**Aberration**: the apparent angular displacement of the observed position of a celestial object from its geometric position, caused by the finite velocity of light in combination with the motions of the observer and of the observed object. (See aberration, planetary).

**Aberration, annual**: the component of stellar aberration (see aberration, stellar) resulting from the motion of the Earth about the Sun.

**Aberration, diurnal**: the component of stellar aberration (see aberration, stellar) resulting from the observer’s diurnal motion about the center of the Earth.

**Aberration, E-terms of**: terms of annual aberration (see aberration, annual) depending on the eccentricity and longitude of perihelion (see longitude of pericenter) of the Earth.

**Aberration, elliptic**: see aberration, E-terms of.

**Aberration, planetary**: the apparent angular displacement of the observed position of a celestial body produced by motion of the observer (see aberration, stellar) and the actual motion of the observed object.

**Aberration, secular**: the component of stellar aberration (see aberration, stellar) resulting from the essentially uniform and rectilinear motion of the entire solar system in space. Secular aberration is usually disregarded.

**Aberration, stellar**: the apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer. Stellar aberration is divided into diurnal, annual, and secular components. (See aberration, diurnal: aberration, annual: aberration, secular).

**Altitude**: the angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith. Altitude is 90° minus zenith distance.

**Anomaly**: angular measurement of a body in its orbit from its perihelion.

**Aphelion**: the point in a planetary orbit that is at the greatest distance from the Sun.

**Apogee**: the point at which a body in orbit around the Earth reaches its farthest distance from the Earth.

**Apparent place**: the position on a celestial sphere, centered at the Earth, determined by removing from the directly observed position of a celestial body the effects that depend on the topocentric location of the observer: i.e., refraction, diurnal aberration (see aberration, diurnal) and geocentric (diurnal) parallax. Thus the position at which the object would actually be seen from the center of the Earth, displaced by planetary aberration (except the diurnal part – see aberration, planetary; aberration, diurnal) and referred to the true equator and equinox.

**Apparent solar time**: the measure of time based on the diurnal motion of the true Sun. The rate of diurnal motion undergoes seasonal variation because of the obliquity of the ecliptic and because of the eccentricity of the Earth’s orbit. Additional small variations result from irregularities in the rotation of the Earth on it axis.

**Aspect**: the apparent position of any of the planets or the Moon relative to the Sun, as seen from Earth.

**Astrometric ephemeris**: an ephemeris of a solar system body in which the tabulated positions are essentially comparable to catalog mean places of stars at a standard epoch. An astrometric position is obtained by adding to the geometric position, computed from gravitational theory, the correction for light-time. Prior to 1984, the E-terms of annual aberration (see aberration, annual; aberration, E-terms of) were also added to the geometric position.

**Astronomical coordinates**: the longitude and latitude of a point on the Earth relative to the geoid. These coordinates are influenced by local gravity anomalies. (See zenith; longitude, terrestrial; latitude, terrestrial).

**Astronomical unit (a.u.)**: the radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2p /k days, where k is the Gaussian gravitational constant. This is slightly less than the semimajor axis of the Earth’s orbit.

**Atomic second**: see second, Systeme International.

**Augmentation**: the amount by which the apparent semidiameter of a celestial body, as observed from the surface of the Earth, is greater than the semidiameter that would be observed from the center of the Earth.

**Azimuth**: the angular distance measured clockwise along the horizon from a specified reference point (usually north) to the intersection with the great circle drawn from the zenith through a body on the celestial sphere.

**Barycenter**: the center of mass of a system of bodies; e.g., the center of mass of the solar system or the Earth-Moon system.

**Barycentric Dynamical Time (TDB)**: the independent argument of ephemerides and equations of motion that are referred to the barycenter of the solar system. A family of time scales results from the transformation by various theories and metrics of relativistic theories of Terrestrial dynamical Time (TDT). TDB differs from TDT only by periodic variations. In the terminology of the general theory of relativity. TDB may be considered to be a coordinate time. (See dynamical time.)

