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Glossary of Astronomical TermsA 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. Accretion An accumulation of dust and gas into larger bodies such as stars, planets and moons, or as discs around existing bodies. 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 it 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. Aperture The unobstructed diameter of the objective lens or primary mirror. Aphelion The position in a heliocentric orbit at which the orbiting object is at its greatest distance from the Sun. Apoapse (also called Apocentre) The position in an orbital path that is the greatest distance from the primary body. Apoapsis The point in an orbit when a planet is farthest from any body other than the Sun or the Earth. 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 arc minute. (1/3600th of a degree.) Argument of the Pericentre The angle, measured around the orbital plane, between the ascending node and the pericentre. Ascending Node The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving northward. Asteroid Also ("planetoid") These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. It is thought that there must be around 100,000 in all. The largest asteroid is Ceres which has a diameter of 579 miles. The smallest detected asteroids have diameters of several hundred feet. Together with comets and meteoroids, asteroids make up the minor bodies of the solar system. They are considered to be left over planetesimals from the formation of our solar system. The gravitational pull of Jupiter is thought to have stopped the members of the asteroid belt from forming a planet. Astigmatism An optical aberration resulting from unequal magnification across different diameters. Astronomical Twilight When the centre of the Sun is between 12º and 18º 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. Aurora A glow in the Earth's ionosphere caused by the interaction between the Earth's magnetic field and charged particles from the Sun (The Solar Wind). It gives rise to the "Northern Lights", or Aurora Borealis, in the Northern Hemisphere, and the Aurora Australis in the Southern Hemisphere. 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. Azimuth The angular distance around the horizon, usually measured from north (although it is sometimes measured from south), of the great circle passing through the object and the zenith.
Barycenter The center 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 center of this mass (called the barycenter of the solar system) is offset somewhat from the sun; "original" and "future" orbits of long-period comets are computed for this barycenter, while perturbed, osculating orbits of currently-observed objects in the inner solar system are computed for heliocentric orbits. Barycentric Dynamical Time (TDB) Differing from TDT only via periodic variations, TDB is used in ephemerides and equations of motion that refer to the barycenter of the solar system. 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 time of the year was not convenient for dynamical studies that utilize Julian dates (see definition for Julian date), 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 Center and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See Julian year.) Barlow lens A diverging lens which has the effect of increasing (usually doubling) the effective focal length of the telescope. Binary Star A system of two stars orbiting around a common centre of mass due to their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud. Blue Moon The second full moon in a calendar month, or the third full moon in a season containing four. Bolometric Magnitude The total radiation received from an object.
CCD Charge-coupled device, a very sensitive electronic device that is revolutionizing astronomy in the 1990s. CCD cameras are composed of silicon chips that are sensitive to light, changing detected photons of light into electronic signals that can then be used to make images of astronomical objects or to analyze how much light is being received from such objects. CCDs require computers for reduction of data, so the expense can be much greater than for, say, photography --- but CCDs can detect much fainter objects than can photographs. Unfiltered CCDs tend to be more red-sensitive than the human eye. 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 Equator The projection of the Earth's equator upon the celestial sphere. It is the reference plane for measurement of the declination in the system of equatorial co-ordinates. Celestial Poles The projection of the Earth's poles onto the celestial sphere. 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. Chromosphere The layer between the photosphere and the corona in the atmosphere of the Sun, or any other star, mainly composed of excited hydrogen atoms. Circumpolar An object that does not set from its observer's latitude. Civil Twilight When the centre of the Sun is less than 6º below the horizon. 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. Comet A celestial body orbiting the sun (though some may be ejected from the solar system by planetary perturbations) that displays (at least during a portion of its orbit) some diffuseness and/or a "tail" of debris that points generally in the anti-solar direction. Both the diffuseness (generally called a coma) and the tail are composed of gas and/or dust of various atomic or molecular compositions, as is ascertained by spectroscopy. The coma and tail material come from a much smaller nucleus that is usually invisible due to the bright surrounding coma activity. Close-up pictures of a cometary nucleus did not occur until spacecraft fly-bys of Halley's comet in 1986. Conjunction There are at least three definitions of conjunction. Bodies are said to be in conjunction when either they have the same Right Ascension or when they have the same ecliptic longitude or when they are at their closest. Planets are said to be "at conjunction" when they are in conjunction with the Sun. (See diagram.) Constellation An arbitrary grouping of stars which form a pattern. The sky is divided into 88 constellations. These vary in size and shape from Hydra, the sea monster, which is the largest at 1,303 square degrees, to Crux, the cross, which is the smallest at 68 square degrees. Corona The outer layer, and hottest part, of the Sun's atmosphere, Coronagraph A special telescope which blocks light from the Sun's disc, thus creating an artificial eclipse, in order to study its atmosphere. Cosmic Ray An extremely fast, energetic and relativistic (high speed) charged particle. Cosmos The Universe: the word is derived from the Greek, meaning 'everything'. 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.
