Astronomical dictionary

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This is by no means a comprehensive glossary of astronomical terms however it should provide some assistance to novices in particular. Click on the symbol for a diagram or picture associated with that word. Cross references within the glossary are indicated by hyperlink text. Click on the letters below to go to the first entry of alphabetical sections.

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Click the link to go to the contents page (lists all of the words in the dictionary) or click on the letters below to go to the first entry of each alphabetical section. Click the link to get back to the site entry page.


Alternatively, use the search box below to search the site for the information that you require.


It is my intention that the site will continue to evolve. My main long term plan is to add deeper layers of information to make the site more useful as a reference source. Please feel free to contact me to suggest additions to the site. The following is an excellent reference book - Space Encyclopedia (Heather Couper(Editor), Nigel Henbest(Editor)), as is Philip's Atlas of the Universe.

Last updated on 20th January 2003


Netscape users: Please note that some of the text formatting does not seem to render correctly. I do not know why, it works fine in IE.

Click here to visit Aries on line, the Derby and District Astronomical Society magazine. It contains a variety of articles including beginners materials.

Aberration (of starlight)
The apparent displacement of a star from it's true position in the sky. It is caused by a combination of the motion of the Earth in orbit round the Sun (about 30 km per sec) and the finite velocity of light (299,792.5 km per sec or , if you prefer imperial units, 186,252.5 miles per second). During the year a star will seem to move in a small ellipse around its true position. This is called annual aberration. There is also a very much smaller daily effect caused by the rotation of the Earth. This is called diurnal aberration. The maximum displacement is 20.5 seconds of arc. This number is called the constant of aberration. For a much more thorough treatment click here.
Absolute Magnitude The magnitude a star would seem to have if viewed from a standard distance of 10 parsecs (32.6 light years).
Accretion Disc A disc of material from which larger bodies may be formed under the influence of gravity. Also the rotating disc of material formed around a Black Hole.
Achondrite A type of stony meteorite containing very little iron or nickel.
Achromatic Lens A lens that is corrected for chromatic aberration, thus reducing the false colour around a star.
Aerolite A stony meteorite.
Airglow The faint luminosity of the night sky, due to the Earth's upper atmosphere.
Airy Disc The apparent size of a star's disc produced by a perfect optical system.
Albedo A measure of how reflective a body is. Albedo is expressed as a percentage, the higher the percentage, the higher the albedo and therefore the more reflective the object is.
Altazimuth Mount A type of telescope mount, which allows the instrument to be moved freely in declination (altitude) and right ascension (azimuth).
Altitude The angle of elevation (height in degrees) of a star or other astronomical object above the observer's horizon.
Analemma If you take a picture of the Sun at the same time each week, from exactly the same position, on a single frame of film, the figure of eight shape that results is called the analemma. See for more detail and photos.
Angular Distance The apparent distance between two celestial objects. It is measured in degrees, arcminutes and arcseconds. On average, the distance from your thumb tip to the tip of your little finger of your outstretched hand at arms length is 20 degrees. The width of your palm will be about 12 degrees and the width of the tip of your little finger is about 1 degree. The angular diameter of the Moon (and the Sun) is more or less 1/2 degree.
The diameter of the main lens or mirror of a telescope.
When something is orbiting the Sun, this is the point of the orbit that is furthest from the Sun. See also perihelion.
When something is orbiting the Earth, this is the point of the orbit that is furthest from the Earth. See also perigee.
Apennines One of the most noticeable mountain range on the Moon, next to Mare Imbrium (Sea of Showers).
Aperture Synthesis A technique used in radio astronomy where by an array of radio telescope dishes are used together, effectively giving the observer a much larger dish size e.g. the VLA in New Mexico (visit the Aries on line site to read an article about the VLA).
Appulse The apparent close approach of one celestial body to another. This is a line of sight effect.
Arcminute A small measure of angular distance. There are 60 arcminutes in one degree.
Arcsecond A small measure of angular distance. There are 60 arcseconds in one arcminute and therefore 3600 arcseconds in one degree.
Areography The physical study of Mars.
Ashen Light When the plant Venus appears as a crescent, the night side sometimes appears dimly luminous. This is the Ashen light. The reason for the Ashen light is not understood.