**Brilliancy**: for Mercury and Venus the quantity ks^{2}/r^{2}, where k=0.5 (1+cos i), i is the phase angle, s is the apparent semidiameter, and r is the heliocentric distance.

**Calendar**: a system of reckoning time in which days are enumerated according to their position in cyclic patterns.

**Catalog equinox**: the intersection of the hour circle of zero right ascension of a star catalog with the celestial equator. (See dynamical equinox; equator.)

**Celestial ephemeris pole**: the reference pole for nutation and polar motion: the axis of figure for the mean surface of a model Earth in which the free motion has zero amplitude. This pole has no nearly-diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system.

**Celestial equator**: the plane perpendicular to the celestial ephemeris pole. Colloquially, the projection onto the celestial sphere of the Earth’s equator. (See mean equator and equinox: true equator and equinox.)

**Celestial pole**: either of the two points projected onto the celestial sphere by the extension of the Earth’s axis of rotation to infinity.

**Celestial sphere**: an imaginary sphere of arbitrary radius upon which celestial bodies may be considered to be located. As circumstances require, the celestial sphere may be centered at the observer, at the Earth’s center, or at any other location.

**Conjunction**: the phenomenon in which two bodies have the same apparent celestial longitude (see longitude, celestial) or right ascension as viewed from a third body. Conjunctions are usually tabulated as geocentric phenomena. For Mercury and Venus, geocentric inferior conjunction occurs when the planet is between the Earth and Sun, and superior conjunction occurs when the Sun is between the planet and Earth.

**Constellation**: a grouping of stars, usually with pictorial or mythical associations, that serves to identify an area of the celestial sphere. Also one of the precisely defined areas of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.

**Coordinated Universal Time (UTC)**: the time scale available from broadcast time signals. UTC differs from TAI (see International Atomic Time) by an integral number of seconds: it is maintained within ± 0.90 second of UT1 (see Universal Time) by the introduction of one second steps (leap seconds). (See leap second).

**Culmination**: passage of a celestial object across the observer’s meridian; also called “meridian passage”. More precisely, culmination is the passage through the point of greatest altitude in the diurnal path. Upper culmination (also called “culmination above pole” for circumpolar stars and the Moon) or transit is the crossing closer to the observer’s zenith. Lower culmination (also called “culmination below pole” for circumpolar stars and the Moon) is the crossing farther from the zenith.

**Day**: an interval of 86 400 SI seconds (see second, Systeme International), unless otherwise indicated.

**Day numbers**: quantities that facilitate hand calculations of the reduction of mean place to apparent place. Besselian day numbers depend solely on the Earth’s position and motion: second-order day numbers, used in higher precision reductions, depend on the positions of both the Earth and the star.

**Declination**: angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object. Declination is usually given in combination with right ascension or hour angle.

**Defect of illumination**: the angular amount of the observed lunar or planetary disk that is not illuminated to an observer on the Earth.

**Deflection of light**: the angle by which the apparent path of a photon is altered from a straight line by the gravitational field of the Sun. The path is deflected radially away from the Sun by up to 1².75 at the Sun’s limb. Correction for this effect, which is independent of wavelength, is included in the reduction from mean place to apparent place.

**Deflection of the vertical**: the angle between the astronomical vertical and the geodetic vertical. (See zenith: astronomical coordinates: geodetic coordinates.)

**Delta T (D T)**: the difference between dynamical time and Universal Time; specifically the difference between Terrestrial Dynamical Time (TDT) and UT1: D T = TDT-UT1.

**Direct motion**: for orbital motion in the solar system, motion that is counterclockwise in the orbit as seen from the north pole of the ecliptic; for an object observed on the celestial sphere, motion that is from west to east, resulting from the relative motion of the object and the Earth.

**Diurnal motion**: the apparent daily motion caused by the Earth’s rotation, of celestial bodies across the sky from east to west.

**D UT1**: the predicted value of the difference between UT1 and UTC, transmitted in code on broadcast time signals: D UT1= UT1-UTC. (See Universal Time; Coordinated Universal Time.)

