This page last modified 2006 February 14
Glossary of Astronomical Terms
This glossary is intended to grow as time permits. If you have any requests for inclusion, suggestions for improvement or clarity of explanation, or corrections, please . In the mean time, you may find what you need in the other tutorials on this web site, where many of the terms below are expanded upon in greater detail and context.
Where a term is a combination of words, it will normally be indexed under the first word of the term. For example, Apparent Magnitude is indexed as Apparent Magnitude, not Magnitude, Apparent.
Aberration of Starlight The apparent displacement of a star's position as a consequence of Earth's motion through space and the finite speed of light. More here.
Ablation The vaporisation of the surface layers of a body entering the atmosphere as a consequence of the heating that results from the compression of air ahead of it.
Absolute Magnitude The apparent magnitude that an object would possess it if was placed at a distance of 10 parsecs from the observer. In this way, absolute magnitude provides a direct comparison of the brightness of stars.
Achromatic Literally "no colour". A lens combination in which chromatic aberration is corrected by bringing two colours to the same focus.
Airy disc The bright central part of the image of a star. It is surrounded by diffraction rings and its size is determined by the aperture of the telescope. About 85% of the light from the star should fall into the Airy disc.
Altazimuth Mount A mounting in which the axes of rotation are vertical and horizontal, i.e. in altitude and azimuth. An altazimuth mount requires motion of both axes to follow an astronomical object, but is simpler to make than an equatorial mount and can, in some forms, be held together by gravity.
Altitude The angle of a body above or below the plane of the horizon negative altitudes are below the horizon.
Albedo The proportion of incident light which a body reflects in all directions. The albedo of Earth is 0.36, that of the Moon is 0.07 and that of Uranus is 0.93. The true albedo may vary over the surface of the object so, for practical purposes, the mean albedo is used.
Analemma The lemniscate-shaped form that results from plotting the position of the Sun at the same time every day.
Anomaly The angle at the Sun between a planet and its perihelion.
Ansae Literally handles. Originally a description of the appearance of Saturn's rings before they were recognised as being a ring system. Now used to describe (i) the extension of Saturn's rings outside the disc of the planet, and (ii) extensions from the central star of some planetary nebulae (due to bipolar outflow of material).
Apastron The position in an orbit about a star at which the orbiting object is at its greatest distance from the star.
Aphelion The position in a heliocentric orbit at which the orbiting object is at its greatest distance from the Sun.
Apochromatic A lens combination in which chromatic aberration is corrected by bringing three colours to the same focus. Some manufacturers use the term to describe achromatic doublets whose false colour is approximately equivalent to that of an apochromatic triplet lens.
Apogee The position in a geocentric orbit at which the orbiting object is at its greatest distance from Earth.
Apparent Magnitude The brightness of a body, as it appears to the observer, measured on a standard magnitude scale. It is a function of the luminosity and distance of the object, and the transparency of the medium through which it is observed.
Apsides The points where the major axis of an elliptical orbit meets the orbital path. The periapse (or pericentre) is the point of closest approach to the primary body; the apoapse (or apocentre) is the point of greatest distance.
Arcminute One sixtieth of a degree.
Arcsecond The second division of a degree of arc. One sixtieth of an arcminute. (1/3600th of a degree.)
Astigmatism An optical aberration resulting from unequal magnification across different diameters.
Astronomical Twilight When the centre of the Sun is between 12° and 18° below the horizon; faint stars become visible.
Astronomical Unit (AU) The mean distance from the Earth to the Sun, i.e. 149,597,870 km or 499.005 light seconds.
Attitude The orientation of a spacecraft or satellite with respect to its direction of motion.
Autoguider A CCD that is optically attached to a guidescope or off-axis guider and electronically attached to the control of the telescope mount. It monitors the position of a guide object on the CCD array and adjusts the telescope's drives so as to keep the object in the same position, thus correcting for any errors in the drive or in polar alignment. It enables long-exposure photography or imaging through the main OTA without the astronomer having to make manual corrections to the drive in response to what he sees in a guidescope.
Barlow lens A diverging lens which has the effect of increasing (usually doubling) the effective focal length of the telescope.
Bolometric Magnitude The total radiation received from an object.
Catadioptric A telescope whose optics, not including the eyepiece, consists of both lenses and mirrors. The most common examples of these are the Schmidt-Cassegrain telescopes, whose "lens" is an aspheric corrector plate, and the Maksutov-Cassegrain telescopes, whose "lens" is a deeply curved meniscus.
Celestial Co-ordinates A system by which the position of a body on the celestial sphere is plotted with reference to a reference plane and a reference direction. For more detail, see the tutorial on positional astronomy. The four systems in use are Ecliptic Co-ordinates, Equatorial Co-ordinates, Galactic Co-ordinates, and Horizon Co-ordinates.
