above the horizon. Part of the horizontal system of co-ordinates. An object directly overhead at the zenith would have an altitude of 90, an object on the horizon an altitude of 0.

Andromeda Galaxy: The largest member of the Local Group of galaxies; roughly twice the size of the Milky Way; also known as M31


Angstrom (unit). One hundredth-millionth of a centimetre.

Angular diameter. The apparent size of an object, usually expressed in degrees, minutes, or seconds of arc.

Angular distance. The apparent distance between two objects on the celestial sphere measured in degrees, minutes or seconds of arc.

Angular momentum. A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the laws of physics, the angular momentum of any orbiting body must remain constant at all points in its orbit; that is, momentum cannot be created or destroyed. If the orbit is elliptical, the radius will vary; and since the mass is constant, the velocity must change. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A rotating body also possesses angular momentum in its spin.


Annular Eclipse. A solar eclipse in which the moon does not fully cover the sun’s disk, allowing observers to see a thin ring of sunlight.


Anti Tail. The name given to a comet's tail when it points toward the sun. This rare event typically occurs when Earth crosses the plane of the comet's orbit and the comet is relatively close to the sun.


Antimatter. Matter consisting of particles that have the same mass and properties as their matter counterparts but opposite electrical charges.


Apastron. The point of greatest separation between two stars that are in orbit around each other. The opposite of periastron.


Aperture. The diameter of a telescope’s primary lens or mirror; the larger the aperture, the greater the telescope’s light-gathering power.


Aphelion. The point at which an orbiting body is furthest from the Sun in its orbit.

Apoapsis. The point in an orbit when the two objects are farthest apart. The opposite of periapsis.


Apochromatic Lens. A lens consisting of three or more elements which gives a greater reduction of chromatic aberration than that of a two-element (Achromatic) lens.

Apogee. The furthest point of the Moon from the Earth in its orbit.

Apollo. 1. The U.S. space program that sent astronauts to the moon in the 1960s and '70s. 2. An asteroid with a perihelion less than 1.017 AU (and thus comes within the orbit of Earth).


Apparent Field of View. The angular diameter of the circle of light that the eye sees through an eyepiece.


Apparent (or Visual) magnitude. The apparent brightness of a celestial object. The lower the magnitude, the less bright the object. Thus, the Sun has an apparent magnitude of -27; the Moon up to -12; Venus up to -4; the brightest stars -1; the faintest stars visible to the naked-eye +6, the faintest objects yet detected about +30. For mainly historical reasons the magnitude scale has the peculiar attribute of having brighter objects at negative values and vice-versa. (See also Absolute magnitude and Magnitude.)

Apparition. The period or time when an object is visible and well placed for observation.

Appulse. The apparent close approach of two celestial bodies as seen from Earth, such as a star and a planet, or two planets for example.

Arc (measurement of). Angles on the celestial sphere, measured in degrees, minutes and seconds. Arc may be an expression of the angular distance between two celestial objects or the angular size of an object.

Arcminute. A unit of angular size equal to 1/60 of a degree; abbreviated by '. Arcminutes are used to measure of the separation between two sky objects or the angular size of an object.


Arcsecond. A unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute); abbreviated by ". Arcseconds are used to measure of the separation between two sky objects or the angular size of an object.


Areography. The proper name for the geography of Mars.

Aspides. The points in the Moon's orbit where it is either closest to Earth (perigee) or farthest from Earth (apogee).


Ashen light. The faint luminosity of the night side of the planet Venus, seen when Venus is in the crescent stage. It is probably a genuine phenomenon rather than a mere contrast effect, but its cause is not certainly known.

Asterism. A pattern of stars larger than a cluster but smaller than a constellation. Examples of an asterism would be Orion's Belt, or The Hyades in Taurus.

Asteroids. One of several names for the minor planet swarms of the Solar System. Asteroids are found in many regions of the solar system but most are to be found orbiting the Sun between the orbits of Mars and Jupiter. This region is often referred to as the Asteroid Belt.

Asteroid Belt. The zone in which most asteroids orbit the sun, located between the orbits of Mars and Jupiter.


Astigmatism. An optical defect where an image is focused into a cross or ellipse and is caused by optics that are not a uniform, spherical shape.

Astrograph. A telescope designed specifically for astrophotography.

Astrological sign. One of twelve sections of the zodiac that are 30 degrees long and that correspond to the positions of the constellations as they were about 2,600 years ago when the astrological system was established. Do not confuse an astrological sign with the astronomical constellation of the same name, as they only partially overlap.


Astrolabe. An ancient instrument used for measuring the altitudes of celestial objects.

Astrometry. The branch of astronomy that deals with precisely measuring the positions of objects on the celestial sphere.

Astronomical Unit (AU). The mean distance between Earth and the Sun. A unit of distance, equal to 149 600 000 kilometres (92 900 000 miles).

Astronomy. The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.


Astrophysics. The study of the physical nature of celestial objects.

Atmosphere. A gaseous envelope surrounding a moon, planet, or star. It can have no definite boundary, but merely thins out until the density is no greater than that of surrounding space.


Atom. The basic structure of which all matter is made. It is made up of three particles; protons and neutrons (which make up the nucleus) and electrons (which orbit the nucleus). (The one exception to this is hydrogen which has only a proton at its nucleus. In its basic state an atom will form an element, the kind of element being dictated by the structure's atomic number. For example, a hydrogen atom will normally have 1 of each particle and its atomic number is 1. A helium atom, which has the atomic number 2, will have two of each particle, and so on. Atoms may be broken up (fission) or fused with another (fusion). Any combination of atoms will form a molecule.

Attitude. The orientation of a spacecraft relative to the direction of its motion.


AU. Astronomical Unit. A measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometres).


Aurora. 'Polar lights' which occur in the Earth's upper atmosphere, caused by particles emitted by the Sun causing gas molecules in Earth's atmosphere to glow. Aurora Borealis are seen above the North Pole, Aurora Australis above the South.

Autumnal Equinox. The moment when the Sun crosses the celestial equator travelling in a southward direction, on or about September 22. In the Northern Hemisphere, it marks the first day of fall. The term is also applied to the Sun’s position in the sky at that moment. It is one of two points where the ecliptic and celestial equator intersect, the other being the vernal equinox.


Axis. A straight line about which an object rotates.


Azimuth (Azi). The measurement of an object's angle, in degrees, along the horizon. Usually measured from North (0) Part of the horizontal system of co-ordinates. For example due south would be 180 azimuth.


Bailly's Beads. Bright points of light seen along the edge of the Moon just before and just after a total eclipse of the Sun. They are caused by sunlight shining though valleys (or between peaks) at the Moon's limb.

Barlow Lens. A lens attached behind the eyepiece of a telescope that increases magnification.


Barnard's Star. A red dwarf in the constellation Ophiuchus that has the highest proper motion than any other known star; it was discovered by E. E. Barnard in 1916


Barred Spiral Galaxy. A spiral galaxy with a central bar consisting of stars and gas.


Barycentre. The centre of mass of a system of bodies, such as the solar system. When a comet, for example, is well outside the orbit of Neptune (the farthest major planet), it sees the sun and major planets essentially as a single object of summed mass, and the centre of this mass (called the barycentre of the solar system) is offset somewhat from the sun; "original" and "future" orbits of long-period comets are computed for this barycentre, while perturbed, osculating orbits of currently-observed objects in the inner solar system are computed for heliocentric orbits.
The centre of gravity of the earth-moon system. Because the earth is 81 times more massive than the moon, the barycentre lies within the terrestrial globe.


Baryon particles. Any of the subatomic particles that interact through the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.


Baseline. The line between two observational points or two telescopes of an interferometer.


Besselian year. A quantity introduced by F. W. Bessel in the nineteenth century that has been used into the twentieth century. Bessel introduced a system whereby it would be convenient to identify any instant of time by giving the year and the decimal fraction of the year to a few places, but the starting times of the year was not convenient for dynamical studies that utilize Julian dates, differing by 0.5 day, and the Besselian year varies slowly. The recent change to Julian year usage in dynamical astronomy (and the J2000.0 equinox) took effect in solar-system ephemerides of the Minor Planet Centre and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See Julian year.)


Big Bang. The hypothetical event that is thought to have created the universe. Estimates vary but many accounts of this event put it as occurring about 15 billion years ago.

Billion. 1 000 000 000 - one thousand million.

Binary Star. A system of two stars that are genuinely associated with each other and are moving around their common centre of gravity.

Black dwarf. A non-radiating ball of gas resulting from either a white dwarf that has exhausted all its energy, or cloud of gas that has contracted but contains too little mass to begin nuclear fusion.


Black Hole. A volume of space in which gravity is so intense that nothing can escape, not even light.

Blazar. A high-energy, variable type of quasar which astronomers believe has a jet of material aimed in our direction that causes it to appear more energetic than other quasars.


Blink Comparator. An instrument that allows astronomers to view two images of the same region of sky simultaneously. Objects that have changed their brightness or position appear to stand out of the plane of the picture.


BL Lacert objects. Objects which are strong emitters of infra-red radiation. They are very luminous and remote and are thought to have similar properties to quasars.

Blue Moon. The second Full Moon of any month. A cycle of Lunar Phases (a Lunation) is completed in approximately 29.5 days so any month except February can have a Blue Moon. Typically, a Blue Moon occurs roughly every two and a half years, hence the expression "Once in a Blue Moon".

Blueshift. A decrease in the wavelength of light coming from an object due to its motion toward Earth.


Bode's law. A mathematical relationship of uncertain significance that links the distances of the planets from the Sun. Strictly speaking it should be called Titius' Law since it was discovered by J. D. Titius several years before J. E. Bode popularised it in 1772. Some refer to it as the Titius-Bode Law.

Bok Globule. A small, dark nebula thought to be a region of star formation.


Bolide. A brilliant exploding meteor.

Bolometer. An instrument used to measure heat radiation.

Brown Dwarf. A gaseous object that forms like a star but lacks the necessary mass to sustain nuclear fusion in its core; a body intermediate in mass between a star and planet.


Bulge. The generally spherical, central region of a spiral galaxy.


Cannibal Coronal Mass Ejections. Fast-moving solar eruptions that appear to overtake and often devour their slower-moving kin.


Carbonaceous Chondrites. A class of stony meteorites and asteroids which contain organic (carbon) compounds and may be the most primitive samples of the early solar system.


Cataclysmic Variable. A close binary system which includes a white dwarf accreting matter from a less massive companion.


Captured rotation. Rotation of an object that spins at the same rate as that object takes to orbit another object. Sometimes referred to as synchronous rotation. The Moon is a good example of an object that has captured rotation.

Carbon stars. Red stars of spectral type 'R' and 'N' which have carbon-rich atmospheres.

Cassegrain reflector. A type of reflecting telescope where the secondary mirror is convex and the reflected light is then passed through a hole in the main or primary mirror. The advantage of this design is that it may be made more compact than a Newtonian reflector of equal focal ratio.

CCD. Charge Coupled Device. An electronic device used in place of conventional photographic film. They consist of a silicon chip which is sensitive to light, the chip being divided up into picture elements (pixels). Light falling onto the chip builds up an electrical signal which is passed on to a processing computer in the form of digital data for later storage and processing. In some cases the signal from the chip is passed direct to a monitor for immediate display. CCD's are very sensitive to light, much more so than photographic film, therefore exposures can be much shorter. The downside is that they are smaller in area and often have less resolution than normal film.


Celestial equator. The projection of Earth’s equator into space. Also a line in the sky midway between the north and south celestial poles. The celestial equator is the line of zero declination in the equatorial coordinate system.


Celestial Pole. The imaginary projection of Earth’s rotational axis onto the celestial sphere.


Celestial sphere. An imaginary sphere that surrounds the Earth, the Earth being at the centre of that sphere.

Central meridian. The imaginary north-south line that bisects a planet. It is used as a reference for estimating the longitude of planetary features as it rotates.

Central meridian transit. The passage of a particular feature on a planet across the planet's central meridian.

Cepheid Variable. A short period variable star with very regular variations in magnitude. The name comes from the prototype star Delta Cephei. Cepheids are important astronomically because there is a clear link between their luminosity and their period of variation. Cepheids are also known to be highly luminous, giant stars than can be seen from great distances - distances can then be calculated by sheer observation alone.