**Dynamical equinox**: the ascending node of the Earth’s mean orbit on the Earth’s true equator: i.e., the intersection of the ecliptic with the celestial equator at which the Sun’s declination is changing from south to north. (See catalog equinox, equinox, true equator and equinox.)

**Dynamical time**: the family of time scales introduced in 1984 to replace ephemeris time as the independent argument of dynamical theories and ephemerides. (See Barycentric dynamical Time: Terrestrial Dynamical Time.)

**Eccentric anomaly**: in undisturbed elliptic motion, the angle measured at the center of the ellipse from pericenter to the point on the circumscribing auxiliary circle from which a perpendicular to the major axis would intersect the orbiting body. (See mean anomaly; true anomaly.)

**Eccentricity**: a parameter that specifies the shape of a conic section; one of the standard elements used to describe an elliptic orbit. (See elements, orbital.)

**Eclipse**: the obscuration of a celestial body caused by its passage through the shadow cast by another body.

**Eclipse, annular**: a solar eclipse (see eclipse, solar) in which the solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun.

**Eclipse, lunar**: an eclipse in which the Moon passes through the shadow cast by the Earth. The eclipse may be total (the Moon passing completely through the Earth’s umbra), partial (the Moon passing partially through the Earth’s umbra at maximum eclipse), or penumbral (the Moon passing only through the Earth’s penumbra).

**Eclipse, solar**: an eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon’s umbra), partial (observer in the Moon’s penumbra), or annular. (See eclipse, annular.)

**Ecliptic**: the mean plane of the Earth’s orbit around the Sun.

**Elements, Besselian**: quantities tabulated for the calculation of accurate predictions of an eclipse or occultation for any point on or above the surface of the Earth.

**Elements, orbital**: parameters that specify the position and motion of a body in orbit. (See osculating elements: mean elements.)

**Elongation, greatest**: the instants when the geocentric angular distances of Mercury and Venus from the Sun are at a maximum.

**Elongation (planetary)**: the geocentric angle between a planet and the Sun, measured in the plane of the planet, Earth and Sun. Planetary elongations are measured from 0° to 180° , east or west of the Sun.

**Elongation (satellite)**: the geocentric angle between a satellite and its primary, measured in the plane of the satellite, planet and Earth. Satellite elongations are measured from 0° east or west of the planet.

**Epact:** the age of the Moon: the number of days since new moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (See Gregorian calendar: lunar phases.)

**Ephemeris**: a tabulation of the positions of a celestial object in an orderly sequence for a number of dates.

**Ephemeris hour angle**: an hour angle referred to the ephemeris meridian.

**Ephemeris longitude**: longitude (see longitude, terrestrial) measured eastward from the ephemeris meridian.

**Ephemeris meridian**: a fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by Terrestrial Dynamical Time (TDT). The ephemeris meridian is 1.002 738 D T east of the Greenwhich meridian, where D T= TDT-UT1.

**Ephemeris time (ET)**: the time scale used prior to 1984 as the independent variable in gravitational theories of the solar system. In 1984, ET was replaced by dynamical time.

**Ephemeris transit**: the passage of a celestial body or point across the ephemeris meridian.

**Epoch**: an arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit).

**Equation of center**: in elliptic motion the true anomaly minus the mean anomaly. It is the difference between the actual angular position in the elliptic orbit and the position the body would have if its angular motion were uniform.

**Equation of the equinoxes**: the right ascension of the mean equinox (see mean equator and equinox) referred to the true equator and equinox; apparent sidereal time minus mean sidereal time. (See apparent place: mean place.)

**Equation of time**: the hour angle of the true Sun minus the hour angle of the fictitious mean sun; alternatively, apparent solar time minus mean solar time.

**Equator**: the great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (See celestial equator.)

**Equinox**: either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; ie., when the apparent longitude (see apparent place; longitude, celestial) of the Sun is 0° or 180° . (See catalog equinox; dynamical equinox for precise usage.)

**Era**: a system of chronological notation reckoned from a given date.

**Fictitious mean sun**: an imaginary body introduced to define mean solar time; essentially the name of a mathematical formula that defined mean solar time. This concept is no longer used in high precision work.