Celestial Sphere The projection of space and the objects therein onto an imaginary sphere surrounding the Earth and centred on the observer.
Central Meridian The imaginary line through the poles of a planet that bisects the planetary disc.
Chromatic Aberration An aberration of refractive optical systems in which light is dispersed into its component colours, resulting in false colour in the image.
Circumpolar An object that does not set from its observer's latitude.
Civil Twilight When the centre of the Sun is less than 6° below the horizon; normal daylight activities are possible.
Collimation The bringing of the optical components of a telescope into correct alignment.
Coelostat A device, usually consisting of two mirrors, that is designed so as to reflect the light from a celestial object into a fixed instrument, where it forms a non-rotating image.
Coma (i) The matter surrounding the nucleus of a comet it results from the evaporation of the nucleus. (ii) An optical aberration in which stellar images are fan-shaped, similar to comets.
Conjunction There are at least three definitions of conjunction. Bodies are said to be in conjunction when they have the same ecliptic longitude (this is the strict definition) or when they have the same Right Ascension or when they are at their closest. Planets are said to be "at conjunction" when they are in conjunction with the Sun. (See diagram.) For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.
Culmination An object culminates when it reaches greatest and least altitudes ( upper culmination and their lower culmination respectively). For non circumpolar objects, the lower culmination is below the horizon. Most objects (the Moon sometimes being a notable exception) culminate when they reach the observer's meridian.
Dichotomy When the phase is exactly 50%.
Diffraction limited A measure of optical quality in which the performance is limited only by the size of the theoretical diffracted image of a star for a telescope of that aperture.
Direct motion Another term for prograde motion.
Dobsonian named for John Dobson, who originated the design. An altazimuth mount constructed usually of plywood or MDF suited to home construction. Also refers to a telescope so mounted.
Eccentricity The eccentricity of an orbit is a measure of its departure from a circle. Elliptical orbits have an eccentricity >0 and <1, parabolic paths have an eccentricity =1, and hyperbolic paths have an eccentricity >1.
Eclipse An alignment of two bodies with the observer such that either the nearer body prevents the light from the further body from reaching the observer (strictly speaking, these are occultations), e.g. solar eclipse or eclipsing binary stars, or when one body passes through the shadow of another, e.g. lunar eclipse, eclipses of Jovian satellites.
Ecliptic The apparent path the Sun on the celestial sphere. It intersects the celestial equator at the equinoxes. It is so named because, when the Moon is on the ecliptic, solar and lunar eclipses can occur.
Ecliptic Co-ordinates A system of celestial co-ordinates that uses the ecliptic as the reference plane and the First Point of Aries as the reference direction. The co-ordinates are given as ecliptic latitude (β) and ecliptic longitude (λ). (These are also called celestial latitude and celestial longitude.)
Elongation The angular distance between the Sun and any other solar system body, or between a satellite and its parent planet. The greatest elongation of an inferior planet is its maximum angular distance from the Sun; at this time the planet sets (greatest elongation east) or rises (greatest elongation west) at the greatest time from sunset or sunrise. (See diagram.) For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.
Equatorial Co-ordinates A system of celestial co-ordinates that uses the celestial equator as the reference plane and the First Point of Aries as the reference direction. The co-ordinates are given as Right Ascension (RA) and Declination (Dec).
Equatorial Mount A mounting in which one of two mutually perpendicular axes is aligned with Earth's axis of rotation, thus permitting an object to be tracked by rotating this axis so that it counteracts Earth's rotation.
Equinox Literally "equal night". it refers to the time of year when day and night are of equal length. (i) The positions where the centre of the Sun crosses the celestial equator. (ii) The dates when the declination of the Sun is zero (i.e. when it is on the celestial equator).
Escape Speed (Escape Velocity) It is the speed at which an object on the surface of a body must be propelled in order not to return to that body under the influence of their mutual gravitational attraction. Alternatively, it may be defined as the speed required to propel an object on the surface of a body into a parabolic trajectory about that body.
Exit pupil The position of the image of the objective lens or primary mirror formed by the eyepiece. It is the smallest disc through which all the collected light passes and is therefore the best position for the eye's pupil.
Extinction Loss of light from an object as a consequence of absorption or scattering by an intervening medium. An example is the atmospheric extinction of light from stars near the horizon.
Eye ring An alternative name for the exit pupil.
Faculae Unusually bright spots on the Sun's surface.
Finder A small telescope, ideally of wide field of view, that is fixed to the main telescope in order to facilitate the finding of objects.
First Point of Aries (FPA) The Vernal Equinox point, i.e. that where the centre of the Sun, moving northwards, crosses the equator. It is the reference direction for the equatorial system of co-ordinates.