Degree A unit used in the measurement of angles, heavily used particularly in astronomy. Due to ancient Babylonian mathematics, we still divide a circle into 360 even units of arc and call each of these units one degree. The entire sky, therefore, spans 360 degrees. Up to about 180 degrees of sky is visible from any given point on earth with an unobstructed horizon (as measured from, say, east to west, or north to south). The degree is used to make measurements of distance, or position (as with declination) in astronomy. In turn, a degree is composed of 60 minutes of arc, and also of 360 seconds of arc DeltaThe upper-case Greek letter used to denote an object's geocentric distance in ephemeris tables; see "ephemeris". (Note that lower-case delta is used to denote declination.) Descending Node The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving southward. 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. Drawtube The moving part of a telescope's focuser. Dwarf Star A star, which lies on the main sequence and is too small to be classified as a giant star or a supergiant star. For example, the Sun is a yellow dwarf star.
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 (e.g. solar eclipse or eclipsing binary stars) or when one body passes through Earth's shadow (e.g. lunar eclipse). 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 (b) and ecliptic longitude (l). (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.) Ephemeris (plural: ephemerides) Pronounced ee-FEM-er-is (ef-fi-MARE-uh-deez). A table listing specific data of a moving object, as a function of time. Ephemerides usually contain right ascension ("R.A." in these web pages) and declination ("Decl." in these web pages), apparent angle of elongation ("Elong." in these web pages) 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), usually given as Roman letter "r" and Greek letter "Delta", respectively; 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 (i) The date at which a the co-ordinates on a star chart will be correct with respect to precession. (ii) The date of reference in astronomical calculations. Equation of Time The correction which must be applied to solar time in order to obtain mean solar time. See the tutorial on Time. 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. 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.
Filament A strand of (relatively) cool gas suspended over the Sun (or star) by magnetic fields, which appears dark against the disc of the Sun. A filament on the limb of the Sun seen in emission against the dark sky is called a solar prominence. 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.
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 galactical longitude. Galaxy A vast star system containing thousands of billions of stars, dust and gas, held together by gravity. Galaxies are the basic building blocks of the Universe. There are three main classes, Elliptical, Spiral and Barred, named after their appearance. Galilean Moons The four Jovian moons first observed by Galileo ( Io, Europa, Ganymede and Callisto). They are observable with small amateur telescopes. Geocentric Earth-centred. Geostationary Orbit The orbit of a satellite which is both geosynchronous and in the equatorial plane. The satellite will appear to remain in a fixed position in relation to the observer. 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 180º, 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. Graticule A system of parallel lines or crossed lines at the telescope's focal plane, used in micrometers. Great circle A circle formed on a 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 between two points on a spherical surface.
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. Hour Angle The angle, measured westwards around the celestial equator, between the observer's meridian and the hour circle of an object. Hour Circle (i) A great circle passing through an object and the celestial poles. (ii) The setting circle on the polar axis of an equatorial mount.
Inferior Conjunction The conjunction of Mercury or Venus when they lie between Earth and the Sun. (See diagram.) 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.
Julian date (JD) The interval of time in days (and fraction of a day) since Greenwich noon on Jan. 1, 4713 BC. The JD is always half a day off from Universal Time, because the current definition of JD was introduced when the astronomical day was defined to start at noon (prior to 1925) instead of midnight. Thus, 1995 Oct. 10.0 UT = JD 2450000.5. 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.
Kepler's Laws The three laws of planetary motion formulated by Johannes Kepler. For more detail see the tutorial on the Heliocentric Revolution. km kilometer = 0.6 mile.