Chandra X-ray Observatory (CXO). One of NASA’s Great Observatories in Earth orbit, launched in July 1999 and named after the Indian physicist Subrahmanyan Chandrasekhar. It was previously named the Advanced X-ray Astrophysics Facility (AXAF).


Chandrasekhar limit. The maximum mass of a white dwarf star, equivalent to 1.4 solar masses.


Charles Messier. A French astronomer and comet hunter who discovered 13 comets independently and co-discovered a half-dozen others. While hunting for comets, Messier compiled a list of fuzzy objects that were not comets in order to avoid them. These catalog entries were later identified as star clusters, nebulae, and galaxies and became the Messier Catalog. Published in various versions beginning in 1771, the catalog grew to 103 objects by 1781. Charles Messier lived from June 26, 1730, to April 12, 1817.


Chondrite. A stony meteorite containing small, round, silicate granules called chondrules.


Chromatic aberration. A defect present in all single lenses because different wavelengths of light come to a focus at different distances from the lens. This defect will show up as a false colour fringe around a star for example. The defect is corrected by using multiple lens arrangements using different types of glass.

Chromosphere. The part of the Sun's atmosphere that lies above the photosphere of the Sun.

Circumpolar star. A star that never sets as seen from a particular location on Earth. For example, the stars of the Great Bear (Ursa Major) never set as seen from England.

CME. Huge eruptions of electrified, magnetic gas ejected from the solar corona; this gas is hurled into space with speeds from 12 to 1,250 miles per second (about 20 to 2,000 kilometres per second); CMEs can produce geomagnetic storms and auroral displays on Earth.


Coated Optics. Optics treated with a thin, uniform coating that greatly reduces scattered light and thus makes the image brighter.


Collapsar. A giant star that collapses of its own weight at the end of its normal lifetime.


Collecting area. The area of a telescope capable of collecting electromagnetic radiation. The collecting area is an important measure of a telescope’s sensitivity: the more radiation it can collect - that is, the larger its collecting area - the more likely it is to detect dim objects.


Collimation. The act of aligning the optical components of an instrument such as a telescope so that the instrument performs correctly.


Colures. Great circles on the celestial sphere. The equinoctial colure, for instance, is the great circle which passes through both celestial poles.

Coma (cometary). The cloud of gas and dust that makes up the head of a comet, the nucleus is at the centre.

Coma (optical). An optical defect where objects close to the edge of a field of view appear to be flared or fanned out.

Comet. A body composed of ice and dust in orbit around the Sun.


Comet Nucleus. A solid, compact mass of rock and ice that heats up when exposed to sunlight and releases gas and dust.


Conjunction. An instant when two celestial objects appear to lie very close together or in a line as seen from a particular viewpoint.

Conjunction, Inferior. A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet between Earth and the Sun. Obviously, a superior planet (Mars, Jupiter, Saturn, Uranus, Neptune or Pluto) cannot be at inferior conjunction.

Conjunction, Superior. A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet on the far side of the Sun.

Constellation. An area of the celestial sphere bounded by internationally decreed lines of Right Ascension and Declination. The whole celestial sphere is divided up into a total of 88 areas of varying size, each with its own name. The smallest is Crux (The Southern Cross) and the largest is Hydra (The Watersnake). The most common concept of a constellation is generally the pattern of stars (usually the brightest ones) that make up a familiar figure or pattern. But, strictly speaking, a constellation actually refers to an area of sky rather than the pattern. Without exception, a constellation's area exceeds the area of the pattern that gave that area its name.

Copernicus, Nicolaus (1473-1543). A Polish astronomer who advanced the theory that Earth and the other planets revolve around the Sun—the heliocentric theory. This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was at the centre of the universe, and all objects, including the Sun, circled it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his alternative. Although, it should be noted, that the heliocentric idea was first put forth by Aristarcus of Samos in the third century B.C., a fact known to Copernicus but long ignored by others prior to him.


Corona. The tenuous, outermost part of the Sun's atmosphere. It is visible to the naked-eye, but only at the time of a total solar eclipse or with the use of specialised instruments.

Coronagraph. An instrument that allows study of the Sun's inner corona at times of non-eclipse.

Coronagraphic Mask. A disk-shaped instrument designed to block light from the disk of a star, allowing the region very close to a target star to be studied.


Coronal Mass Ejection. A huge eruption of electrified, magnetic gas ejected from the solar corona; this gas is hurled into space with speeds from 12 to 1,250 miles per second (about 20 to 2,000 kilometres per second); CMEs can produce geomagnetic storms and auroral displays on Earth.


Cosmic Background Radiation. Microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang.


Cosmic Microwave Background. Microwave radiation that permeates the universe and represents the still-cooling heat generated from the Big Bang.


Cosmic rays. High velocity particles reaching Earth from outer space. The heavier cosmic ray particles are broken up when they enter Earth's upper atmosphere.

Cosmogony. The study of the origin and evolution of the universe.

Cosmological Constant. A term in the equations of general relativity that represents a repulsive force in the universe.


Cosmological redshift. An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. See also doppler effect.


Cosmology. The study of the universe considered as a whole.

Cosmos. A synonym for universe.


Counterglow. The English name for the skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Usually known by its German name, the Gegenschein.

Crescent. The phase of a planet or moon during which less than half the surface is illuminated.


Critical Density. The density of the universe that provides just enough gravity to bring the expansion to a halt after an infinite time.


Crust. The thin, outermost geological layer of a planet, moon, or asteroid.


Cryovolcanism. The eruption of water and other liquid or vapor-phase volatiles, together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating.


Culmination. The maximum altitude that a celestial object attains above the horizon.


CXO. See Chandra X-ray Observatory.




Damocloid. A rare type of asteroid with an elliptical, comet-like orbit; named for the first one discovered, asteroid 5335 Damocles.


Dark Adaptation. The process by which the human eye becomes well adjusted to seeing dim objects in the dark.


Dark Energy. A type of "negative gravity" that seems to play a role in the acceleration of universal expansion.


Dark Matter. Matter that exerts gravitational force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown.


Dark Nebula. A cloud of dust grains that is thick enough to obscure the light from background stars.


Dawes limit. The maximum practical limit for the resolving power of a telescope. It is 4.56 / d, where d is the aperture in the instrument in inches. For example, the Dawes limit of a of a four inch telescope is 1.1 seconds of arc. A twelve inch instrument would have a Dawes limit of 0.4 seconds of arc. In other words, the Dawes limit is the theoretical point at which the telescope is able to separate two objects that distance apart. In practice this is seldom achieved.

Day, Sidereal. The interval between two successive meridian passages (or culminations) of the same star. A sidereal day is equal to 23 hours, 56 minutes and 4.091 seconds. This is the true rotational period of the Earth.

Day, Solar. The mean interval between successive meridian passages of the Sun. It is equal to 24 hours, 3 minutes and 56.55 seconds. The solar day is slightly longer than the sidereal day because the Sun seems to moves eastward against the stars, on average at roughly one degree per day.

Declination. The angular distance of an object relative to the celestial equator, expressed in degrees. The celestial equivalent of latitude on Earth’s surface. Distances north of the celestial equator are positive; distances south are negative. The declination of the celestial equator is 0; the declination of the north celestial pole is +90, and the declination of the south celestial pole is -90.

Deep-Sky. The part of space that lies beyond the Solar System. Star clusters, galaxies and nebulae are frequently termed as 'Deep-sky objects'.

Deep-Sky Objects. Objects located beyond the solar system; consist of stars, nebulae, star clusters, and galaxies.


Degree (Deg). Symbol , 1/360th of a circle.

Density. The mass of an object per unit of its volume. Density is usually expressed as a numerical ratio to that of water which has a density of 1. An object denser than water would have a figure of greater than 1, and vice-versa.

Deuterium. An isotope of hydrogen; its nucleus, consisting of one proton and one neutron, has double the mass of the nucleus of ordinary hydrogen.


Dichotomy. The instant when an object is exactly half lit as seen from Earth. It is usually applied to the Moon or an inferior planet. The exact half-phase of Mercury, Venus or the Moon.

Differential rotation. The rotation of a body such as a gaseous planet or the Sun so that different parts are rotating at different speeds. For example, a star or planet which rotates faster at its equator than it does at its poles.

Diffraction. The spreading out of light as it passes the edge of an obstacle.


Diffraction rings. Concentric rings surrounding the image of a star as seen in a telescope.


Direct motion. Movement of revolution or rotation of a celestial object in the same sense as that of the Earth. For example, the Sun moves across the sky each day from east to west, an effect of the Earth's rotation on its axis. But, against the background of stars the Sun actually moves from west to east over the course of a year, the effect of Earth's orbit around the Sun. This west to east movement is direct motion. The Moon behave in exactly the same way. The planets also exhibit this west to east motion most of the time but owing to our perspective of the planets they occasionally appear to move in the reverse sense, known as retrograde motion.

Disk. A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. Also, the apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.

Diurnal. Another name for daily.


Diurnal motion. The apparent daily rotation of the sky from east to west. It is due to the real rotation of the Earth from west to east.

Dobsonian Telescope. A telescope with a simple but stable altazimuth mount that rotates easily.


Doppler effect. The change in frequency (or wavelength) of light (or other radiation) caused by the motion of an object or the observer. An object receding from an observer would exhibit a frequency shift toward a lower frequency (red shift) and vice-versa.

Double star. A star made up of two components. They are either genuinely associated (binary stars) or they appear close by chance (optical pair or binary).

Dust. The irregularly shaped grains of carbon and silicates measuring a fraction of a micron across that are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.

Dwarf Galaxy. A small galaxy containing a few million stars; the most common type of galaxy in the universe.


Dwarf Star. A main-sequence or smaller star


Early-type star. Any hot star of spectral class O, B or A. Sometimes referred to as 'early spectral-type.'

Earthshine. The faint luminosity seen of the night side of the Moon, especially when the Moon appears at a crescent phase. It is caused by light reflected from the Earth onto the Moon.

Eccentricity. The measure of how non-circular an object's orbit is. 0 = a perfect circle; any figure between 0 and 1 = an ellipse; 1 = a parabola; any figure greater than 1 = a hyperbola. Eccentricity may sometimes be expressed as a percentage. Eccentricity may be calculated by dividing the distance between the two foci of the ellipse and the length of the major axis of the ellipse.

Eclipse, Lunar. The passage of the Moon through Earth's shadow. Lunar eclipses may be either total or partial. Totality may last up to one and three quarter hours although the period of totality is on average shorter.

Eclipse, Solar. The passage of the Moon in front of the Sun so that the Moon is directly in front of the Sun. Totality may last for a little over 7 minutes under favourable conditions. Partial eclipses occur when the Sun is incompletely covered. Annular eclipses occur when the Moon it near the farthest part of its orbit and hence appears smaller. In this case a bright ring of sunlight is seen around the dark body of the Moon. Technically speaking, a solar eclipse could be termed as an occultation of the Sun by the Moon.

Eclipsing binary (or eclipsing variable). A binary star system where one star passes in front of the other causing a variation in the overall brightness of the star system.

Ecliptic. The Sun’s apparent annual path through the fixed stars. Also the orbit of Earth, if it could be seen in the sky. The constellations through which the ecliptic passes are called the constellations of the Zodiac. The projection of the Earth's orbit on to the celestial sphere.

Ecliptic co-ordinate system. The system of specifying positions in the sky that uses the ecliptic—the Sun’s apparent annual path through the fixed stars—as the fundamental reference plane. Ecliptic co-ordinates are useful when specifying positions in the solar system and especially positions relative to the Sun.


Ecliptic latitude. The angular distance of an object relative to the ecliptic, expressed in degrees. Distances north of the ecliptic are positive; distances south are negative. The ecliptic latitude of the Sun is always zero.

Ecliptic longitude. The angular distance of an object eastward from the vernal equinox, measured in degrees along the ecliptic. The ecliptic longitude of the Sun is zero when the Sun is on the vernal equinox and increases through the year by very nearly one degree per day.


Ecosphere. The region around the Sun in which the temperatures are neither too hot nor too cold for life to exist under suitable conditions. Venus lies near the inner edge of the Ecosphere; Mars is near the outer edge.


Ejecta. Material that is ejected. Used mostly to describe the content of a massive star that is propelled outward in a supernova explosion. Also used to describe the material that is blown radially outward in a meteor impact on the surface of a planet or moon.