A distinctive group of stars that is not one of the recognised constellations. The Plough is a good example, it is not a constellation, it is an asterism that is part of the constellation Ursa Major.

These are relatively small bodies that orbit the Sun mainly between the orbits of Mars and Jupiter. They are though to be material of the Solar System that never accreted to form a planet. There are others who believe that they are the remains of a somehow destroyed planet that lay between Mars and Jupiter. In recent years it has been realised that there are a number of asteroids that cross the orbit of the Earth. Some of these are quite large and would cause great problems if they impacted the Earth. Life on this planet could conceivable be eradicated by a large enough impact. It is now generally accepted that mass extinctions of life in the past e.g. the demise of the dinosaurs, could have been caused by such an impact. There is now an observing programme to locate and evaluate the dangers of this hazard (NEAR). I have put some links to books on the subject below.


Comet and Asteroid Impact Hazards on a Populated Earth

Rogue Asteroids and Doomsday Comets : The Search for the Million Megaton Menace That Threatens Life on Earth

Impact! The Threat of Comets and Asteroids

Astrograph A telescope designed for the sole purpose of astrophotography.


A pseudo science based on ignorance and superstition in which the stars are believed to influence daily personal events on the Earth!
Astrometric Binary A binary system in which the fainter component can not be seen, but is observable from the gravitational effects on the proper motion of the brighter companion.
Astrometry The branch of astronomy dealing with the movements and positions of celestial bodies.
Astronomical Twilight The period between sunset and the time when the sun has dropped 18 degrees below the horizon.

Astronomical Unit

The mean (average) distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU). It is a convenient way of describing distances within our Solar System.
Astrophysics The branch of astronomy that deals with the physics and chemistry of the stars.
Atmosphere The gaseous mantle surrounding a planet or other body. It is thought that the atmosphere of the Earth is a secondary atmosphere. The theory is that the original (primary) atmosphere was lost during the T-Tauri stage of the Sun's evolution. Volcanoes gradually replaced this with an atmosphere of methane, carbon dioxide and water vapour. The current atmosphere evolved from this. The oceans were formed as the water vapour condensed as Earth cooled down. When plants containing chlorophyll evolved, they used the carbon dioxide for photosynthesis and introduced oxygen into our atmosphere.
Atmospheric refraction This is the bending of light the Earth's atmosphere. This causes an increase in the apparent height (altitude) of an object above the horizon. This increase is zero at the zenith (because the observer is looking out from the Earth at 90 degrees) to about half a degree (the width of the full Moon) at the horizon.
Atom The smallest unit of a chemical element which retains its own characteristics. An excellent book on the atoms and Einstein's theory of relativity is "Mr Tompkins in paperback" by George Gamow

A glow in the northern (aurora borealis) or the southern (aurora australis) sky caused by particles from the solar wind interacting with the atmosphere. The particles are held in the Van Allen belts. When there is a solar flare, there is a huge increase in the density of the Solar wind. This overloads the Van Allen belts and causes high energy particles to spill into the atmosphere. As these interact with the gases of the atmosphere, light of different wavelengths is emitted and we see this as an aurora.

Book: The nature of the Aurora was worked out by the Norwegian Scientist Kristian Berkeland. I was riveted by the book Northern Lights, an account of how the mysteries of the Aurora were unravelled and learn about the man who invented a weapon developed in the USA's SDI (Star Wars) early in the twentieth century.


The angle measured from the south point of the horizon toward the west to a point at the foot of a star's vertical circle. See also celestial sphere.
Baily's Beads

Brilliant points of light along the edge of the moon disc, just before and just after a total solar eclipse.