L Light pollution The emission of stray light or glare from lighting fixtures in manners that counter the purpose of the light. Light Year The distance travelled by light in one year. 9.4607x 1012 km or 63,240 AU or 0.3066 parsecs. Limb The edge of the disc of a celestial body. Line of nodes The imaginary line passing through the ascending and descending nodes of an orbit. It is the line of intersection of the orbital plane with the reference plane. Longitude of the Ascending Node The angular distance, measured around the reference plane, between the FPA and the ascending node of an orbit. 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). Lunation The period between successive New Moons.
Maksutov, Maksutov-Cassegrain, Maksutov-Newtonian Forms of catadioptric telescope. Magnetosphere The region of space in which a planet's magnetic field dominates that of the solar wind. Magnification The increase in the angle subtended by an object. See the tutorial on telescope function. Magnitude The brightness of a celestial body on a numerical scale. See also absolute magnitude, apparent magnitude, bolometric magnitude and integrated magnitude. Mean Anomaly The anomaly which would exist if a planet orbited at a uniform speed in a circular orbit. Mean Solar Time Time based on an imaginary "mean Sun". See the tutorial on Time. Meridian The great circle passing through the celestial poles and the observer's zenith. Meteor The incandescent trail of a meteoroid as it enters Earth's atmosphere. Meteorite A meteoroid that reaches Earth's surface. Meteoroid A fragment of matter which may turn into a meteor or a meteorite if it strikes Earth. 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. Moon A naturally occurring satellite, or relatively large body, orbiting a planet.
Nautical Twilight When the centre of the Sun is between 6º and 12º below the horizon; the marine horizon becomes invisible. Near-Earth Asteroid (NEA) An asteroid whose orbit brings it close to Earth's orbit. Near-Earth Object (NEO) A comet or asteroid whose orbit brings it close to Earth's orbit. The criterion is a perihelion distance < 1.3 AU. Nebula A term used to describe celestial objects which have a fuzzy, or nebulous, appearance (from the Latin for cloud.), such as gas, or dust, clouds. Galaxies were once described thus.. Newtonian Telescope A reflecting telescope with a paraboloidal primary mirror, and a flat elliptical diagonal secondary mirror that directs the focal plane out of the side of the telescope tube. Nova An existing star which suddenly increases its brightness by more than 10 magnitudes and then slowly fades.
Obliquity of the Ecliptic The angle between the plane of the ecliptic and that of the celestial equator. 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. Open Cluster A group of young stars, possibly bound together by gravity, that formed together. 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.) Optical Binary A pair of stars which happen to lie close to one another on the celestial sphere because of a chance alignment. They are not physically associated with one another and lie at vastly different distances. Optical binaries are also known as visual binaries. Orbit The path of one body around another due to the influence of gravity. 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 (w), and the longitude of the ascending node (W). 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.
Parsec The distance at which a star would have a parallax of one arcsecond. (3.2616 light years, 206,265 astronomical units, 30.857 x 10^12 km). 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. Periapse (also called Pericentre) The position in an orbital path that is the least distance from the primary body. 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. Perihelion The position in a heliocentric orbit at which the orbiting object is at its least distance from the Sun. Perturbations 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. Phase The percentage illumination, from the observer's perspective, of an object (normally planet or Moon). 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. Photometry In astronomy, the measurement of the light emitting from astronomical objects, generally in the visible or infrared bands, in which a specific or general wavelength band is normally specified. Photosphere The visible surface of the Sun. Plage Bright regions in the Sun's chromosphere. 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 disks 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 Abbreviation for primary body or primary mirror . 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. Primary mirror In a reflecting telescope, the mirror that collects the light and focuses it to the focal plane. Prime Meridian The polar great semi-circle adopted as the reference direction for measurement of longitude. Earth's prime meridian is the Greenwich meridian. Prograde The apparent eastward motion of a planet with respect to the stars. Prominence A cloud, or plume, of hot, luminous gas in the Sun's corona. It appears bright when seen against the cool blackness of space. When they are in silhouette against the disc they are known as filaments. They are mainly composed of hydrogen, helium and calcium Proper motion The apparent motion of a star with respect to its surroundings.
Quasars Compact, extra galactic, objects at extreme distances, which are highly luminous. They are thought to be active galactic nuclei. The name is an acronym for quasi-stellar radio source. A quasar is very similar to a QSO (quasi-stellar object) but gives out radio waves also.