Electromagnetic Radiation. The various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays.


Electromagnetic Spectrum. The spectrum encompassing the entire range of electromagnetic radiation (light).


Electron. An atomic particle that carries a negative charge. It orbits the nucleus of an atom.

Electron degeneracy. Occurs when electrons are compressed into a very tiny volume. Electron degeneracy is the force that supports a white dwarf against its own gravity, preventing it from collapsing. A neutron star is supported by neutron degeneracy. If a star is massive enough, not even neutron degeneracy can support its weight. The result is a black hole.


Electron Volt. A unit of energy equal to the energy gained by an electron that falls through a potential difference of one volt; 1.60 x 10^-19 joule.


Element. A fundamental unit of matter; consists of a fixed number of protons, although the number of neutrons and electrons can vary.


Element, optical. Any single part of an optical train such as a mirror or lens. In the case of lenses or eyepieces they may comprise several pieces of glass, each one known as an element. The combination of elements is used to correct the faults that would often be present in a single element lens or eyepiece.

Ellipse. An oval. The fact that the orbits of the planets are ellipses and not perfect circles was discovered by Johannes Kepler, working with the careful observations by Tycho Brahe.


Elliptical Galaxy. Ellipsoidal agglomerations of stars outside our galaxy which usually do not contain much interstellar matter. One of the two major types of galaxies, the other being a spiral galaxy.


Elongation. The angular distance between the Sun and a planet, or between a planet and a satellite, as seen from Earth.

Emersion. A term used to describe when an object re-emerges after an occultation or eclipse.

Emission. The discharge of electromagnetic radiation from an object.


Emission lines. Specific wavelengths of light that are brighter than adjoining wavelengths seen in spectra.

Emission Nebula. A cloud of very hot gas that is being illuminated from within by the radiation of energetic, young stars.


Ephemeris. A table or list of the predicted position of an object such as a planet. Ephemerides usually contain right ascension and declination, apparent angle of elongation from the sun (in degrees), and magnitude (brightness) of the object; other quantities frequently included in ephemerides include the objects distances from the sun and earth (in AU), phase angle, and moon phase.


Ephemeris Time (ET). Determined in principle from the sun's apparent annual motion, ET is the numerical measure of uniform time, which is the independent variable in the gravitational theory of the earth's orbital motion, coming from Simon Newcomb's Tables of the Sun. In practice, ET was obtained by comparing observing positions of the Moon with gravitational ephemerides calculated from theories. In 1992, standard (apparent geocentric) ephemerides of comets and minor planets changed from using Ephemeris Time to Terrestrial Dynamical Time (TDT, or TT).

Epoch. The particular date for which astronomical positions in a book or table are accurate. Most books give star positions that are valid for the J2000—January 1, 2000—epoch.

Equation of time. The difference between true solar time (determined by the Sun’s position in the sky) and mean solar time (the time told by your watch). The two times can vary by as much as 16 minutes over the course of a year.


Equator, celestial. The projection of the Earth's equator onto the celestial sphere. It divides the sky into two equal hemispheres.

Equatorial co-ordinate system. A system of specifying positions in the sky that uses the celestial equator—the projection of Earth’s equator into space—as the fundamental reference plane.


Equatorial mounting. A type of mounting for a telescope (or other instrument) which is set up so that one axis of motion is parallel to the Earth's axis. This arrangement means that only one axis is required to be driven to keep an object in the field of view.

Equinox. The equinoxes are the two points at which the ecliptic intersects with the celestial equator. The Vernal equinox (or First Point of Aries) is where the ecliptic crosses the celestial equator from south to north, the Sun reaches this point around the 21st March. The opposite equinox, the autumnal equinox, is where the ecliptic crosses the celestial equator from north to south. The Sun reaches this point around the 22nd September. The term equinox stems from Latin, equi = equal, nox = night. At the equinoxes the day and night are of equal duration.

Escape Velocity. The minimum speed that an object must attain to escape from the surface of the planet or other body without being given any extra impetus. The escape velocity of Earth is 11.2 kilometres per second.

Evening Star. The planet Venus when it appears in the evening sky.


Event Horizon. The distance from a black hole within which nothing can escape. Once a particle has entered this horizon, nothing can prevent it from hitting the singularity in a very short amount of proper time. In this sense, the event horizon is considered the point of no return.


Evolved star. A star near the end of its lifetime, when most of its fuel has been used up. This period of the star’s life is characterized by loss of mass from its surface in the form of stellar wind.


Exit Pupil. The image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece.


Exobiologist. A person who studies the origin, development, and distribution of 'living' systems that may exist outside of Earth.


Exosphere. The outermost part of the Earth's atmosphere. It is an ill-defined, highly rarified zone which starts at a height of about 700km (435 miles) and extends into the vacuum of space.

Extinction. The apparent reduction in brightness of a celestial object when it is low in the sky and much of its light is absorbed by Earth's atmosphere. Extinction becomes a severe problem for astronomers when objects are viewed close to (especially within 20 degrees of) the local horizon. There are various methods that have been developed for astronomers to try and compensate for this extinction, but it is always best to make measurements of astronomical objects when they are as high in the sky as possible (to minimize errors).

Extragalactic. Beyond the Milky Way Galaxy.


Extrasolar. Beyond the sun.


Extraterrestrial. Beyond Earth


Eye Relief. The distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view.


Eyepiece (or Occular). The lens, or lens combination, at the eye end of a telescope. It is responsible for the magnification of the object under scrutiny. There are a variety of eyepiece types with different characteristics. Eyepieces will be stated as having a certain focal length and it is this figure, usually in millimetres, which when divided into the focal length of the telescope, dictates the magnifying factor that a given eyepiece will yield.


Faculae. Bright patches seen of the photosphere during solar observation, usually (though not always) associated with sunspots.

Far Ultraviolet. Ultraviolet radiation with the shortest wavelengths ("farthest" from visible light in the electromagnetic spectrum).


Field of View. The angular width of sky that can be seen with an optical instrument. Field of view is measured in degrees, arcminutes, and arcseconds.


Filter. A device that transmits light of only certain wavelengths; used by astronomers to observe specific wavelengths or to reduce the light of exceptionally bright objects.


Finder Scope. A small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky.


Fireball. A very brilliant meteor, generally brighter than magnitude –4.

First Quarter. The phase of the moon a quarter of the way around its orbit from new moon; the eastern half is illuminated during this phase.


Flare stars. Faint red dwarf stars which may brighten up by several magnitudes over a period of a few minutes, fading back to their usual brightness within an hour or so. Typical flare stars are UV Ceti and AD Leonis.


Flares, Solar. Brilliant eruptions in the outer part of the Sun's atmosphere. They can usually only be seen with the aid of special filters and instruments. They are often associated with areas of activity on the Sun such as sunspots. Occasionally, these eruptions may cause charged particles to reach the Earth which may in turn cause auroral displays.

Flocculi. Patches on the Sun's surface visible with spectroscopic equipment. Bright flocculi are composed of calcium; dark flocculi are made up of hydrogen.

Flux. A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Flux measurements make it easy for astronomers to compare the relative energy output of objects with very different sizes and ages.


Focal length. The distance between a lens or mirror and the point where it brings light to a focus. The focal length divided by the aperture of the mirror or lens is termed the focal ratio.

Focal ratio. The focal length of a telescope divided by its aperture (opening) or primary mirror diameter.

Focus. The point at which rays of light passing through a lens (or reflecting off a mirror) converge.


Focuser. The device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus.


Fork Mount. An equatorial mount in which the telescope swings in declination between the two prongs of a fork.


Foucault pendulum. A pendulum that varies in the direction of its swing as the Earth rotates. Used to demonstrate that it is Earth that rotates and not the sky.


Fraunhofer Lines. Dark absorption lines seen in the spectrum of the Sun, named in honor of the German optician J. von Fraunhofer, who first studied and mapped them in 1814.

Frequency. The number of waves passing a point in a given time, usually 1 second. Frequency is measured in hertz and is equal to the speed of the wave divided by their wavelength. Longer wavelengths have lower frequency and vice-versa.


Fringe region. The upper part of the exosphere . Atomic particles in the fringe region have little chance of collision with each other, and to all intents and purposes they travel in free orbits, subject to the Earth's gravitation.


Full Moon. The phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated.

Fusion. The process by which atomic nuclei collide so fast that they stick together, form new atoms, and emit a large amount of energy. In the centre of most stars, hydrogen fuses into helium. The energy emitted by fusion prevents the star’s enormous mass from collapsing in on itself and causes the star to glow.


Galactic Cluster. Another name for an open star cluster. They are often termed Galactic Clusters because they are found mainly in the plane of our galaxy (The Milky Way). If you were to view our galaxy from afar, you would find that all the open/galactic clusters lie within the spiral arms of the galaxy. See also Open Cluster.


Galactic Co-ordinate System. The system of specifying positions in the sky that uses the plane of the Milky Way as the fundamental reference plane.


Galactic Disk. The disk of a spiral galaxy.

Galactic halo. A spherical region surrounding the centre of a galaxy. This region may extend beyond the luminous boundaries of the galaxy and contain a significant fraction of the galaxy’s mass. In terms of cosmological distances, objects in the halo of our galaxy are relatively close.


Galactic Nucleus. The central region of a galaxy; often contains a high density of stars and gas, and a supermassive black hole.


Galactic Plane. The projection of the Milky Way’s disk on the sky.


Galaxy. A system made up of stars, nebulae and interstellar matter. Many galaxies, but not all, are spiral in form. A component of our universe made up of gas and a large number (usually more than a million) of stars held together by gravity.

Galaxy, The. The star system of which the Sun is a member. The Galaxy is also known as the Milky Way galaxy. It is spiral in shape, contains about 100 000 million stars and is approximately 100 000 light years across. The nucleus of the Galaxy is roughly 30 000 light years thick tapering away to about 10 000 light years thickness in the spiral arms. The whole system is slowly spinning taking about 200 million years to rotate once. The Galaxy is the second largest member of the Local Group of galaxies, a loose cluster of galaxies roughly dumbbell in shape and about 4 million light years long.

Galaxy Cluster. A system of galaxies containing from a few to a few thousand member galaxies that are all gravitationally bound to each other.


Galilean Moons. Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610.


Galilean Satellites. Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto; discovered by Galileo Galilei in 1610.


Galilei, Galileo (1564-1642): An Italian scientist renowned for his contributions to physics, astronomy, and scientific philosophy. He is regarded as the chief founder of modern science. He developed the telescope, with which he found craters on the Moon and discovered the largest moons of Jupiter. Galileo was condemned by the Catholic Church for his view of the cosmos, which was based on the heliocentric theory of Copernicus.


Gamma-rays. The form of electromagnetic radiation with the highest energy and the shortest wavelength. Any photon having an energy greater than about 100,000 electronvolts (eV). In comparison, visible light has an energy of 1.65 to 3.1 eV, and x-rays have an energy of 124 eV and upward.

Gamma-ray Burst (GRB). A burst of gamma rays from space lasting from a fraction of a second to many minutes. There is no clear scientific consensus as to their cause. Recently, a determination of their distances, placed the origins of the bursts in other galaxies.


Gas Giant. A large planet made primarily of gas, such as Jupiter, Saturn, Uranus, and Neptune.


Gauss. A unit of measurement of a magnetic field. Earth has a surface magnetic field strength of between 0.3 and 0.6 Gauss.

Gegenschein. A very faint skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Often known by its English name, Counterglow.

General Relativity. The geometric theory of gravitation developed by Albert Einstein, incorporating and extending the Theory of Special Relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent. The theory has consequences for the bending of light by massive objects, the nature of black holes, and the fabric of space and time.


Geocentric. Meaning: As seen from the centre of Earth. A system of co-ordinates.

Geodesy. The study of the shape, mass, size and other features of the Earth.

Giant Molecular Cloud (GMC). Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.


Giant stars. Stars that are swelling in size as they approach the end of their lives. Giant stars are often no more massive than the Sun but they have expanded to great size and are therefore less dense but highly luminous.

Gibbous. The phase of the Moon or a planet when it is between half and fully illuminated.

Globular Cluster. A roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxy’s halo. The stars of a cluster were born together and travel through space together. M13 and M22 are familiar examples.


Gravitation. The force of attraction which exists between all particles of matter in the universe. Particles attract each other with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.


