Glossary
The definitions on this page are consistent with those in the 2023 edition ofThe Astronomical Almanac.
ΔT: the difference betweenTerrestrial Time (TT) andUniversal Time (UT): \(\Delta {\rm T} = {\rm TT} - {\rm UT}1\).
ΔUT1 (or ΔUT): the value of the difference betweenUniversal Time (UT) andCoordinated Universal Time (UTC): \(\Delta {\rm UT1} = {\rm UT1} - {\rm UTC}\).
aberration (of light): the relativistic apparent angular displacement of the observed position of a celestial object from itsgeometric position, caused by the motion of the observer in the reference system in which the trajectories of the observed object and the observer are described. (Seeaberration, planetary.)
aberration, annual: the component of stellar aberration resulting from the motion of the Earth about the Sun. (Seeaberration, stellar.)
aberration, diurnal: the component of stellar aberration resulting from the observer'sdiurnal motion about the center of the Earth due to Earth's rotation. (Seeaberration, stellar.)
aberration, E-terms of (obsolete): the terms of annual aberration which depend on theeccentricity and longitude ofperihelion of the Earth. (Seeaberration, annual;perihelion.)
aberration, elliptic (obsolete): see aberration, E-terms of.
aberration, galactic: the apparent angular displacement of the observed position of an extra-galactic celestial object from itsgeometric position, arising from the motion of the solar system about the galactic center.
aberration, planetary: the apparent angular displacement of the observed position of a solar system body from its instantaneous geometric direction as would be seen by an observer at the geocenter. This displacement is produced by the combination of aberration of light andlight-time displacement.
aberration, secular: the component of stellar aberration resulting from the essentially uniform and almost rectilinear motion of the entire solar system in space. Secular aberration is usually disregarded. (Seeaberration, stellar.)
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. (Seeaberration, annual;aberration, diurnal;aberration, secular.)
altitude: the angular distance of a celestial body above or below thehorizon, measured along the great circle passing through the body and thezenith. Altitude is 90° minus thezenith distance.
annual parallax: seeparallax, heliocentric.
anomaly: the angular separation of a body in itsorbit from itspericenter.
anomaly, eccentric: in undisturbed elliptic motion, the angle measured at the center of theorbit ellipse frompericenter to the point on the circumscribing auxiliary circle from which a perpendicular to the major axis would intersect theorbiting body. (Seeanomaly, mean;anomaly, true.)
anomaly, mean: the product of themean motion of an orbiting body and the interval of time since the body passed thepericenter. Thus, the meananomaly is the angle from thepericenter of a hypothetical body moving with a constant angular speed that is equal to themean motion. In realistic computations, with disturbances taken into account, the meananomaly is equal to its initial value at anepoch plus an integral of themean motion over the time elapsed since theepoch. (Seeanomaly, eccentric;anomaly, mean at epoch;anomaly, true.)
anomaly, mean at epoch: the value of the mean anomaly at a specificepoch, i.e., at some fiducial moment of time. It is one of the sixKeplerian elements that specify anorbit. (SeeKeplerian elements;orbital elements.)
anomaly, true: the angle, measured at the focus nearest thepericenter of anelliptical orbit, between thepericenter and theradius vector from the focus to the orbiting body; one of the standardorbital elements. (Seeanomaly, eccentric;anomaly, mean;orbital elements.)
aphelion: the point in anorbit that is the most distant from the Sun.
apocenter: the point in anorbit that is farthest from the origin of the reference system. (Seeaphelion;apogee.)
apogee: the point in anorbit that is the most distant from the Earth. Apogee is sometimes used with reference to the apparentorbit of the Sun around the Earth.
apparent place (or position): theproper place of an object expressed with respect to the true (intermediate) equator and equinox of date.
apparent solar time: seesolar time, apparent.
appulse: the least apparent distance between two celestial objects from the observer's point of view. The time of appulse is close to that ofconjunction inecliptic longitude for objects moving on or near theecliptic.
Aries, First point of: another name for thevernal equinox.
aspect: the position of any of the planets or the Moon relative to the Sun, as seen from the Earth.
asteroid: asmall solar system body orbiting the Sun that is not massive enough to be adwarf planet. Unlike acomet, asteroids rarely exhibit the ejection of volatile material. The term ''asteroid'' is sometimes restricted to bodies with orbital semi-major axes less than or approximately equal to that of Jupiter, and is often used interchangeably with the term ''minor planet''.
astrometric ephemeris: anephemeris of a solar system body in which the tabulated positions are astrometric places. Values in an astrometricephemeris are essentially comparable to catalog mean places of stars after the star positions have been updated forproper motion andparallax.
astrometric place (or position): the position of a solar system body formed by applying corrections forlight-time displacement to thegeometric position. This position is directly comparable with the catalog positions of nearby background stars after those positions have been updated forproper motion andparallax. There is no correction for aberration ordeflection of light. It is assumed that these correction are nearly identical for both the solar system body and background stars.
astronomical coordinates: the longitude and latitude of the point on Earth relative to thegeoid. These coordinates are influenced by local gravity anomalies. (Seelatitude, terrestrial;longitude, terrestrial;zenith.)