**Flattening**: a parameter that specifies the degree by which a planet’s figure differs from that of a sphere; the ratio f = (a-b)/a, where a is the equatorial radius and b is the polar radius.

**Frequency**: the number of cycles or complete alternations per unit time of a carrier wave, band, or oscillation.

**Frequency standard**: a generator whose output is used as a precise frequency reference; a primary frequency standard is one whose frequency corresponds to the adopted definition of the second (see second, Systeme International), with its specified accuracy achieved without calibration of the device.

**Gaussian gravitational constant (k= 0.017 202 098 95)**: the constant defining the astronomical system of units of length (astronomical unit), mass (solar mass) and time (day), by means of Kepler’s third law. The dimensions of k^{2} are those of Newton’s constant of gravitation: L^{3}M^{-1}T^{-2}.

**Gegenshein**: faint nebulous light about 20° across near the ecliptic and opposite the Sun, best seen in September and October. Also called counterglow.

**Geocentric**: with reference to, or pertaining to, the center of the Earth.

**Geocentric coordinates**: the latitude and longitude of a point on the Earth’s surface relative to the center of the Earth; also celestial coordinates given with respect to the center of the Earth. (See zenith; latitude, terrestrial, longitude, terrestrial.)

**Geodetic coordinates**: the latitude and longitude of a point on the Earth’s surface determined from the geodetic vertical (normal to the specified spheroid). (See zenith; latitude, terrestrial; longitude, terrestrial.)

**Geoid**: an equipotential surface that coincides with mean sea level in the open ocean. On land it is the level surface that would be assumed by water in an imaginary network of frictionless channels connected to the ocean.

**Geometric position**: the geocentric position of an object on the celestial sphere referred to the true equator and equinox, but without the displacement due to planetary aberration. (See apparent place; mean place; aberration, planetary.)

**Greenwich sidereal date (GSD)**: the number of sidereal days elapsed at Greenwich since the beginning of the Greenwich sidereal day that was in progress at Julian date 0.0.

**Greenwich sidereal day number**: the integral part of the Greenwich sidereal date.

**Gregorian calendar**: the calendar introduced by Pope Gregory XIII in 1582 to replace the Julian calendar; the calendar now used as the civil calendar in most countries. Every year that is exactly divisible by four is a leap year, except for centurial years, which must be exactly divisible by 400 to be leap years. Thus 2000 is a leap year, but 1900 and 2100 are not leap years.

**Height**: elevation above ground or distance upwards from a given level (especially sea level) to a fixed point. (See altitude).

**Heliocentric**: with reference to, or pertaining to, the center of the Sun.

**Horizon**: a plane perpendicular to the line from an observer to the zenith. The great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith is called the astronomical horizon.

**Horizontal parallax**: the difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon.

**Hour angle**: angular distance on the celestial sphere measured westward along the celestial equator from the meridian to the hour circle that passes through a celestial object.

**Hour circle**: a great circle on the celestial sphere that passes through the celestial poles and is therefore perpendicular to the celestial equator.

**Inclination**: the angle between two planes or their poles; usually the angle between an orbital plane and a reference plane; one of the standard orbital elements (see elements, orbital) that specifies the orientation of an orbit.

**International Atomic Time (TAI)**: the continuous scale resulting from analyses by the Bureau International des Poids et Mesures of atomic time standards in many countries. The fundamental unit of TAI is the SI second (see second, Systeme International), and the epoch is 1958 January 1.

**Invariable plane**: the plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system.

**Irradiation**: an optical effect of contrast that makes bright objects viewed against a dark background appear to be larger than they really are.

**Julian calendar**: the calendar introduced by Julius Caesar in 46 B.C. to replace the Roman calendar. In the Julian calendar a common year is defined to comprise 365 days, and every fourth year is a leap year comprising 366 days. The Julian calendar was superseded by the Gregorian calendar.

**Julian date (JD)**: the interval of time in days and fraction of a day since 4713 B.C. January 1, Greenwich noon, Julian proleptic calendar. In precise work the timescale, e.g., dynamical time or universal time, should be specified.