Gravitational collapse. Occurs when a massive body collapses under its own weight. For example, interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.


Gravitational Lens. A massive object which magnifies or distorts the light from a more distant object along the same line of sight.


Gravitational Lensing. The distortion or amplification of an object’s light due to the presence of a massive object in the light path.


Gravitationally bound. Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around the Earth are gravitationally bound to this planet since they can’t escape its gravity. By contrast, the Voyager spacecraft, which explored the outer solar system, was launched with enough energy to escape Earth’s gravity altogether, and hence it is not gravitationally bound.


Gravity. The force of attraction between two or more masses. This force is dependant on both the masses themselves as well as the distance between them. Being a force, gravity is strictly speaking measured in Newton's but is commonly measured in Kilogrammes.

Gravity Waves. Weak, wavelike disturbances which represent the radiation related to the gravitational force; produced when massive bodies are accelerated or otherwise disturbed.


Great circle. A circle on the surface of a sphere whose plane passes through the centre of that sphere.


Greatest eastern elongation. The greatest angular distance to the east of the Sun reached by Mercury or Venus. When a planet is at its eastern elongation, it sets after the Sun and is at its best visibility in the evening sky.


Habitable Zone (or Ecosphere). The zone around a star in which a planet can maintain liquid water on its surface.


Halo, galactic. A roughly spherical shaped region around the main part of the Galaxy.

Heliacal Rising. The period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun.


Heliocentric. As seen from the centre of the Sun. A system of co-ordinates. A heliocentric orbit is one based on the sun as one of the two foci of the (elliptical) orbit (or as the centre of a circular orbit); a heliocentric magnitude is the brightness of an object as would be seen from a heliocentric distance of 1 AU (which means a distance of 1 AU from the sun).

Heliosphere. A vast region around the sun dominated by the solar wind.


Helium. The second lightest element; consists of two protons, and usually two neutrons and two electrons; about 8 percent of the atoms in the universe are helium.


Herschel, William (1738-1822). Sir William Herschel was a renowned astronomer who first detected the infrared region of the electromagnetic spectrum in 1800.


Hertz. A unit of frequency equal to one cycle per second.


Hertzsprung-Russell diagram. A diagram in which stars are plotted according to their spectral type and their absolute magnitude. A diagram that plots luminosity against temperature for a group of stars.

HII Region. An area filled with clouds of ionized hydrogen; the ionization is usually caused by radiation from newborn stars.


Horizon. The great circle on the celestial sphere which is everywhere 90 degrees from the observers zenith, the point directly overhead the observer.

Hours, Minutes, Seconds (h, m, s). Measure of time or right ascension.

HST. The Hubble Space Telescope makes its observations from above Earth's atmosphere. The telescope orbits 600 kilometers (375 miles) above Earth, working around the clock. It was originally designed in the 1970s and launched in 1990. The telescope is named for astronomer Edwin Hubble.


Hubble, Edwin P. (1889-1953). An American astronomer whose observations proved that galaxies are what he called “island universes” outside our galaxy and not nebulae within our galaxy. His greatest discovery, called Hubble’s Law, was the linear relationship between a galaxy’s distance and the speed with which it is moving. The Hubble Space Telescope is named in his honour.


Hubble's Constant. The rate of increase of the recession of a galaxy with increased distance from the Earth. This figure varies depending on which observational data one uses but is often around 50 miles per second per Megaparsec. The principle that a distant galaxy’s recessional velocity is proportional to its distance from Earth. The constant H0—discovered in 1925 by Edwin Hubble—determines the relationship between the distance to a galaxy and its velocity of recession due to the expansion of the Universe. After many years in which the Hubble constant was only known to be somewhere between 50 and 100 km/s/Mpc (kilometers per second per megaparsec) it has recently been determined by the Hubble Space Telescope’s Key Project team to be 70 km/s/Mpc 7 km/s/Mpc. Advances in cosmology have shown that since the Universe is self gravitating, H0 is not truly constant. Astronomers thus seek its current value.


Hubble’s Law: The relationship—discovered in 1925 by Edwin Hubble—between a galaxy’s distance from us and its velocity through space. The farther away a galaxy is, the faster it is receding from us. The constant of proportionality is the Hubble constant, H0. Hubble’s Law is interpreted as evidence that the universe is expanding.


Hydrazine. A colorless liquid which burns rapidly and is used as a common rocket and missile fuel.


Hydrogen. The simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen.


Hypered Film. Film that has been treated, usually with gas, to enhance its response to low light levels.


Igneous Rock. Rock formed by the solidification of magma.


Immersion. The entry of an object into shadow during an eclipse, or the covering of an object during an occultation.

Inclination. The inclination of a planet’s orbit is the angle between the plane of its orbit and the ecliptic. The inclination of the orbit of a planet’s moon is the angle between the plane of the moon’s orbit and the plane of the planet’s equator.

Inertia. The ability of an object to resist acceleration or deceleration. Inertia can be broadly equated with mass.

Inferior Conjunction. The passage of Mercury or Venus between Earth and the Sun. The outer planets cannot pass between Earth and the Sun and therefore cannot come to inferior conjunction.


Inferior Planets. The planets (Mercury and Venus) that lie closer to the Sun than Earth.

Inflation. A brief and extraordinarily rapid period of expansion a fraction of a second after the Big Bang.


Infra-Red Radiation. Electromagnetic radiation at wavelengths that are longer than those the red end of the visible-light spectrum and shorter than microwaves (roughly between 1 and 100 microns). Very little infrared light reaches the surface of the Earth, although some can be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of Mauna Kea in Hawaii).

Interacting Galaxies. Galaxies caught in each other’s gravitational embrace, often results in galactic mergers or extreme star formation.


Interference Fringes. A wave-like pattern resulting from the successful combination of two beams of light which amplifies the light.


Interferometer. A system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope.


Interferometry. The technique of using two or more widely separated telescopes to achieve the resolving power of a much larger telescope.


Intergalactic. The space between the galaxies.


International Space Station (ISS). A global cooperative program between the United States, Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization of a permanently habitated space station in low-Earth orbit.


Interplanetary. The space between the planets.


Interstellar. The space between the stars of a galaxy.


Interstellar Medium. The gas and dust between the stars that fills the plane of the galaxy. For centuries, scientists believed that the space between the stars was empty. It wasn’t until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it is not uniformly distributed through space, but rather has its own unique structure.


Ion. An atom or molecule that has lost one or more of its electrons (a positive ion) or gained one or more electron (a negative ion).

Ionic Gas. Also known as ionized gas. Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose electrons.


Ionisation. The process where electrons are added to or stripped from an atom or molecule turning it into an ion.

Ionosphere. The region of the Earth's atmosphere that lies above the stratosphere and below the exosphere. The ionosphere is an ionised (charged) layer of the atmosphere (hence the name) that lies at a height of 60 to 1000km (38 to 620 miles) above the Earth. The layer is thought to be a product of the absorption of the Sun's ultraviolet radiation. The ionosphere lies below the exosphere and above the stratosphere.


Irregular Galaxy. A galaxy without a clearly defined spiral or elliptical shape.

Isotope. Forms of an element in which the atoms all have the same number of protons but different numbers of neutrons.


Jet. A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.


Jet Propulsion Laboratory. The lead U.S. centre for robotic exploration of the solar system, located in Pasadena, California; JPL spacecraft have visited all known planets except Pluto.


Jet Stream. A high-speed, wandering wind current in the upper troposphere that blows from west to east and affects weather.


Jovian. Pertaining to the planet Jupiter.

Julian day. The number of days (and fractions thereof) that have elapsed since noon on January 1, 4713 BC (Greenwich Mean Time). It is used to simplify the calculation of the time interval between two events. For example, 9:00 P.M. P.S.T. on January 1, 2000, was Julian day 2,451,545.71.


Julian year. Exactly 365.25 days, in which a century (100 years) is exactly 36525 days and in which 1900.0 corresponds exactly to 1900 January 0.5 (from the Julian-date system, which is half a day different from civil time or UT). The standard epoch J2000.0, now used for new star-position catalogues and in solar-system-orbital calculations, means 2000 Jan. 1.5 Barycentric Dynamical Time (TDB) = Julian Date 2451545.0 TDB. When this dynamical, artificial "Julian year" is employed, a letter "J" prefixes the year.


Kelvin (degrees). A measurement of temperature, symbol 'K'. Kelvin is measured in degrees from absolute zero. So, 0 degrees Kelvin equals minus -273.16 degrees Centigrade.

Kepler, Johannes (1571-1630). A German astronomer and mathematician. Considered a founder of modern astronomy, he formulated three famous laws of planetary motion that comprised a quantitative formulation of Copernicus’s theory that the planets revolve around the Sun.


Kepler’s First Law. A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler’s Second Law. A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.

Kepler’s Third Law. The square of the period of a planet’s orbit is proportional to the cube of that planet’s semimajor axis; the constant of proportionality is the same for all planets.

Kiloparsec. One thousand parsecs, equal to 3260 light years.

Kirkwood gaps. Regions in the asteroid belt where very few asteroid are found. The gaps are caused by Jupiter's gravitational influence which shifts asteroids out of orbit if their orbital period is a precise fraction of Jupiter's orbit.


Kuiper Belt. The Kuiper Belt lies outside the orbit of Neptune at a distance from the Sun of 30 to 100 AU and contains an estimated 35,000 objects with diameters greater than 100 kilometers (60 miles). The first Kuiper Belt object was detected in 1992. Most periodic comets - that is, those comets that orbit the Sun on a regular schedule - come from the Kuiper Belt.


L Chondrite. A chondrite (a stony meteorite containing small, round, silicate granules called chondrules) that has a low amount of iron.


Lagrangian points. Five places where small bodies can exist in stable orbits in the plane of two larger bodies. Three of these points lie in a line joining the two large bodies; one point between the two larger bodies (L1); the other two points either side of them (L2 & L3). The remaining points lie 60 ahead of and behind one of the larger bodies in its orbit around the other (L4 & L5).

Large Megellanic Cloud. An irregular galaxy that orbits the Milky Way Galaxy.


Last Quarter. The phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated.


Latitude, celestial. The angular distance of a celestial object from the nearest point on the ecliptic.

Lens. A curved piece of glass that brings light to a focus.


Lenticular Galaxy. A galaxy possessing a large bulge and small disk.


Libration. The apparent tilting of the Moon as seen from Earth. The result is that over a period of time it is possible to see 59% of the surface of the Moon from Earth, though of course, only 50% at any one time.

Light. The common term for electromagnetic radiation, usually referring to that portion of the electromagnetic spectrum that is visible to the human eye. Other bands of the EM spectrum, however, are also often referred to as different forms of light.


Light-Gathering Power. The ability of a telescope to collect light; the larger a telescope’s aperture, the greater its light-gathering power.


Light Pollution. The brightening of the night sky due to artificial light. Light pollution makes it impossible to view many dim objects that can only be seen in a very dark sky.


Light Year (l.y.). The unit of distance in which light travels in one year - 9 464 566 100 km (5 878 612 500 miles).

Light, Speed of. 299 792.5 km (186 291 miles) per second in a vacuum, the fastest speed in the Universe.

Limb. The apparent edge of a body such as the Moon as seen from Earth.

Limiting Magnitude. The apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using.


LINER Galaxy. A low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions.


Local group. A term used to describe the local cluster of galaxies of which the Milky Way galaxy is part. The largest member of the Local Group is the Andromeda Galaxy, Messier 31.

Local Supercluster. The galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member.


Locked rotation. The condition in which a moon has the same period of rotation as its period of revolution around its parent body. This means that the moon always shows the same face to its parent. Our Moon is in locked rotation around Earth.


Long-Period Comet. Comets that have orbital periods greater than 200 years.


Longitude. The angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England.


Luminosity. An expression of the true brightness of a star as compared to the Sun. The Sun’s luminosity is 1.0 by definition. Sirius, for example, has a luminosity of 23. Rigel has a luminosity of about 50,000.


Lunar Eclipse. An eclipse of the Moon caused when the Moon moves partially or wholly into the shadow of Earth and grows dark for up to a few hours. A lunar eclipse can be seen by everyone on the side of Earth facing the Moon.


Lunar Month. The period of one complete revolution of the moon around Earth, 29.5 days.


Lunation. One complete cycle of phases by the Moon, 29.53 days. A lunation is also known as a Synodic month.