Focal length The distance from the centre of a lens or mirror to its point of focus.
Focal plane The plane (usually this is actually the surface of a sphere of large radius) where the image is formed by the main optics of the telescope. The eyepiece examines this image.
Focuser The part of the telescope which varies the optical distance between the objective lens or primary mirror and the eyepiece. This is usually achieved by moving the eyepiece in a drawtube, but in some catadioptric telescopes it is the primary mirror that is moved.
Fork mount A mount where the telescope swings in declination or in altitude between two arms. It is suited only to short telescope tubes, such as Cassegrains, and variations thereof. It requires a wedge to be used equatorially.
Galactic Co-ordinates The system of celestial co-ordinates in which the galactic plane as the reference plane and the galactic centre as the reference direction. The positions are given in galactic latitude and galactic longitude.
Galilean Moons The four Jovian moons first observed by Galileo ( Io, Europa, Ganymede and Callisto). They are observable with small amateur telescopes.
Geosynchronous Orbit The orbit of a satellite in which the orbital period of the satellite is equal to Earth's period of rotation. If the orbit is in the equatorial plane, the satellite will be geostationary; if the orbit is inclined to the equatorial plane the satellite will appear to trace a lemniscate in the sky.
German Equatorial Mount (GEM) A common equatorial mount for small and medium sized amateur telescopes, suited to both long and short telescope tubes. The telescope tube is connected to the counter-weighted declination axis, which rotates in a housing that keeps it orthogonal to the polar axis. Tracking an object across the meridian requires that the telescope be moved from one side of the mount to the other, which in turn requires that both axes are rotated through 180°, thus reversing the orientation of the image. This is not a problem for visual observation, but is a limitation for astrophotography.
Gnomon (i) The "pointer" in a sundial. (ii) Vertical stick, rod or pillar, the length and direction of whose shadow indicates the altitude of the Sun and the time of day.
Granulation The "grains of rice" appearance of the Sun's surface, which results from convection cells within the Sun.
Great circle A circle formed on the surface of a sphere which is formed by the intersection of a plane which passes through the centre of a sphere. A great circle path is the shortest distance on a spherical surface between two points.
Horizon Co-ordinates The system of celestial co-ordinates in which the observer's horizon is the reference plane and the north point is the reference direction. The positions are given in altitude and azimuth.
Inferior Planets Planets (i.e. Mercury and Venus) whose orbits lie inside Earth's orbit.
Integrated Magnitude The magnitude which would apply if all the light energy from an extended object was coming from a point source.
Kepler's Laws The three laws of planetary motion formulated by Johannes Kepler. For more detail see the tutorial on the Heliocentric Revolution.
Limb The edge of the disc of a celestial body.
Luminosity The amount of energy radiated into space per second by a star. The bolometric luminosity is the total amount of radiation at all frequencies; sometimes luminosity is given for a specific band of frequencies (e.g. the visual band).
Maksutov, Maksutov-Cassegrain, Maksutov-Newtonian Forms of catadioptric telescope.
Magnification The increase in the angle subtended by an object. See the tutorial on telescope function.
Micrometer A device, of which various types exist, that is used in a telescope for measuring small angular distances between objects.
Minor Planet Another term for an asteroid.
Nautical Twilight When the centre of the Sun is between 6° and 12° below the horizon; the marine horizon becomes invisible.
Near-Earth Asteroid (NEA) An asteroid whose orbit brings it close to Earth's orbit.
Occultation An alignment of two bodies with the observer such that the nearer body prevents the light from the further body from reaching the observer. The nearer body is said to occult the further body. A solar eclipse is an example of an occultation.
Opposition The position of a planet such that Earth lies between the planet and the Sun. Planets at opposition are closest to Earth at opposition and thus opposition offers the best opportunity for observation. (See diagram.) For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.
Orbital Elements The six numerical values that completely define the orbit of one body about another of known mass. They are the semi-major axis (a), the eccentricity (e), the inclination to the reference plane (i), the mean anomaly (M), the argument of the pericentre (ω), and the longitude of the ascending node (Ω). The elements vary with time as a consequence of perturbations of other bodies, so their epoch is important. For comets and asteroids, the perihelion conditions are often of interest, so the date of perihelion (T) and perihelion distance (q) are usually used instead of M and a. (At T, M=0; q = a(1-e) )
Osculating Orbit The orbit that a body would follow if the only gravitational force acting on it was that of the primary body, i.e. if its motion was not perturbed by the presence of other bodies.
OTA Abbreviation for Optical Tube Assembly. It is normally considered to consist of the tube itself, the focuser and the optical train from the objective lens (refractor), primary mirror (reflector), or corrector plate (catadioptrics) up to, but not including, the eyepiece.