**Julian date, modified (MJD)**: the Julian date minus 2400000.5.

**Julian day number (JD)**: the integral part of the Julian date.

**Julian proleptic calendar**: the calendric system employing the rules of the Julian calendar, but extended and applied to dates preceding the introduction of the Julian calendar.

**Julian year**: a period of 365.25 days,. This period served as the basis for the Julian calendar.

**Laplacian plane**: for planets see invariable plane; for a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component.

**Latitude, celestial**: angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object.

**Latitude, terrestrial**: angular distance on the Earth measured north or south of the equator along the meridian of a geographic location.

**Leap second**: a second (see second, Systeme International) added between 60s and 0s at announced times to keep UTC within 0.90 s of UT1. Generally, leap seconds are added at the end of June or December.

**Librations**: variations in the orientation of the Moon’s surface with respect to an observer on the Earth. Physical librations are due to variations in the orientation of the Moon’s rotational axis in inertial space. The much larger optical librations are due to variations in the rate of the Moon’s orbital motion, the obliquity of the Moon’s equator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth’s surface.

**Light, deflection of**: the bending of the beam of light due to gravity. It is observable when the light from a star or planet passes a massive object such as the Sun.

**Light-time**: the interval of time required for light to travel from a celestial body to the Earth. During this interval the motion of the body in space causes an angular displacement of its apparent place from its geometric place (see geometric position). (See aberration, planetary.)

**Light-year**: the distance that light traverses in a vacuum during one year.

**Limb**: the apparent edge of the Sun, Moon, or a planet or any other celestial body with a detectable disc.

**Limb correction**: correction that must be made to the distance between the center of mass of the Moon and its limb. These corrections are due to the irregular surface of the Moon and are a function of the librations in longitude (see longitude, celestial) and latitude (see latitude, celestial) and the position angle from the central meridian.

**Local sidereal time**: the local hour angle of a catalog equinox.

**Longitude, celestial**: angular distance on the celestial sphere measured eastward along the ecliptic from the dynamical equinox to the great circle passing through the poles of the ecliptic and the celestial object.

**Longitude, terrestrial**: angular distance measured along the Earth’s equator from the Greenwich meridian to the meridian of a geographic location.

**Luminosity class**: distinctions among stars of the same spectral class. (See Spectral types or classes.)

**Lunar phases**: cyclically recurring apparent forms of the Moon. New moon, first quarter, full moon and last quarter are defined as the times at which the excess of the apparent celestial longitude (see longitude, celestial) of the Moon over that of the Sun is 0° , 90° , 180° and 270° , respectively.

**Lunation**: the period of time between two consecutive new moons.

**Magnitude, stellar**: a measure on a logarithmic scale of the brightness of a celestial object considered as a point source.

**Magnitude of a lunar eclipse**: the fraction of the lunar diameter obscured by the shadow of the Earth at the greatest phase of a lunar eclipse (see eclipse, lunar), measured along the common diameter.

**Magnitude of a solar eclipse**: the fraction of the solar diameter obscured by the Moon at the greatest phase of a solar eclipse (see eclipse, solar), measured along the common diameter.

**Mean anomaly**: in undisturbed elliptic motion, the product of the mean motion of an orbiting body and the interval of time since the body passed pericenter. Thus the mean anomaly is the angle from pericenter of a hypothetical body moving with a constant angular speed that is equal to the mean motion. (See true anomaly; eccentric anomaly.)

**Mean distance**: the semimajor axis of an elliptic orbit.

**Mean elements**: elements of an adopted reference orbit (see elements, orbital) that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations.

**Mean equator and equinox**: the celestial reference system determined by ignoring small variations of short period in the motions of the celestial equator. Thus the mean equator and equinox are affected only by precession. Positions in star catalogs are normally referred to the mean catalog equator and equinox (see catalog equinox) of a standard epoch.

**Mean motion**: in undisturbed elliptic motion, the constant angular speed required for a body to complete one revolution in an orbit of a specified semimajor axis.