M. The prefix used for the 109 objects that were catalogued by Charles Messier.

Magnetic pole. Either of the two regions in a magnetic field at which the field is most intense. The two regions have opposing polarities, which we label north and south, after the two magnetic poles on Earth.


Magnetograph. An instrument that maps the strength, distribution, and direction of magnetic fields on the sun's disk.


Magnetosphere. The region of the magnetic field of a planet or other solar system body. Only Mercury, Earth, Jupiter, Saturn, Uranus and Neptune are known to have a magnetosphere.

Magnetometer. An instrument used to measure the strength and direction of a magnetic field.


Magnetopause. The boundary between Earth's magnetic field and the solar wind.


Magnetosphere. The dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind.


Magnitude (Brightness). The brightness of a celestial object. The lower the magnitude, the less bright the object and vice-versa. For mainly historical reasons magnitude has the peculiar attribute of having brighter objects at negative values and vice-versa. Each whole number of magnitude is equal to a factor of 2.5:1. So, an object of magnitude +1 is 2.5 times brighter than an object of magnitude +2 but is 2.5 times fainter than an object of magnitude 0. Five whole units of magnitude are equal to a factor of approximately 100 times. So, a magnitude +6 object is 100 times fainter than an object of magnitude +1. (See also Absolute magnitude and Apparent magnitude.)

Main Belt. A collection of thousands of rocky and metallic bodies revolving more or less together between the orbits of Mars and Jupiter, roughly 2.0 to 3.5 AU from the Sun. Most asteroids are part of the Main Belt.


Main Sequence. A band within the Hertzsprung-Russell diagram that contains the majority of normal stars except for giant stars and white dwarfs.

Maksutov Telescope. A type of telescope that uses mirrors and lenses. Maksutov's are in fact Cassegrain telescopes: In the case of a Maksutov the light enters the front of the telescope via a curved corrector plate or lens (curved towards the interior of the telescope) which directs the light onto the concave primary mirror (which has a hole at its centre) which lies at the rear of the telescope. The primary mirror then reflects the light back toward the corrector plate which has a small convex secondary mirror coated onto the centre of the corrector plate. This secondary mirror then directs the light through the hole in the centre of the primary mirror and on into the eyepiece which is attached to the rear of the telescope. Like all Cassegrain telescopes the Maksutov has the advantage to being able to have a longer focal ratio than a Newtonian reflector of equal optical size but at the expense of a more complicated optical train.

Mantle. The portion of a planet’s interior above the core but below the crust.


Mare. A dark and relatively smooth area on the surface of the moon or a planet.


Mass. A measure of a body's inertia (resistance to acceleration), the amount of matter that a body contains. Strictly speaking, mass is not the same as weight or gravity, although on Earth they are often regarded as the same thing. Mass is measured in Kilogrammes. Apart from speeds approaching that of light, the mass of a body remains constant whereas weight or gravity is dependant on the masses of two or more bodies and the distance between them.

Mass Loss. The loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova.


Massing. The close alignment of three or more planets—or two or more planets and the Moon—as seen from Earth. This occurs when all the bodies involved in the massing have similar ecliptic longitudes.


Mean Sun. An imaginary Sun travelling at a speed equal to the average rate that the real Sun travels along the ecliptic.

Mean. The average of a series of values.

Megaparsec. One million parsecs, a distance equal to 3,260,000 light years.

Meridian. The imaginary line in the sky that extends from the southern point on the horizon through the zenith to the northern point on the horizon, bisecting the sky into an eastern and western half. Objects are at their highest when they cross the meridian. The Sun is on the meridian at noon, local time. Also a line on the surface of Earth (or another body) that extends from pole to pole.

Messier Catalog. A catalog of 107 bright deep-sky objects that belong to a catalog compiled by French astronomer Charles Messier in the 1700s.


Messier, Charles (1730-1817). The 18th-century French astronomer who compiled a list of 110 fuzzy, diffuse objects that appeared at fixed positions in the sky. Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the sky.


Messier object. One of the 110 objects in the catalog compiled by Charles Messier. Most Messier objects are galaxies, star clusters, or nebulae.


Metalicity. Elements in a star heavier than helium.


Meteor. A streak of light, lasting a few seconds at most, produced when a meteoroid enters the Earth's atmosphere. Most meteoroids are barely the size of a grain of sand and are thought to be cometary debris. In most cases the object is destroyed by friction in Earth's ionosphere at a height of about 100 to 160 km (60 to 100 miles).

Meteor Shower. A period of enhanced meteor activity that occurs when Earth collides with a swarm of meteoroids; an individual shower happens at the same time each year and has all its meteors appearing to radiate from a common point.


Meteor Storm. Meteor storms are rare events that occur when Earth encounters dense regions within a meteor stream. Such encounters can increase normal meteor rates by more than 1,000 meteors per minute.


Meteorite. The solid particle, either stone or iron, that falls through the atmosphere to produce a meteor. Most meteorites are fragments of asteroids. Science museums display meteorites that survived their falls.

Meteoroid. Any small solid object in space. Most meteoroids are barely bigger than a grain of sand.

Metonic Cycle. The period of 6 939.6 days, or 19 calendar years, after which the Moon's phases recur on the same day of the year. This period is also equal to 253 lunations.

Microgravity. A condition in which the force of gravity is very low, producing a near-weightless environment.


Microlensing. The effect of gravity from a small astronomical body or bodies focusing light rays, similar in manner to lenses.


Micrometeorite. A very small particle of interplanetary debris, too small to cause the luminous flash associated with meteors.

Micron. One thousandth of a millimetre. The symbol used is (mu).

Microwaves. The most energetic form of radio waves.


Midnight Sun. The Sun when visible at midnight, which occurs only in the summer north of the Arctic Circle and south of the Antarctic Circle.


Milky Way, The. (1) Another name for The Galaxy. (2) The broad band of faint light that stretches across the sky (when the time is favorable) as seen from a dark site. It is the result of countless millions of stars of The Galaxy seen edge on from the viewpoint of Earth. The Milky Way is brightest in the Northern hemisphere in the constellation of Cygnus, and in the Southern hemisphere in the constellation of Crux.

Million. 1 000 000 - one thousand, thousand.

Millisecond Pulsar. A neutron star that rotates hundreds of times per second, which typically accretes matter from a stellar companion.


Minor Planet. A rocky body that orbits the sun; also known as an asteroid.


Mock Sun. An effect caused by ice crystals in Earth's atmosphere which refract sunlight and cause the appearance of two diffuse patches of light 22 either side of the Sun. These patches are termed Parhelia or Sundogs and often appear on the rim of a halo surrounding the Sun. The patches often have coloured fringes, red on one side, blue on the other.

Molecule. A combination of two or more atoms that represents the smallest part of a compound that has the chemical properties of that compound.


Month, Anomalistic. The interval between two successive perigee passages of the Moon, equal to 27.55 days.

Month, Nodical or Draconic. The interval between successive passages of the Moon through one of its nodes, equal to 27.21 days.

Month, Sidereal. The revolution period of the Moon relative to the stars, equal to 27.32 days.

Month, Synodic. The interval between two successive New Moon's (a lunation), equal to 29.53 days.

Month, Tropical. The time taken for the Moon to return to the same celestial longitude (7 seconds shorter than the sidereal month).

Moon. A smaller body orbiting a larger body; often refers to Earth’s moon.


Moon, Blue. The second Full Moon that occurs during a calendar month. Since the Moon takes 29.53 days to complete a cycle of phases a Blue Moon cannot occur in February. The last Blue Moon was on 30th November 2001, there are none during 2002 and 2003. The next Blue Moon occurs on 31st July 2004. Blue Moon's occur roughly every two and a half years so are not very common. Hence the term "Once in a Blue Moon".

Moon, Harvest. The Full Moon that occurs nearest the time of the autumnal equinox. So-called because this will occur around the time of harvest.

Moon, Hunter's. The Full Moon that occurs after Harvest Moon. So-called bacause this will occur around the start of the hunting season.


Morning Star. The planet Venus when it appears in the morning sky.

Multiple Star System. A gravitationally bound system in which two or more stars orbit a common centre of mass.


Nadir. The point on the celestial sphere directly below the observer. The nadir is directly opposite the zenith.

Naked Eye. Something visible or accomplished without the aid of binoculars or a telescope (e.g. a naked-eye object or naked-eye observing).


Near Infrared. Light from the part of the infrared band of the electromagnetic spectrum closest to the visible range.


Nebula, Nebulae. Any cloud of gas and dust in space. The three nebulae types listed below may appear on their own but are frequently seen in combination.

Nebula, Dark. A dense cloud of gas and dust in space that appears as a dark silhouette against a brighter background. In many cases dark nebulae are part of a larger nebulae complex which provides the light background against which the dark nebulae appears. At infra-red wavelengths young stars not visible at visual wavelengths are often found to be forming. Dark nebulae sometimes appears as a dark feature against a starry background but without any other associated nebulae.

Nebula, Emission. A gas cloud which is close to a star or stars. The radiation from the stars excites the gas so that it emits light of its own. Most of this gas is hydrogen which emits the red/orange light which is typical of this type of nebulae.

Nebula, Reflection. A cloud of gas and dust which lies close to a star or stars which reflects light from those stars. This type of nebulae is often part of a larger nebulae complex (although it is sometimes seen without the other nebulae types above) and the reflection is typically blue in colour due to the dust within the nebulae.

Neutrino. A fundamental particle produced in massive numbers by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without any interaction with the material that makes up this planet.

Neutron. A particle with approximately the same mass of a proton, but with zero charge. Commonly found in the nucleus of atoms.


Neutron star. The remnant of a very massive star that has undergone a supernova explosion. Neutron stars send out rapidly changing radio emissions and are frequently termed as pulsars.

Neutron. An atomic particle with no charge and a mass almost equal to a proton.

New Moon. The phase in which the moon is in the same direction as the sun in Earth’s sky, so it is unilluminated and invisible.


Newton, Isaac 1642-1727. The English cleric and scientist who discovered the classical laws of motion and gravity. The bit with the apple is probably apocryphal.


Newton’s Law of Universal Gravitation. Two bodies attract each other with equal and opposite forces. The magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centres of mass of the two bodies.


Newton’s First Law of Motion. A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton’s Second Law of Motion. For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed. The constant of proportionality is the inertial mass of the body.

Newton’s Third Law of Motion. In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.


Newtonian reflector. A reflecting telescope in which the light is collected by a concave primary mirror at the rear of the telescope and then reflected back to a flat secondary mirror angled at 45 degrees mounted near the front of the telescope. The light is then reflected through the side of the telescope tube and on into an eyepiece. It is the simplest type of reflecting telescope.

NGC-IC. The New General Catalogue (NGC) was originally compiled and published by J. L. E. Dreyer in 1888. Two additional supplements—called the Index Catalogue (IC)—were published in 1895 (IC I) and 1908 (IC II). These catalogues were an early attempt to create a single list containing all the non-stellar objects known at the time. The original NGC/IC catalogue contains 13,226 entries. Starry Night uses Wolfgang Steinicke’s corrected NGC/IC catalogue, which contains 13,993 entries and is regarded as a more complete and reliable database that includes all the objects listed in Dreyer’s original catalogue and its two supplements, plus many new objects (named with extension letters) and companions.

Night vision. The increased ability to see dim object, such as faint stars, due to a sensitization of the eye's 'rod' receptors. Exposure to bright light desensitizes the rods and therefore reduces night-vision.


Nodes. The points at which the orbit of the Moon, a planet or a comet intersect with the plane of the ecliptic; south to north (Ascending Node), north to south (Descending Node).

North Celestial Pole. The sky’s north pole. The point in the sky directly above Earth’s north pole.


Nova. A star that suddenly flares up to several times its normal brightness. Typically they remain bright for a short period before fading back into obscurity.

Nuclear Fission. A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the parts. The difference in mass is converted to energy according to Einstein’s famous equation, E=mc2 where he discovered that the energy contained in such a reaction (E) is equivalent to the difference in mass (m) times the square of the speed of light (c) which is a very, very large number. This is the source of the Sun’s energy.


Nucleosynthesis. The creation of heavy elements from lighter ones by nuclear fusion.


Nucleus. The central part of an atom containing two particles; a proton and (with the exception of hydrogen) and a neutron.