Penumbra Literally "next to the umbra". (i) The shadow that results when only part of the bright object is occulted; e.g. an observer will see a partial eclipse when he is in the penumbra of the shadow of the moon. (ii) The lighter area surrounding a sunspot.
Periastron The position in an orbit about a star at which the orbiting object is at its least distance from the star.
Perigee The position in a geocentric orbit at which the orbiting object is at its least distance from Earth.
Planisphere The projection of a sphere (or part thereof) onto a plane. It commonly refers to a simple device which consists of a pair of concentric discs, one of which has part of the celestial sphere projected onto it, the other of which has a window representing the horizon. Scales about the perimeters of the discs allow it to be set to show the sky at specific times and dates, enabling its use as a simple and convenient aid to location of objects.
Precession A rotation of the direction of the axis of rotation. Normally refers to the precession of the equinoxes, a consequence of the effect of the Sun's gravity on Earth's equatorial bulge. Earth's axis of rotation precesses with a period of about 25,770 years, during which time the equinoxes make a complete revolution about the celestial equator. Because the Vernal Equinox is the reference direction for the equatorial co-ordinate system, the co-ordinates of "fixed" objects changes with time and must therefore be referred to an epoch at which they are correct.
Primary body The body that is being orbited. E.g. the Sun is the primary of the orbits of the planets and comets. With respect to multiple star systems, it is the most massive star.
Prograde The apparent eastward motion of a planet with respect to the stars.
Proper motion The apparent motion of a star with respect to its surroundings.
Rayleigh criterion (Rayleigh limit) Lord Rayleigh, a 19th century physicist, showed that a telescope optic would be sensibly indistinguishable from a theoretical perfect optic if the light (strictly, the wavefront) deviated from the ideal condition by no more than one quarter of its wavelength.
Red Shift The lengthening of the wavelength of electromagnetic radiation resulting from one or more of three causes: Doppler redshift: resulting from bodies moving away from each other in space. Gravitational redshift: resulting from strong gravitational fields. Cosmological redshift: resulting from the expansion of space-time itself.
Reflector A telescope whose optics, apart from the eyepiece, consist of mirrors.
Refractor A telescope whose optics consist entirely of lenses.
Resolution A measure of the degree of detail visible in an image. It is normally measured in arcseconds.
Reticle A system of lines and/or concentric circles at the focal plane of a telescope, used for positioning or guiding the telescope, or polar-aligning an equatorial mount. Is usually incorporated into an eyepiece and may be illuminated in order to render the lines visible against a dark background sky.
Retrograde Apparent westward movement off a planet with respect to the stars.
Schmidt, Schmidt-Cassegrain, Schmidt Newtonian Forms of catadioptric telescope.
Scintillation The twinkling of stars, resulting from atmospheric disturbance.
Secondary Abbreviation for secondary mirror. Small mirror that directs the light from the primary mirror to the eyepiece.
Semi-major Axis Half the distance across an ellipse measured along a line through its foci.
Solstice Literally "sun still". It refers to the apparent standstill of sunrise and sunset points at midsummer and midwinter. (i) The most southerly and northerly declinations of the Sun. (ii) The date on which the Sun attains its greatest declination.
Spherical Aberration An optical aberration in which light from different parts of a mirror or lens is brought to different foci.
Superior Planets Those planets whose orbits lie outside Earth's orbit.
Terminator The boundary of the illuminated part of the disc of a planet or moon.
Topocentric Referred to a position on the surface of the Earth (cf geocentric, which is referred to the centre of the Earth.)
Transit (i) The passage of Mercury or Venus across the disc of the Sun (ii) The passage of a planet's moon across the disc of the parent planet (iii) The passage of a planetary feature (such as Jupiter's Great Red Spot) across the central meridian of the planet. (iv) The passage of an object across the observer's meridian (see culmination). In the latter case, for extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.
Twilight The period of decreasing sky brightness after sunset, or of increasing sky brightness before sunrise. There are three definitions of twilight: Civil Twilight, Nautical Twilight, and Astronomical Twilight. Twilight lasts longer in higher latitudes. For more information, see the tutorial on Twilight.
Worm drive Probably the most common drive on equatorial mounts. It consists of a spirally cut cylinder (the "worm") which rotates longitudinally such that its thread engage with the specially shaped teeth on the circumference of a disc (the "worm wheel"), which in turn drives the shaft of the mount.
Zenithal Hourly Rate (ZHR) The theoretical hourly rate of meteors which would be observed at the peak of a shower, by an experienced observer, with the radiant at the zenith, under skies with a limiting naked eye magnitude of 6.5.
Thanks are due to the following for suggestions and corrections:
Martin Frey (who did some of the diagrams)
Dr John Stockton