How to observe eclipses

Barlow lens Put a Barlow lens between the main mirror or lens of your telescope and its eyepiece and increase the magnification. Effectively it increases the focal length of the main mirror or lens. The most usual magnification for a Barlow lens is X2. In practice, they are rarely used since they cause a large light loss in the telescope. It is a cheap way of getting a short focal length eyepiece. Better to spend more on an eyepiece, you get better quality.
Big Bang Theory The theory that the universe came into existence at one set moment in time, between 15,000 and 20,000 million years ago.
Binary (star) A double star in which the components orbit one another.
BL Lacertae object Named after the first discovered member of this type of object. It appeared to be a variable star at first but it was realised that they were much more luminous than stars. They are less luminous than quasars and one idea is that a BL Lacertae object is a quasar seen from a narrow angle.
Black Body A body that absorbs all the radiation that it receives, giving it an albedo of zero i.e. it is 100% efficient at absorbing radiation. A black body also radiates with 100% efficiency, it is therefore a theoretical object although many stars come very close to this theoretical position.
Black Drop An appearance seen at the beginning of a transit of Venus. As the planet moves across the Sun's disc it seems to draw a strip of blackness after it.
Black Dwarf A dead star, which has used up all its reserves of energy. The ultimate fate of a White Dwarf.
Black Hole A localised region of space from which not even light can escape, due to a super massive star collapsing in on itself. This is because the gravitational field of a black hole is so strong that the escape velocity is greater than the speed of light. Astronomers believe that the location of large black holes can be observed because of radiation emitted from the accretion disc as matter is pulled into the black hole. The event horizon marks the outer limits of these objects.
Blue shift If an astronomical body is moving towards the observer, the light will seem to be shifted to the blue end of the spectrum. The faster the movement, the greater the blue shift. It occurs because the wavelength of light is slightly compressed by the Doppler effect as the body moves towards the observer. Blue shift is measured by looking at the key spectral lines. For an object moving towards the Solar System, they will appear closer to the blue end than normal. The faster the object is approaching, the greater the blue shift will be.
Bok Globules The Dutch astronomer Bart Bok first drew attention to these small black objects. They appear in gaseous emission nebulae and are thought to be protostars that are still forming but have not yet become hot enough to shine.
Bolide An extremely bright meteor, also called a fireball.
Carbon stars Red stars of spectral types R and N, containing an unusual amount of carbon in their atmosphere.
Cassegrain Reflector A type of reflecting telescope where the main mirror has a central hole. Light from an object is reflected off the primary mirror up to the secondary mirror and back through the hole in the primary to be focused in the eyepiece tube. The primary mirror is a spherical mirror and is therefore easier and cheaper to make than a parabolic (slightly elliptical) mirror. Correction for spherical aberration is made by having a parabolic secondary mirror. The path of the light is also folded on itself which makes its tube much shorter, lighter and more portable than a refracting or a Newtonian reflector of the equivalent aperture.
Cassini Division The principal division in Saturn's ring system, separating ring A from ring B.
Catadioptic Telescope A type of telescope, which uses both refraction and reflection to form an image at the prime focus. This type of telescope is generally based on the Cassegrain design but uses a corrector plate to prevent spherical aberration. This means that both the primary and secondary mirror can be spherical rather than parabolic. See Cassegrain Telescope for the advantages.
Celestial equator A great circle on the celestial sphere that is midway between the two poles of rotation. It really amounts to the projection of the Earth's equator onto the sky. It can be thought of as an imaginary ring, 90 degrees from either pole in the sky.
Celestial latitude The angle north or south of the ecliptic to an object.
Celestial longitude The angle that is measured eastward along the ecliptic, from the vernal equinox, to the foot of a circle that is perpendicular to the ecliptic and passing through the object.
Celestial sphere This does not actually exist. Early humans believed that stars were fixed to a crystal sphere in the sky, at a great distance from Earth. This is because there is no sense of distance in the night sky and the stars always seemed to be fixed in position relative to each other. The idea is a handy one when dealing with positioning and angles between objects in the sky. See also celestial latitude, longitude, azimuth.