Nutation. A slow 'nodding' of the Earth's axis due to the gravitational tug of the Moon on the Earth's protuberant equator.


O-Type Star. A hot, massive blue star that emits strongly at ultraviolet wavelengths and has a surface temperature between about 28,000 to 40,000 kelvins.


OB Association. A loose grouping of O and B stars, which are the most luminous, most massive, and shortest-lived stars.


Objective. A telescope’s primary lens or mirror that gathers light and brings it to a focus.


Oblateness. The measure of how much a rotating object deviates from being a perfect sphere. A perfect sphere would have an oblateness of 0.0 (0%). The Earth for example is oblate to a factor of 0.0034 (0.34%), while Saturn, the most oblate of the planets has a figure of 0.108 (10.8%). Generally, the planets have an equatorial bulge and in the case of Saturn, its equatorial diameter is some 10.8% greater than its polar diameter.

Obliquity of the ecliptic. The angle between the ecliptic and the celestial equator: 23 degrees, 26 minutes and 45 seconds. This angle is also an expression of the tilt of Earth's axis. This determines the angle the ecliptic makes with the celestial equator as they intersect in the sky.

Occultation. The covering-up of one celestial object by another. For example, a total solar eclipse is an occultation of the Sun by the Moon.

Omega. 1. The ratio of the density of the universe to the critical density 2. The 24th letter of the Greek alphabet.


Omega Centauri. A massive globular cluster in the southern constellation Centaurus located about 17,000 light-years from Earth; also known as NGC 5139.


Omega Nebula. One of the Milky Way’s numerous stellar nurseries, the Omega Nebula is about 5,000 light-years from Earth and can be seen in the constellation of Sagittarius the Archer. It is also known as the Swan Nebula, M17, NGC 6618, the Horseshoe Nebula, and the Lobster Nebula.


Oort Cloud. A cloud of objects up to 50,000 AU from the Sun. There are likely billions of comets in the Oort Cloud. The Oort Cloud is the source of non-periodic comets. Non-periodic comets are comets that swing around the Sun once and are flung off into space, never to return.


Open Cluster. A loose association of stars with no clearly defined shape. Often referred to as 'Galactic Clusters' since they are all found close to the galactic plane (of our galaxy). There are a little over 1000 of these clusters and they may contain just a dozen stars or as many as 500. The clusters occupy a region of space of only a few tens of light years across and are formed in the same region of space from a nebula, only in time do the stars disperse. In some instances traces of the original nebula can still be seen.

Opposition. The position of a planet when it is opposite the Sun in the sky. At this point the Sun, Earth and the planet are roughly in line and the planet will rise at about the same time that the Sun sets: the planet is thus visible virtually all night. A good example is when the Moon is Full, it could be said to be in opposition.

Optical Double. Two stars at different distances that lie along nearly the same line of sight and thus appear close together.


Orbit. The path of a celestial body around its parent body.

Orbital Period. The length of time it takes one body to orbit another.


Orrery. A model showing the Sun and planets. The model is usually capable of being moved mechanically so that the planets move at the correct speed around the Sun relative to each other. Some versions of an Orrery have just the Sun, Earth and Moon.


Outgassing. The release of gas from a rocky body.


PAHs. Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules. They are flat molecules made only of carbon and hydrogen atoms. PAH molecules are quite common and highly carcinogenic. They are one of the by-products of combustion from automobiles and airplanes, and some are present in charcoal broiled hamburgers.


Parallax, Trigonometric. The apparent shift of an object when viewed from two different directions. This shift may be used to gauge the distance of an object out to distances of about 1 000 light years or more. The object is observed six months apart and the amount it shifts relative to the background stars is measured in arcseconds. No star subtends an angle of one arcsecond, so the angles measured this way are extremely small. However, if the angle is measurable it is possible, by using trigonometry, to measure the distance to a star.

Parsec (pc). Parallax second. The distance at which one astronomical unit subtends an angle of one arcsecond. The distance at which a star would show parallax of one second of arc. One parsec is equal to 3.26 light years, or 206 265 astronomical units, or 30.86 million million kilometres.

Patera. An irregular, saucer-shaped volcanic structure.


Penumbra. (1). The lighter part of a sunspot. (2). The area of partial shadow either side of the main umbra cone of shadow cast by the Earth.

Penumbral Eclipse. An event that occurs when the moon passes into the outer ring of Earth's shadow (penumbra), causing a slight shading in the moon's appearance.


Periapsis. The point in an orbit when the two objects are closet together. The opposite of apoapsis.


Periastron. The point of closest approach between two stars that are in orbit around each other. The opposite of apastron.


Perigee. The point in the Moon's or a satellite’s orbit when it is closest to Earth.

Perihelion. The point in an object’s orbit when it is closest to the Sun. Earth is at perihelion each year around January 3. The opposite of aphelion.

Period. The time an object takes to complete a certain motion and return to its original state—for example, the period of revolution or the period of rotation.

Periodic Comet. A comet that has been seen to orbit the sun more than once (Comet Halley was the first recognized as periodic).


Perturbations. The disturbances in the orbit of a celestial object produced by the gravitational effects of other bodies. Gravitational influences ("tugging" and "pulling") of one astronomical body on another. Comets are strongly perturbed by the gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in orbit computations, and they lead to what are known as "osculating elements" (which means that the orbital element numbers change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is necessarily stated to denote the particular date that the elements are valid.

PGC/LEDA. A catalogue containing primary information for over one million galaxies.


Phase angle. For a solar system object besides the earth and sun, the angle between the earth and the sun (or the earth's elongation from the sun) as seen from that third object. The phase angle is given in ephemerides on IAU Circulars and Minor Planet Circulars is denoted by either of the lower-case Greek letters beta or phi.


Phases. The apparent change in shape of the Moon and inferior planets from Full to New. Mars sometimes displays a gibbous phase but no other planets show a significant phase as seen from Earth.

Photometer. A device that measures the intensity of light from a particular source.

Photometry. The measurement of light intensities, generally in the visible or infrared bands, in which a specific or general wavelength band is normally specified.


Photon. The smallest—quantum - unit of visible light or other electromagnetic energy.

Photosphere. The bright visible surface of the Sun.

Photovoltaic. Conversion of light energy into electricity.


Pixel. Short for “picture element,” the individual light detectors on a CCD chip.


Planck Scale. The smallest units of measurement scientists use to describe the universe; a Planck unit of length is 10^-33 centimetres.


Planetary nebula. A shell of gas that surrounds a small, dense, hot star. The name is misleading because planetary nebulae have no connection with planets. They probably got their name because some of these nebulae appear disk-like and may have been initially mistaken for planets.

Planet. Any large body orbiting a star. A somewhat arbitrary term since there appears to be no defining size that clearly differentiates between a planet or asteroid. For example, Pluto is widely regarded as the ninth planet of the Solar System yet its parameters do not clearly conform to the other eight planets of the Solar System. Although it seems unlikely to lose its planetary status, Pluto does appear to be an oddity when you consider such things as its orbit, size and other known data.

Planetesimals. Asteroid-size bodies in a young planetary system that collide to form larger bodies.


Planetoid. Another name for an asteroid or minor planet.

Planisphere. A circular map of the stars covered by a rotating mask that has a window or opening in it. The map and mask are the same size and have time and date markings on the outside. Lining up the time/date marks then expose a given area of the map through the mask so that the stars visible for a given time can then be seen. Planispheres are usually devised so that they can only be used within a certain latitude range.

Plasma. A gas that has been heated to a state where it contains ions and free-floating electrons; also known as ionized gas.


Plasmasphere. A region of cold, high-density plasma above the ionosphere.


Plate Tectonics. A theory that describes how Earth’s crust is broken into plates and how those plates move across Earth’s surface.


Polar Cap. An icy region at the north or south pole of a planet.


Polarization. A state in which the directions of the electric or magnetic field in an electromagnetic wave changes in a regular pattern; light from celestial objects is often polarized.


Poles, celestial. The north and south points of the celestial sphere.

Populations, stellar. There are essentially two types of star regions. I - in which the brightest stars are hot and white; and II - in which the brightest stars are old Red Giants. This is an odd distinction because the first stars to have populated a given region would now be classified as Population II! In our own galaxy Population II stars are more numerous toward the centre of the galaxy while Population I stars are more numerous in the outer regions of the galaxy and spiral arms.

Position Angle. The direction in the sky of one celestial object from another, measured eastward from due north.


Power. The ability of a telescope or binoculars to increase the apparent size of a distant object.


Poynting-Robertson Effect. A drag on interplanetary particles caused by their interaction with solar radiation, which causes the particles to lose orbital momentum and spiral into the sun.


Precession. This is the slow movement of the celestial poles tracing out large circles on the celestial sphere. It is caused by a slow wobble in the Earth's axis due to the gravitational effects of the Sun and Moon on the Earth's equatorial bulge. A reasonable analogy is that of a spinning top - As the top slows down it will start to wobble. This 'Earth wobble' is very slow, one 'wobble' takes about 26 000 years to complete. The other effect is that the equinoxes are slowly moving or processing, at the rate of about 50 arcseconds a year. The Vernal Equinox is also called the First Point of Aries but due to the effects of precession the Vernal Equinox is now in the constellation of Pisces and moving towards Aquarius.

Precession of the equinoxes. The slow wobbling of Earth’s axis in a 25,800-year cycle, caused by the gravitational attraction of the Moon on Earth’s equatorial bulge. Precession causes the vernal equinox (and all other points on the ecliptic) to move westward along the ecliptic, slowly changing the equatorial co-ordinate grid.


Primal Glow. See cosmic background radiation.


Primary. A term used to describe the larger or brighter member of a pair of celestial objects.

Primary Lens. A telescope's main lens which gathers light and brings it to a focus.


Primary Mirror. A telescope's main mirror which gathers light and brings it to a focus.


Prime Meridian. The meridian on Earth's surface which passes through Greenwich Observatory. It is taken to be the dividing line between the east and west hemispheres, 0 degrees longitude.

Prism. A wedge-shaped piece of glass that breaks white light into its constituent colors.


Prograde. Objects that move or appear to move in the same direction of most solar system bodies, or for moons, the same direction as the planet rotates.


Prominences. A mass of glowing gas, mainly hydrogen, that rises from the surface of the Sun.

Proper motion. The motion of the stars relative to each other, caused by their actual motion in different directions at different speeds through space.

Proton. An atomic particle, part of the nucleus, that has a positive electrical charge.

Protoplanet. A body that is accreting gas, dust, and rocks en route to becoming a full-fledged planet. Protoplanets are moon-sized planets, or larger embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide. According to planet formation theory, protoplanets perturb each other's orbits slightly and thus collide in giant impacts to gradually form the real planets.


Protoplanetary Disk. A disk of gas and dust that surrounds a newborn star; planets form from collisions of particles inside the disk.


Protostar. A cloud of hot, dense gas and dust that is gravitationally collapsing to form a star. The very dense region, or core, of a molecular cloud where a star is in the process of forming.


Proxima Centauri. The nearest star to the sun at a distance of 4.2 light-years.


Ptolemy (circa 100 to circa 170 A.D.). Also known as Claudius Ptolemaeus. Ptolemy believed the planets and the Sun orbited the Earth in the order Mercury, Venus, the Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system. He authored a book called Mathematical Syntaxis (widely known as the Almagest). The Almagest included a star catalog containing 48 constellations, using names we still use today.


Pulsar. A neutron star that is rapidly rotating and emitting radio waves. Pulsars are believed to vector the radio emissions so that if Earth lies in the line of sight they appear like a lighthouse (when seen by a radio telescope, that is).


Quadrature. The position of the Moon or another planet when it is at right angles to the Sun as seen from Earth. An example would be when the Moon is seen at First or Last Quarter phases.

Quadrillion. 1,000,000,000,000,000


Quantum Mechanics. The physical laws that describe the behavior of matter at the atomic and subatomic level.


Quasar. A very remote, highly luminous body. They are now known to be the cores of very energetic galaxies although precisely what makes them so luminous is uncertain.


r. The alphabetic letter ("variable") used to denote the distance between the sun and the object being discussed, also called the object's heliocentric distance; in most ephemerides of objects such as comets and minor planets, r is given in AU. Similarly, the upper-case Greek letter Delta gives the distance between the object and the earth (its geocentric distance).


Radial Velocity. The movement of a celestial body either away from (a positive value) or toward (a negative value) the observer.