**Mean place**: the geocentric position, referred to the mean equator and equinox of a standard epoch, of an object on the celestial sphere centered at the Sun. A mean place is determined by removing from the directly observed position the effects of refraction, geocentric and stellar parallax, and stellar aberration (see aberration, stellar), and by referring the coordinates to the mean equator and equinox of a standard epoch. In compiling star catalogs it has been the practice not to remove the secular part of stellar aberration (see aberration, secular). Prior to 1984, it was additionally the practice not to remove the elliptic part of annual aberration (see aberration, annual; aberration, E-terms of).

**Mean solar time**: a measure of time based conceptually on the diurnal motion of the fictitious mean sun, under the assumption that the Earth’s rate of rotation is constant.

**Meridian**: a great circle passing through the celestial poles and through the zenith of any location on Earth. For planetary observations a meridian is half the great circle passing through the planet’s poles and through any location on the planet.

**Month**: the period of one complete synodic or sidereal revolution of the Moon around the Earth; also a calendrical unit that approximates the period of revolution.

**Moonrise, moonset**: the times at which the apparent upper limb of the Moon is on the astronomical horizon: i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90° 34¢ + s – p , where s is the Moon’s semidiameter, p is the horizontal parallax, and 34¢ is the adopted value of horizontal refraction.

**Nadir**: the point on the celestial sphere diametrically opposite to the zenith.

**Node**: either of the points on the celestial sphere at which the plane of an orbit intersects a reference plane. The position of a node is one of the standard orbital elements (see elements, orbital) used to specify the orientation of an orbit.

**Nutation**: the short-period oscillations in the motion of the pole of rotation of a freely rotating body that is undergoing torque from external gravitational forces. Nutation of the Earth’s pole is discussed in terms of components in obliquity and longitude (see longitude, celestial.)

**Obliquity**: in general the angle between the equatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of the ecliptic is the angle between the planes of the equator and the ecliptic.

**Occultation**: the obscuration of one celestial body by another of greater apparent diameter; especially the passage of the Moon in front of a star or planet, or the disappearance of a satellite behind the disk of its primary. If the primary source of illumination of a reflecting body is cut off by the occultation, the phenomenon is also called an eclipse. The occultation of the Sun by the Moon is a solar eclipse (see eclipse, solar.)

**Opposition**: a configuration of the Sun. Earth and a planet in which the apparent geocentric longitude (see longitude, celestial) of the planet differs by 180°from the apparent geocentric longitude of the Sun.

**Orbit**: the path in space followed by a celestial body.

**Osculating elements**: a set of parameters (see elements, orbital) that specifies the instantaneous position and velocity of a celestial body in its perturbed orbit. Osculating elements describe the unperturbed (two-body) orbit that the body would follow if perturbations were to cease instantaneously.

**Parallax**: the difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points. Geocentric (diurnal) parallax is the difference in direction between a topocentric observation and a hypothetical geocentric observation. Heliocentric or annual parallax is the difference between hypothetical geocentric and heliocentric observations; it is the angle subtended at the observed object by the semimajor axis of the Earth’s orbit. (See also horizontal parallax.)

**Parsec**: the distance at which one astronomical unit subtends an angle of one second of arc; equivalently the distance to an object having an annual parallax of one second of arc.

**Penumbra**: the portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the umbra.

**Pericenter**: the point in an orbit that is nearest to the center of force. (See perigee; perihelion).

**Perigee**: the point at which a body in orbit around the Earth most closely approaches the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth.

**Perihelion**: the point at which a body in orbit around the Sun most closely approaches the Sun.

**Period**: the interval of time required to complete one revolution in an orbit or one cycle of a periodic phenomenon, such as a cycle of phases. (See phase.)

**Perturbations**: deviations between the actual orbit of a celestial body and an assumed reference orbit; also the forces that cause deviations between the actual and reference orbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the reference orbit to obtain the precise coordinates.

**Phase**: the ratio of the illuminated area of the apparent disk of a celestial body to the area of the entire apparent disk taken as a circle. For the Moon; phase designations (see lunar phases) are defined by specific configurations of the Sun, Earth and Moon. For eclipses, phase designations (total, partial, penumbral, etc.) provide general descriptions of the phenomena. (See eclipse, solar; eclipse, annular; eclipse, lunar.)