Cepheid Variable An important type of variable star. Stars of this type have short periods, from a few days to a few weeks; these stars are perfectly regular. The period of a Cepheid depends on its absolute magnitude. So, when the period of variability is known, its absolute magnitude can be deduced. Comparing this to its apparent magnitude (how bright it is when seen from Earth) gives the distance. They are sometimes referred to as "standard candles" because of their usefulness in determining the distances of galaxies. They are named after the star Delta Cephei, which was the first one of this type of star to be discovered.
Charge Coupled Device (CCD) A very sensitive electronic device used in astrophotography. For further details see the article in Aries on line.
Chromatic aberration A defect of a lens that creates a fringe of colour round an object.
Chromosphere The part of the sun's atmosphere lying above the bright photosphere, but below the corona.
Circumpolar This is a region of the sky that is always visible round the celestial pole closest to the observer. An object in this area will therefore never set, at any time of the night (or day of course) and can be observed at any time of the year e.g. the Plough asterism is in the circumpolar region from the UK and can be seen in all four seasons, Orion is not circumpolar and so can only be observed for part of the year.
Cluster (stars) A cluster is a group of stars whose members are genuinely associated. A cluster of stars is formed from the same gas/dust cloud. There are two main types: open and globular.
Cluster (Spacecraft) Four spacecraft that have been placed in Earth orbit in order to study the interactions between particles ejected from the Sun and the Earth's magnetic field. The first attempt to put Cluster into space in 1996 failed when the Ariane rocket exploded.
Collapsar The end product of a very massive star, which has collapsed to form a very high density object.
Colour index A measure of a star's colour, which helps astronomers to tell its surface temperature. It is the difference between the magnitude of a star measured in two different areas of the spectrum. The areas are B (blue), V (violet) and U (ultraviolet) regions. The B-V is the most common index used and is close to zero for a white star. It is extremely useful in the classification of stars, it can tell astronomers if the star is a main sequence star, a giant star or a supergiant star.
Coma There are several astronomical meanings. It can be used to describe the hazy looking patch that surrounds the nucleus of a comet or the blurred effect surrounding the images of stars on a photographic plate, or in the observers field of view in a telescope (or binoculars) due to defects in the lenses.
Comes This is pronounced 'Koh-meez' and means the fainter companion of a double or binary star.
Comet A body of our Solar System. Comets are composed of rocks, dust and ices. They are believed to come from the Oort cloud and possibly contain material from the earliest days of the Solar System. As they approach the Sun, the ices begin to evaporate forming a coma and one or more tails.
Conjunction This is when a planet has the same longitude as the Sun. A planet can only be observed at conjunction if there is a total eclipse or if it transits the Sun. Unlike the other planets, Mercury and Venus have two types of conjunction, inferior when the planet lies between the Sun and the Earth, superior when it lies on the opposite side of the Sun to the Earth.
Constellation A pattern of stars named after an object, animal or mythical person. The stars in a constellation have no physical connection, they are a line of sight effect.
Convection The transfer of energy by moving currents in gases or liquid. Partly responsible for the transfer of heat from the interior of the Sun to the photosphere.
Copernican system The system proposed (in 1543) by Nicholas Copernicus in which the Sun is the central body, with the Earth and the other bodies moving around it. This model superseded the Ptolemaic system which had persisted for nearly 2000 years. Copernicus' idea was not new, it had been proposed in about 300 BC by Aristarchus of Samos, a Greek philosopher. Both astronomers have Lunar craters named after them.
Corona Faint extensions of the Suns outer atmosphere seen during total eclipses.
Coronal hole A region of very low density and temperature in the Sun's corona. Electrified particles can escape through coronal holes to produce the solar wind.
Cosmic rays High speed particles that reach the Earth from outer space. cosmic rays diag Some low energy cosmic rays come from the Sun, but high energy cosmic rays are thought to originate from outside of the Solar System. These possibly originate from supernovae. The highest energy cosmic rays may originate in quasars.
Cosmological red shift The effect of the expansion of the universe in producing the red shifts in the spectra of galaxies and quasars.
Cosmology The study of the origin of the Universe and its subsequent evolution.
Counter glow The English name for Gegenschein.
Culmination The maximum altitude of a celestial body above the horizon. In other words, when an astronomical body transits the meridian.
Dark Adapted This is what every astronomer's eyes should become before they begin observing. On leaving a brightly lit area and entering a dark area, you will notice a rapid increase in visibility over a short period of time. It takes about 20 - 30 minutes to become fully dark adapted. In low light levels, the chemicals in the eye increase the ability to see faint objects. See also the Purkinje effect
Declination The angle north or south of the celestial equator to a star.
Doppler effect