Radiant. The point in the sky where meteors of a given shower seem to originate or radiate from.

Radiation. Electromagnetic waves.


Radiation Pressure. A very small amount of pressure exerted on a surface by light or other electromagnetic radiation.


Radio Galaxy. A galaxy that emits an unusually large amount of radio waves.


Radio Telescope. A telescope designed to detect radio waves coming from space.


Radio Waves. Electromagnetic radiation with a wavelength longer than infrared. The form of light with the longest wavelength and the least energy.

Radiometer. A device that measures the total energy or power from an object in the form of radiation, especially infrared radiation.


Red Dwarf. A low-mass, main-sequence star much smaller, cooler, and less luminous than the sun. Most red dwarfs are about one-tenth the mass and diameter of the Sun.


Red Giant. A star that is large and cool compared with the Sun but is many times the diameter of the Sun. These are stars that are nearing the end of their lives.

Red Supergiant. A cool, massive star near the end of its life that has expanded to a size from a hundred to a thousand times the diameter of the sun.


Redshift. An increase in the wavelength of light coming from an object due to its motion away from Earth, the expansion of the universe, or a strong gravitational field. An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer.


Reflection Nebula. A cloud of gas and dust that is visible because the dust reflects a nearby star’s light.


Reflector. A telescope that uses a curved mirror to gather light. Nearly all large telescopes in use today by amateur and professional astronomers are reflecting telescopes.


Refractor. A telescope that uses a glass lens to gather light. Binoculars are a type of refractor. In general, refractors are much more expensive to build and buy than are reflectors.

Regolith. 1/ The unconsolidated residual or transported rock and soil that overlies solid bedrock on Earth, the moon, or another planet. 2/ The powdery soil of the moon produced by meteorite impacts


Regression of the nodes. This is a slow movement of the lunar nodes caused by the gravitational pull of the Sun. The lunar nodes move slowly westward, taking 18.6 years to complete one revolution.

Relativity. The theories of motion developed by Albert Einstein, for which he is justifiably famous. Relativity describes the motions of bodies more accurately than does Newtonian mechanics in cases where bodies are in strong gravitational fields or travelling at speeds approaching that of light. All experiments done to date agree with relativity’s preproductnameions to a high degree of accuracy. Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the Photoelectric Effect.


Resolution, Spatial. In astronomy, the ability of a telescope to differentiate between two objects in the sky that are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the spatial resolution of the telescope.


Resolving Power. The ability of a telescope or camera to pick out fine detail.


Retardation. This is the time difference between Moon rise one day, and Moon rise the following day.

Reticule. A grid or pattern of two or more fine wires set in the focal plane of a telescope eyepiece and used in determining the position and/or size of a celestial object.


Retrograde. The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic. Any motion that is in the opposite sense from the great majority of solar system bodies.


Retrograde motion. The temporary backwards or westward motion of a superior planet against the background of the stars that results when the faster-moving Earth on on it’s shorter orbit passes that of the the superior planet.

Reusable launch vehicle (RLV). A single-stage-to-orbit spacecraft that may be reused on successive missions.


Reversing layer. The layer of the Sun's atmosphere above the photosphere.

Revolution. The orbiting of one body around another body, as the Moon revolves around Earth, and Earth revolves around the Sun.


Ribonucleic Acid. A nucleic acid that transmits genetic information.


Rich Clusters. Large galaxy clusters with unusually high population densities.


Rich-Field Telescope. A telescope designed to show a large field of view at low magnification.


Right ascension (R.A.). The angular distance of a celestial body from the Vernal Equinox, measured eastwards. This is one of the equatorial system of co-ordinates and it is measured in hours, minutes and seconds. Right ascension is also the difference in time between the culmination of the Vernal Equinox and the culmination of that object.


In the equatorial co-ordinate system, the angular distance of an object eastward from the zero point (which is the vernal equinox), usually expressed in hours and minutes (which represents Earth’s rotation from the vernal equinox to the object). It is the celestial equivalent of longitude on Earth’s surface.


One element of the astronomical coordinate system on the sky, which can be thought of as longitude on the earth projected onto the sky. Right ascension is usually denoted by the lower-case Greek letter alpha and is measured eastward in hours, minutes, and seconds of time from the vernal equinox. There are 24 hours of right ascension, though the 24-hour line is always taken as 0 hours. More rarely, one sometimes sees right ascension in degrees, in which case there are 360 degrees of right ascension to make a complete circuit of the sky. When specifying a comet's location on the sky, one must state the right ascension and declination (with equinox), along with date and time (since a comet moves with respect to the background stars).

RNA. A nucleic acid that transmits genetic information.


Roche limit. The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At less than this distance the tidal forces of the larger object would break apart the smaller object.


Rotation. The spin of a galaxy, star, planet, moon, or asteroid about a central axis.

Rotation Period. The length of time it takes a body to complete one rotation.


Saros. The period after which the Sun, Earth and Moon return to approximately the same positions, this period being 18 years and 11.3 days. This period is very useful for eclipse prediction since one eclipse will be repeated by an almost identical event precisely one Saros later.

Satellite. A small body that orbits a planet or asteroid.


Schmidt Camera. A catadioptric telescope used as a camera to take wide-angle photos of the sky.


Schmidt-Cassegrain Telescope. A compact telescope in which light passes through a correcting lens at the front of the telescope, then reflects off a primary mirror back up to a secondary mirror, which directs the light through a hole in the primary and out the back of the scope; a popular telescope for backyard observers.


Schwarzchild radius. The radius that a body must have if its escape velocity is to be equal to the velocity of light. This term is frequently encountered when discussing the properties of black holes.

Scintillation. Another term used to describe the twinkling of a star. It is caused by the Earth's atmosphere distorting the light from a star, mainly when the star is at a low altitude. Planets may also scintillate when close to the horizon but much less so than stars.

Secondary. The smaller or dimmer component of a pair of celestial objects that orbits the primary object.

Secondary Mirror. A relatively small mirror used in a telescope to redirect the light gathered by the primary mirror.


Seeing. The quality of observing conditions induced by turbulence in Earth’s atmosphere, which blurs the images of astronomical objects.


Selenography. The study of the Moon's surface.

Semimajor Axis. The semimajor axis of an ellipse (a planetary orbit, for example) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary.


SETI. The search for extraterrestrial intelligence.


Setting Circles. Circular scales on the two axes of an equatorial mount that help an observer point a telescope to a specific right ascension and declination.


Seyfert Galaxies. A kind of so-called 'active' galaxy. Seyfert galaxies have fairly small but bright nuclei and weak spiral arms. These galaxies are also strong emitters of radio wave energy.

Shock Wave. A powerful wave caused by a sudden change in density, pressure, or temperature that travels though a medium faster than sound travels through that same medium.


Short-Period Comet. Comets that have orbital periods of less than 200 years.


Sidereal. Relating to or measured with respect to the stars.


Sidereal Day. The time it takes Earth to rotate once relative to the stars or 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a solar day, since during one sidereal day the Sun also moves 1 east along the ecliptic, and Earth has to rotate 4 additional minutes to complete one rotation relative to the Sun in one 24-hour solar day.


Sidereal period. The revolution period of a planet round the Sun. Also the revolution period of a moon around a planet.

Sidereal time. The local time measured according to the apparent rotation of the celestial sphere. When the Vernal Equinox crosses the observer's meridian the sidereal time is 0 hours.

Sidereal Year. The amount of time it takes one body to revolve about another with respect to the stars.


Siderostat. A flat mirror that can be moved to reflect light from a celestial object to a specific spot.


Singularity. A point at which space and time are infinitely distorted, such as the central point of a black hole where matter is concentrated into an area of zero volume and infinite density. The centre of a black hole, where the curvature of spacetime is at its maximum. At the singularity, the gravitational tides diverge. Theoretically, no solid object can even survive hitting a singularity.


Small Megellanic Cloud. A small, irregular dwarf galaxy that orbits the Milky Way Galaxy.


Solar day. The time it takes Earth to spin once relative to the Sun, or exactly 24 hours (by definition).


Solar Eclipse. An eclipse of the Sun by the Moon that occurs when the Moon passes in front of the Sun. Solar eclipses can be partial, total, or annular. Only the few people in the narrow path of totality see a solar eclipse as total.


Solar Filter. A filter used to block almost all of the sun’s light so our star can be viewed safely and comfortably.


Solar Flares. Violent eruptions of gas on the Sun’s surface.


Solar Irradiance. The radiant energy emitted by the sun over all wavelengths that falls each second on one square meter of Earth's atmosphere.


Solar Mass. A unit of mass equivalent to the mass of the Sun. One solar mass is 1.98914 1030 kilograms (2.1924 1027 tons) or 333,000 times the mass of our puny planet.


Solar System. The system containing the sun and all the smaller bodies in orbit around it.


Solar Wind. The flow of particles from the Sun in every direction. The 'wind' is an ever present feature of the Sun but the intensity of the wind is dependant on Solar activity.

Solstices. The time when the Sun is at its maximum declination either north or south of the celestial equator. There are two solstices; summer solstice (around 22nd June each year) when the Sun is 23 and a half degrees north of the celestial equator. At this point the Earth's northern hemisphere will have its longest day and the Sun's noon altitude will be at its highest; and winter solstice (around 22nd December each year) when the Sun is 23 and a half degrees south of the celestial equator. At this point the Earth's southern hemisphere will have its longest day and the Sun's noon altitude will be at its highest.

South Celestial Pole. The point in the sky to which Earth's Geographical South Pole points.


Space Weathering. The process of altering the surface of an object in space by such phenomena as micrometeoroid impacts, cosmic rays, and the solar wind.


Space-Time. The intertwining of the three dimensions of space with one dimension of time within which events can be specified exactly.


Special Relativity. The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity, and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as it consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence which most people know by the equation E=MC2.


Specific gravity. The density of a substance taking water to equal 1. For example, Saturn has a specify gravity, or density, of 0.71. Therefore an equal volume of water would weigh more than the planet - if you had a big enough bowl of water, Saturn would float in it! On the other hand, Earth has a specific gravity of 5.5, thus Earth would weigh 5.5 times as much as an equal volume of water.

Spectra. Plural of "spectrum" (the energy emitted by a radiant source).


Spectroscope. An instrument used to analyze the light from a star or other luminous object.


Spectral Class. The designation of a star based on its spectrum, which is determined by its surface temperature.


Spectral Line. A particular wavelength of light corresponding to the energy transition of a specific atom or molecule.


Spectral Type. The designation of a star based on its spectrum, which is determined by its surface temperature.


Spectrograph. An instrument attached to a telescope to record the spectrum of an astronomical object.


Spectroheliograph. A device for photographing the sun in a single wavelength of light.


Spectrometer. An instrument attached to a telescope to record the spectrum of an astronomical object.


Spectroscope. A device that splits the light coming from an object. This enables the study of the properties of that light source as well as the detection of elements and molecules present.

Spectroscopic binary. A binary star system where the two (or more) components are too close to be separated visually. Observation by spectroscopic means however can reveal the presence of other components of the binary system that normal optical means cannot.

Spectroscopy. The study of spectra from astronomical objects.


Spectrum. 1. The energy emitted by a radiant source 2. The entire range of electromagnetic radiation (light).


Speed of Light. The fastest possible speed in a vacuum. The speed at which electromagnetic radiation propagates in a vacuum. It is has been measured at 299,792,458 meters per second (186,212 miles per second). Einstein’s Theory of Relativity implies that nothing can go faster than the speed of light.


Spherical Aberration. An optical defect caused when the inner and outer parts of a lens or mirror have different focal lengths and results in blurred images.

Spiral Arm. A concentration of gas, dust, and young stars that winds its way out from the nuclear region of a spiral galaxy.


Spiral Galaxy. The second major type of galaxy, characterized by a central bulge and a number of spiral arms extending from the bulge. See also elliptical galaxy.


Standard Candle. An astronomical object of known luminosity; can be used to determine distances.


Star. A self-luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores.


Star Atlas. A collection of maps that marks the positions of stars, nebulae, galaxies, and other astronomical objects on a coordinate system.


Star cluster. A collection of stars - ranging in number from a few to hundreds of thousands—that are bound to each other by their mutual gravitational attraction.


Star Hopping. The technique of using recognizable patterns of stars to “hop” from one part of the sky to another; useful in observing both with the naked eye and a telescope.