**Phase angle**: the angle measured at the center of an illuminated body between the light source and the observer.

**Photometry**: a measurement of the intensity of light usually specified for a specific frequency range.

**Planetocentric coordinates**: coordinates for general use, where the z-axis is the mean axis of rotation, the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian, and the y-axis completes a right-hand coordinate system. Longitude (see longitude, celestial) of a point is measured positive to the prime meridian as defined by rotational elements. Latitude (see latitude, celestial) of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point.

**Planetographic coordinates**: coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude (see longitude, celestial) of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude (see latitude, celestial) of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.

**Polar motion**: the irregularly varying motion of the Earth’s pole of rotation with respect to the Earth’s crust. (See celestial ephemeris pole.)

**Precession**: the uniformly progressing motion of the pole of rotation of a freely rotating body undergoing torque from external gravitational forces. In the case of the Earth, the component of precession caused by the Sun and Moon acting on the Earth’s equatorial bulge is called lunisolar precession; the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is called general precession. (See nutation.)

**Proper motion**: the projection onto the celestial sphere of the space motion of a star relative to the solar system; thus the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century.

**Quadrature**: a configuration in which two celestial bodies have apparent longitudes (see longitude, celestial) that differ by 90° as viewed from a third body. Quadratures are usually tabulated with respect to the Sun as viewed from the center of the Earth.

**Radial velocity**: the rate of change of the distance to an object.

**Refraction, astronomical**: the change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.

**Retrograde motion**: for orbital motion in the solar system, motion that is clockwise in the orbit as seen from the north pole of the ecliptic; for an object observed on the celestial sphere, motion that is from east to west, resulting from the relative motion of the object and the Earth. (See direct motion.)

**Right ascension**: angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination.

**Saber’s Beads**: detached points of light seen along the limb of very young and old lunar crescents. The necklace of staggered brightness peaks near New Moon is reminiscent of the moments before and after a total solar eclipse.

**Satellite**: natural body revolving around a planet.

**Satellite, artificial**: device launched into a closed orbit around the Earth, another planet, the Sun, etc.

**Second, Systeme International (SI):** the duration of 9 192 631 770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

**Selenocentric**: with reference to, or pertaining to, the center of the Moon.

**Semidiameter**: the angle at the observer subtended by the equatorial radius of the Sun, Moon or a planet.

**Semimajor axis**: half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit (see elements, orbital.)

**Sidereal day**: the interval of time between two consecutive transits of the catalog equinox. (See sidereal time.)

**Sidereal hour angle**: angular distance on the celestial sphere measured westward along the celestial equator from the catalog equinox to the hour circle passing through the celestial object. It is equal to 360° minus right ascension in degrees.

**Sidereal time**: the measure of time defined by the apparent diurnal motion of the catalog equinox; hence a measure of the rotation of the Earth with respect to the stars rather than the Sun.

**Solstice**: either of the two points on the ecliptic at which the apparent longitude (see longitude, celestial) of the Sun is 90° or 270° ; also the time at which the Sun is at either point.

**Spectral types or classes**: catagorization of stars according to their spectra, primarily due to differing temperatures of the stellar atmosphere. From hottest to coolest, the spectral types are O, B, A, F, G, K and M.

**Standard epoch**: a date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year (see year, Besselian). Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0.

**Stationary point (of a planet)**: the position at which the rate of change of the apparent right ascension (see apparent place) of a planet is momentarily zero.

**Sunrise, sunset**: the times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90° 50¢ , based on adopted values of 34¢ for horizontal refraction and 16¢ for the Sun’s semidiameter.

**Surface brightness** (of a planet): the visual magnitude of an average square arc-second area of the illuminated portion of the apparent disk.

**Synodic period**: for planets, the mean interval of time between successive conjunctions of a pair of planets, as observed from the Sun; for satellites, the mean interval between successive conjunctions of a satellite with the Sun, as observed from the satellite’s primary.

**Synodic time**: pertaining to successive conjunctions; successive returns of a planet to the same aspect as determined by Earth.