The classic example is the change in tone of the noise of a vehicle engine as it approaches, passes and leaves the observer. As the vehicle approaches, more sound waves per second enter the ears of the observer. The sound appears to be higher pitched that if the vehicle and observer were stationary. This is because more sound waves per second = a higher frequency and higher frequency sound is heard as a higher pitch.

As the vehicle passes the observer and moves away, the engine tone is heard to drop. This is because slightly fewer waves per second enter the ear. Thus, the tone is heard to fall (lower frequency). The same happens to all waves, including light waves and radio waves. In the case of light waves, higher frequencies are bluer and lower frequencies are redder. An object with a negative radial velocity (moving towards the Solar System) will be blue shifted and vice versa will be red shifted.

Double star When you observe some stars through a telescope or binoculars, they appear to be two as opposed to one when seen with the naked eye. Sometimes this is a line of sight effect where the two stars are in reality totally unconnected with each other. In other cases the two stars are a genuine pair, orbiting one another (see binary star).
Eccentricity The degree of flattening of an ellipse. The term is applied to planetary orbits since these are ellipses.

The obscuring of one celestial body by another either by passing directly in front of it or by casting a shadow. Solar eclipses can be total (whole Sun is obscured), partial (part of the Sun is obscured) or annular (a complete ring of sunlight is seen). Lunar eclipses are either total or partial.

Book: Africa and Madagascar Total Eclipse 2001 & 2002

Ecliptic This is the plane of the Earth's orbit projected onto the celestial sphere. Effectively this is the apparent path of the Sun through the sky. Since the main planets of the solar system orbit in more or less the same plane, give or take a few degrees, they are always found close to the ecliptic.
Ellipse An oval. The planets do not orbit in perfect circles, as was believed before Kepler's Laws of planetary motion, but rather in slightly oval (elliptical) orbits. Their eccentricity is low so the orbits are not far off being circles. An ellipse has two foci (plural of focus), in the Solar System, the Sun lies at one of these.
Elongation These terms are generally applied to Mercury and Venus. When seen from Earth, when either planet reaches its greatest angular distance from the Sun, they are said to have reached eastern (or western) elongation.
Escape velocity The velocity required by an object to escape the gravitational pull. The gravitational attraction of a black hole is so great that an object would need to be travelling faster than the speed of light in order to escape. That is why black holes are essentially invisible, even light cannot travel fast enough to escape the immense pull of the gravity of a black hole.
Equinox Literally "equal night". In other words, in Spring (Vernal) or Autumn (Autumnal) when the daylight hours are equal to the darkness hours. Technically speaking, the point at which the ecliptic and celestial equator intersect.
Event horizon The boundary that marks the point of no return of a black hole. Once light (or matter) passes this, the gravitational forces are so strong that the escape velocity woild be greater than the speed of light. See also Schwarzchild radius.
Eyepiece The combination of lenses that magnifies the image formed by the objective (main mirror or lens). There are different types of eyepiece, each has its merits. The most common type seems to be the Plössl, which is a good general eyepiece. Huygens and Kellner are the names of two other types of eyepiece.

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