Star Party. A gathering of people to observe the night sky.


Starburst Galaxy. A galaxy undergoing an extremely high rate of star formation.


Stellar Evolution. The life cycle of stars.


Stellar Wind. The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star’s wind is strongest near the end of its life when it has consumed most of its fuel.


Stereocomparator. An instrument that allows astronomers to view two images of the same region of sky simultaneously. Objects that have changed their brightness or position appear to stand out of the plane of the picture.


Stratosphere. The layer of the Earth's atmosphere that lies 10km to 40km (6 to 25 miles) above the surface of the Earth. Within the layer the temperature is almost constant at -55C (-67F). The stratosphere lies above the troposphere, the lowest layer of Earth's atmosphere.

Sublimation. The transition of a solid substance evaporating into a gas without passing through a liquid phase. This happens in the vacuum of space with comets, as the heating effects of solar radiation cause ices in comets to "steam off" as gasses into space. The ice molecules present in the nucleus actually break up (or dissociate) into atoms or smaller molecules after leaving the nucleus in gas form.


Summer Solstice. The moment when the Sun reaches its greatest distance north of the celestial equator, on or about June 21. In the northern hemisphere this marks the first day of summer; in the southern hemisphere it marks the first day of winter.


Sunspot. A dark, temporary, relatively cool spot on the surface of the sun. Their temperature is around 4000C (as against about 6000C for the general photosphere), so that they are dark only by contrast; if they could be seen shining on their own, their surface brilliance would be greater than that of an arc-light.

Sunspot Cycle. A cycle averaging 11 years in which the number of sunspots increases and decreases.


Supercluster. An enormous congregation of galaxy clusters that stretches across hundreds of millions of light-years.


Superfluid. An unusual state of matter characterized by apparently frictionless flow, found only in liquid helium cooled to near absolute zero.


Superior Conjunction. The configuration of an inferior planet when it lies on the far side of the sun.


Superior Planets. The planets Mars through Pluto, so-called because their orbits are beyond Earth’s orbit around the Sun and thus superior to Earth in terms of distance from the Sun.

Superluminal Motion. Motion that appears to be faster than the speed of light.


Supermassive Black Hole. A black hole at the core of a galaxy that contains millions or billions of solar masses.


Supernova. The final explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At its peak light output, a supernova explosion can outshine an entire galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant or SNR.

Supernova Remnant. An expanding cloud of gas that represents the outer layers of an exploded star.


Synchronous Rotation. When a satellite rotates at the same rate at which it revolves around a more massive object; a body with synchronous rotation shows only one hemisphere to the object it orbits.


Synchronous Emission. Electromagnetic radiation from high-energy electrons moving in a magnetic field.


Synodic Month. The time it takes the Moon to complete one cycle of phases—from new moon to new Moon - an average of 29.53059 days.


Synodic Period. The interval between successive oppositions of a superior planet.

Synodic. With respect of the Earth.

Syzygy. The position of the Moon in its orbit when it is either Full or New.


Telescope. The main instrument used to collect the light from celestial bodies, thereby producing an image which can be magnified.


Tera. A trillion (1,000,000,000,000)


Terminator. The dividing line between light and dark on a moon or planet.

Terrestrial. Of or relating to Earth.


Terrestrial Dynamical Time (TDT or TT). Time scale used in orbital computations; TDT is tied to atomic clocks (International Atomic Time, TAI), whereas Universal Time is tied to observations. Prior to 1992, Ephemeris Time (ET) was used in publications of the ICQ/CBAT/MPC; since then, TT has been used. The difference between TDT and UTC in 1994 was 60 seconds (i.e., UT + 60 seconds = TDT).


Terrestrial Planet. A small, rocky planet such as Mercury, Venus, Earth, and Mars.


Thermal Radiation. Electromagnetic radiation emanating from any object not at absolute zero.


Tidal Force. The difference in gravitational force between two points on an object caused by the gravity of another object; the tidal force often leads to a deformation of an object.


Time Dilation. The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both Special and General Relativity and is quite pronounced for speeds approaching the speed of light and in regions of high gravity.


Topocentric. Meaning: As seen from the surface of the Earth. Most celestial co-ordinates used are topocentric.

Total (visual) magnitude. Total, integrated magnitude of a comet's head (meaning coma + nuclear condensation). This can be estimated visually, as the comet's "total visual magnitude". The variable m(sub)1, usually found in comet ephemerides, is used to denote the total (often predicted) magnitude.


Transit. (1). The passage of a body across the observer's meridian. (2). The passage of Mercury or Venus across the face of the Sun.

Transparency. The clarity of the sky.


Trans-Neptunion Object. An object in our solar system lying beyond the orbit of Neptune; abbreviated TNO.


Trapezium. An open cluster of young stars, protostars, gas, and dust in the Orion Nebula featuring four prominent stars that form a trapezium.


Tremolite. A common mineral in some metamorphic rocks, composed mainly of calcium and magnesium; it occurs from the conversion of dolomite (a sedimentary rock), silica, and water.


Triple conjunction. The close alignment of a planet and a star at three distinct times, caused by the retrograde motion of the planet. The planet passes the star once in its forward motion, once more in its retrograde motion, and a third time when it resumes its forward motion.


Trojan. An asteroid that lies in or near one of the Lagrange points 60 degrees ahead or behind Jupiter along the planet's orbit; Trojan asteroids have also been found accompanying Mars and Neptune.


Tropical Year. The time it takes Earth to revolve around the sun with respect to the vernal equinox. The tropical year is identical to our standard year.


Troposphere. The lowest layer of the Earth's atmosphere, lying at an average height of up to 11km (6.5 miles). Above the troposphere lies the stratosphere, above that the ionosphere, above that the exosphere. In the troposphere the temperature drops steadily except for localised layers of temperature inversion.

True Field of View. The angle of sky seen through an eyepiece when it is attached to a telescope; the true field equals the apparent field divided by the magnification.


Tully Database. An exclusive Starry Night database of 28,000 galaxies plotted in three-dimensional space.


Twilight. By astronomical definition, the state of the sky when the Sun is below the horizon but by no more than 18 degrees.


Type 1a Supernova. The explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the Chandrasekhar limit.

Type II Quasars. A quasar enshrouded in gas and dust that emits very little visible light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum.


Type II Supernova. The explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole.


Ultraviolet Light/Radiation (UV). Electromagnetic radiation with a wavelength shorter than visible light but longer than X-rays.


Umbra. (1). The darkest part of the shadow cast by Earth into space. (2). The darker portion of a sunspot. The area traced on a planet during an eclipse where the eclipsed light source is completely blocked. Observers in the umbral shadow of a solar eclipse, for instance, see a total eclipse. See also penumbra.

Unidentified Infrared Bands (UIBs). Mysterious objects in space that give off as yet unidentified infrared emission patterns.


Universal Time (UT, UTC). Co-ordinated Universal Time. The time standard by which Greenwich Mean Time became known as for scientific purposes in 1928. UTC is the time given by broadcast time since 1972. The time-scale is widely known as Greenwich Mean Time but astronomically speaking the term GMT is no longer used.


Universe. Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain. The entire cosmos. It is best to admit that we have no knowledge whatsoever about the origin of the universe. We do not know how matter came into existence; neither does it help to claim that the universe has always existed, so that there was no actual moment of creation.


Van Allen Belts. Two belts of charged particles from the solar wind that have been trapped by Earth’s magnetic field above Earth’s atmosphere.


Variable stars. A star that varies in brightness over a period of time. There are many types of variable stars, some vary over hundreds of days while other display minute variation over a matter of minutes. Another demarcation of variable stars is extrinsic, and intrinsic. Extrinsic variable are not true variables, their fluctuations are caused by eclipse events as in an eclipsing variable. Intrinsic variable are true variables. Their fluctuations are due to physical processes taking place in the stars themselves.

Vernal Equinox. The moment when the Sun crosses the celestial equator travelling in a northward direction, on or about March 21. In the northern hemisphere, it marks the first day of spring. The term is also applied to the Sun’s position in the sky at that moment. It is one of two points where the ecliptic and the celestial equator intersect, the other being the autumnal equinox.


The point on the celestial sphere where the sun crosses the celestial equator moving northward, which corresponds to the beginning of spring in the northern hemisphere and the beginning of autumn in the southern hemisphere (in the third week of March). This point corresponds to zero (0) hours of right ascension.


Vignetting. Uneven or reduced illumination over the image plane in a telescope or camera, causing distortion such as dimming near the edge of an image.


Virgo Cluster. A group of about 2,500 known galaxies lying near the north galactic pole in the constellation Virgo.


Visible Light. The portion of the electromagnetic spectrum visible to the human eye.


Visual (or Apparent) magnitude. The apparent brightness of a celestial object. The lower the magnitude, the less bright the object. Thus, the Sun has an apparent magnitude of -27; the Moon up to -12; Venus up to -4; the brightest stars -1; the faintest stars visible to the naked-eye +6, the faintest objects yet detected about +30. For mainly historical reasons the magnitude scale has the peculiar attribute of having brighter objects at negative values and vice-versa. (See also Absolute magnitude and Magnitude.)

Voids. Enormous regions of relatively empty space between galaxy superclusters.


Volatiles. Chemical compounds that are gaseous at low temperatures.


Volume. The amount of space occupied by a body or fluid.


Waning Crescent. The phase of the Moon between third quarter and new moon. Waning means declining or fading.


Waning Gibbous. The phase of the Moon between full moon and last quarter.

Wavelength. The distance between a given point on one wave to the same point on the next wave.

Waxing. The period between new moon and full moon.


Waxing Crescent. The phase of the Moon between new moon and first quarter. Waxing means increasing.


Waxing Gibbous. The phase of the Moon between first quarter and full moon.


Weight. The 'heaviness' of an object, the amount of attraction between two or more masses. Weight is often measured in Kilograms but the proper (SI) unit of weight is the Newton. Weight is not the same as mass which is a measure of how much matter or inertia an object has. Weight on the other hand is dependant on two or more masses and is a measure of the force of gravity acting on those two objects. For example, an astronaut will experience weightlessness in space because he is distant (though not entirely free) from the Earth's gravitational influence. His mass however will still be the same as it would be on Earth. He is just as massive as he was before - but he weighs little or nothing!

White dwarf. A very small, dense star that has used up its nuclear energy. Stars of this kind are at the end of their evolution. Typically, a white dwarf contains roughly the same mass as that of our Sun in a volume one hundredth the size. This gives a white dwarf a density about one million times that of water!

Winter Solstice. The moment when the Sun reaches its greatest distance south of the celestial equator, on or about December 22. In the Northern Hemisphere this marks the first day of winter; in the Southern Hemisphere it marks the first day of summer.


Wolf-Rayet stars. Stars that are very hot, reaching temperatues as high as 90,000 kelvins, and are surrounded by an expanding gaseous envelope. They appear greenish-white in colour and their spectra show distinctive bright emission lines.


X-Rays. Electromagnetic radiation with a wavelength shorter than ultraviolet light but longer than gamma rays.


X-Class Flares. The brightest and most energetic type of solar flares.


Year, Anomalistic. The period for successive perihelion passages of the Earth, a little less than 5 minutes longer than the sidereal year.

Year, Calendar. The mean length of the year according to the Gregorian calendar, 365 days, 5 hours, 49 minutes and 12 seconds.

Year, Sidereal. The period taken by the Earth to complete one orbit of the Sun, 365.26 days.

Year, Tropical. The period taken for successive passages of the Sun across the Vernal Equinox, 365.24 days.


Zenith. The point on the celestial sphere that lies directly overhead an observer, exactly 90 degrees away from every part of the observer's horizon.

Zenithal Hourly Rate. The number of meteorites expected to be seen per hour when a meteor shower's radiant is at an observer's zenith; abbreviated ZHR.


Zenith distance. The angular distance of an object from the zenith.

Zodiac. The band of constellations through which the Sun travels each year. The Zodiac is actually a band across the sky, 8 degrees either side of the ecliptic. With the exception of Pluto, all the planets and the Sun and Moon will be found within the Zodiac.

Zodiacal light. A cone of light stretching from the horizon along the ecliptic. It is only seen during good sky conditions when the Sun is a few degrees below the horizon. It is caused by fine, thinly spread interplanetary material lying close to the plane of the solar system.