**Terrestrial Dynamical Time (TDT)**: the independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00sTAI, the value of TDT was exactly 1977 January 1.0003725 d. The unit of TDT is 86 400 SI seconds at mean sea level. For practical purposes TDT= TAI + 32.184 s. (See Barycentric Dynamical Time; dynamical time; International Atomic Time.)

**Terminator**: the boundary between the illuminated and dark areas of the apparent disk of the Moon, a planet or a planetary satellite.

**Topocentric**: with reference to, or pertaining to, a point on the surface of the Earth.

**Transit**: the passage of the apparent center of the disk of a celestial object across a meridian: also the passage of one celestial body in front of another of greater apparent diameter (e.g., the passage of Mercury or Venus across the Sun or Jupiter’s satellites across its disk); however, the passage of the Moon in front of the larger apparent Sun is called an annular eclipse (see eclipse, annular). The passage of a body’s shadow across another body is called a shadow transit; however, the passage of the Moon’s shadow across the Earth is called a solar eclipse. (See eclipse, solar.)

**True anomaly**: the angle, measured at the focus nearest the pericenter of an elliptical orbit, between the pericenter and the radius vector from the focus to the orbiting body; one of the standard orbital elements (see elements, orbital). (See also eccentric anomaly; mean anomaly.)

**True equator and equinox**: the celestial coordinate system determined by the instantaneous positions of the celestial equator and ecliptic. The motion of this system is due to the progressive effect of precession and the short-term, periodic variations of nutation. (See mean equator and equinox.)

**Twilight**: the interval of time preceding sunrise and following sunset (see sunrise, sunset) during which the sky is partially illuminated. Civil twilight comprises the interval when the zenith distance, referred to the center of the Earth, of the central point of the Sun’s disk is between 90° 50¢ and 96° , nautical twilight comprises the interval from 96° to 102° , astronomical twilight comprises the interval from 102° to 108° .

**Umbra**: the portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed.

**Universal Time (UT)**: a measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun and serves as the basis of all civil timekeeping. UT is formally defined by a mathematical formula as a function of sidereal time. Thus UT is determined from observations of the diurnal motions of the stars. The time scale determined directly from such observations is designated UT0: it is slightly dependent on the place of observation. When UT0 is corrected for the shift in longitude (see longitude, terrestrial) of the observing station caused by polar motion, the time scale UT1 is obtained.

**Vernal equinox**: the ascending node of the ecliptic on the celestial equator; also the time at which the apparent longitude (see apparent place; longitude, celestial) of the Sun is 0° . (See equinox.)

**Vertical**: apparent direction of gravity at the point of observation (normal to the plane of a free level surface.)

**Week**: an arbitrary period of days, usually seven days; approximately equal to the number of days counted between the four phases of the Moon. (See lunar phases.)

**Year**: a period of time based on the revolution of the Earth around the Sun. The calendar year (see Gregorian calendar) is an approximation to the tropical year (see year, tropical). The anomalistic year is the mean interval between successive passages of the Earth through perihelion. The sidereal year is the mean period of revolution with respect to the background stars. (See Julian year: year, Besselian.)

**Year, Besselian**: the period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as standard epoch, is denoted by the suffix “.0”. Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0).

**Year, tropical**: the period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year (see year, Besselian) by 0.148T s, where T is centuries from B1900.0.

**Zenith**: in general, the point directly overhead on the celestial sphere. The astronomical zenith is the extension to infinity of a plumb line. The geocentric zenith is defined by the line from the center of the Earth through the observer. The geodetic zenith is the normal to the geodetic ellipsoid at the observer’s location. (See deflection of the vertical.)

**Zenith distance**: angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude.

**Zodiacal light**: a nebulous light seen in the east before twilight and in the west after twilight. It is triangular in shape along the ecliptic with the base on the horizon and its apex at varying altitudes. It is best seen in middle latitudes (see latitude, terrestrial) on spring evenings and autumn mornings.

*** Note**: This glossary is taken from The Astronomical Almanac (1994), M1-M13, after requesting a permission.