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 Giant A spectral type K or M star nearing the end of its life having a low surface temperature and large diameter eg Betelgeuse in Orion.. 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. Reticule A system of lines forming a pattern of squares at the focal plane of a telescope, used in micrometers. Retrograde Apparent westward movement off a planet with respect to the stars. Right Ascension (RA) The angle, measured eastward on the celestial equator, between the First Point of Aries and the hour circle through the object.
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. Secular motion Secular variations in the motions of the planets are those that have very slow changes that proceed through ages (secula) in a way such that they are nearly proportional with time for a relatively large number of years. Precession is considered a secular variation, arising from the motions of the mean equator and the mean ecliptic. Compare this with periodic variations, which are rather rapid changes; nutation is a periodic variation. Semi-major Axis Half the distance across an ellipse measured along a line through its foci. Shepherd Moon Is a moon which constrains the extent of a planetary ring by means of gravitational forces. It is believed that shepherd moons are responsible for Saturn's rings. Sidereal Month The 27.32166 day period of the Moon's orbit. Sidereal Time The hour angle of the First Point of Aries. It is time measured with respect to the stars. See the tutorial on Time. Solar Cycle The 11-year variation in sunspot activity. Solar Flare A sudden, short lived, burst of energy on the Sun's surface, lasting from minutes to hours. Solar Time Time measured with respect to the Sun. See the tutorial on Time. Solar Wind A stream of charged particles emitted from the Sun which travel into space along lines of magnetic flux. 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. Spectral Classification A method of classifying stars which is based upon the appearance of the absorption lines in their spectra. Spherical Aberration An optical aberration in which light from different parts of a mirror or lens is brought to different foci. Star Cluster A loose association of stars within the Milky Way. Sublimation The change of a solid (such as ice) directly into a gaseous state (bypassing the liquid state). 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 smaller atoms and molecules after leaving the nucleus in gas form. Sunspot A cooler region of the Sun's photosphere (which, thus, appears dark) seen as a spot, on the Sun's disc. They are caused by concentrations of magnetic flux, typically occurring in groups or clusters. The number of sunspots varies according to the Sun's 11 year cycle. More sunspots are seen at the Maxima of solar cycles, with few being observed during the Minima between. Superior Conjunction The conjunction of Venus and Mercury when they are more distant than the Sun. Superior Planets Those planets whose orbits lie outside Earth's orbit. Supernova An exploding star. Syzygy The alignment of any three celestial bodies. Planets are at syzygy when they are at opposition or conjunction (aligned with Sun and Earth).
Terminator The boundary of the illuminated part of the disc of a planet or moon. 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). Topocentric Referred to a position on the surface of the Earth (cf geocentric, which is referred to the centre of the Earth.) 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 m1, usually found in comet ephemerides, is used to denote the total (often predicted) magnitude. See also definition for "Magnitude", above. 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). 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.
Universal Time (UT, or UTC)A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the sun. UT is determined from observations of the diurnal (daily) motions of the stars for an observer on the earth. UT is usually used for astronomical observations, while Terrestrial Dynamical Time (TDT, or simply TT) is used in orbital and ephemeris computations that involve geocentric computations. Coordinated Universal Time (UTC) is that used for broadcast time signals (available via shortwave radio, for example), and it is within a second of UT.
Vernal equinox (i) The equinox (on or near March 20) when the Sun is at its ascending node. (ii) The First Point of Aries.
White Dwarf A whitish star, of up to 1.4 Solar masses, and about the size of the Earth with consequential very high density, characterised by a high surface temperature and low brightness. 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. Worm Hole A hypothetical shortcut through the space time continuum.
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. Zodiac The apparent path, in the sky, followed by the sun, moon and most planets, lying within 10 degrees of the celestial equator. Ancient Astrologers (nothing to do with modern astronomy! ) divided it into 12 groups, the Signs of the Zodiac, though there are actually 13 astronomical constellations which lie on the zodiac, since the Sun passes through Ophiuchus each December. Ophiuchus is not recognised by astrologers. Zodiacal light A faint conical glow occurring above the horizon along the ecliptic during twilight, best seen when the ecliptic is at a steep angle to the horizon.
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