astronomical refraction: seerefraction, astronomical.
astronomical unit (au): a conventional unit of length equal to 149 597 870 700 m exactly. Prior to 2012, it was defined as the radius of a circularorbit in which a body of negligible mass, and free ofperturbations, would revolve around the Sun in \(2 \pi /k\) days, \(k\) being the Gaussian gravitational constant. This is slightly less than theorbitalsemi-major axis of the Earth'sorbit.
astronomical zenith: seezenith, astronomical.
atomic second: seesecond, Système International (SI).
augmentation: the increase in thetopocentric apparentsemidiameter of a celestial body compared to its apparentsemidiameter when viewed from the geocenter.
autumnal equinox: seeequinox, autumnal.
azimuth: the angular distance measured eastward along thehorizon from a specified reference point (usually north). Azimuth is measured to the point where the great circle determining thealtitude of an object meets thehorizon.
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: with reference to, or pertaining to, thebarycenter (usually of the solar system).
Barycentric Celestial Reference System (BCRS): a system ofbarycentric space-time coordinates for the solar system within the framework of General Relativity. The metric tensor to be used in the system is specified by theIAU 2000 resolution B1.3. For all practical applications, unless otherwise stated, the BCRS is assumed to be oriented according to the ICRS axes. (SeeBarycentric Coordinate Time (TCB).)
Barycentric Coordinate Time (TCB): the coordinate time of theBarycentric Celestial Reference System (BCRS), which advances by SI seconds within that system. TCB is related toGeocentric Coordinate Time (TCG) andTerrestrial Time (TT) by relativistic transformations that include a secular term. (Seesecond, Système International (SI).)
Barycentric Dynamical Time (TDB): a time scale defined by theIAU in 1976, named in 1979, and revised in 2006 for use as an independent argument ofbarycentric ephemerides and equations of motion. TDB is a linear function ofBarycentric Coordinate Time (TCB) that on average tracksTT for an extended time period around the current standardepoch, JD 245 1545.0. The difference betweenTT and TDB remains less than 2 ms for several thousand years around the thisepoch. (Seesecond, Système International (SI).)
Besselian elements: quantities tabulated for the calculation of accurate predictions of aneclipse oroccultation for any point on or above the surface of the Earth.
calendar: a system of reckoning time in units of solar days. The days are enumerated according to their position in cyclic patterns usually involving the motions of the Sun and/or the Moon.
calendar, Gregorian: Thecalendar introduced by Pope Gregory XIII in 1582 to replace theJulian calendar. Thiscalendar is now used as the civilcalendar in most countries. In the Gregoriancalendar, everyyear that is exactly divisible by four is a leapyear, except for centurialyears, which must be exactly divisible by 400 to be leapyears. Thus 2000 was a leapyear, but 1900 and 2100 are not leapyears.
calendar, Julian: thecalendar introduced by Julius Caesar in 46 B.C. to replace the Romancalendar. In the Juliancalendar a commonyear is defined to comprise 365 days, and every fourthyear is a leapyear comprising 366 days. The Juliancalendar was superseded by theGregorian calendar.
calendar, proleptic: the extrapolation of acalendar prior to its date of introduction.
catalog equinox: seeequinox, catalog.
Celestial Ephemeris Origin (CEO): the original name for theCelestial Intermediate Origin (CIO) given in theIAU 2000 resolutions. Obsolete.
celestial equator: the plane perpendicular to theCelestial Intermediate Pole (CIP). Colloquially, the projection onto thecelestial sphere of the Earth'sequator. (Seemean equator and equinox;true equator and equinox.)
Celestial Intermediate Origin (CIO): the non-rotating origin of theCelestial Intermediate Reference System. Formerly referred to as theCelestial Ephemeris Origin (CEO).
Celestial Intermediate Origin Locator (CIO Locator): denoted by \(s\), is the difference between theGeocentric Celestial Reference System (GCRS)right ascension and the intermediateright ascension of the intersection of the GCRS and intermediate equators.
Celestial Intermediate Pole (CIP): the reference pole of the P03precession andIAU 2000Anutation models. The motions of the CIP are those of theTisserand mean axis of the Earth with periods longer than two days. (Seenutation;precession.)
Celestial Intermediate Reference System: ageocentric reference system related to theGeocentric Celestial Reference System (GCRS) by time-dependent rotations forprecession andnutation. It is defined by the intermediateequator of theCelestial Intermediate Pole (CIP) and theCelestial Intermediate Origin (CIO) at a specificepoch.
celestial pole: seepole, celestial.
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.
Centaur: asmall solar system body orbiting the Sun that is not massive enough to be adwarf planet with aperihelion greater than Jupiter's orbitalsemi-major axis and an orbitalsemi-major axis less than Neptune's.
center of figure: that point so situated relative to the apparent figure of a body that any line drawn through it divides the figure into two parts having equal apparent areas. If the body is oddly shaped, the center of figure may lie outside the figure itself.
center of light: same as center of figure except referring only to the illuminated portion.
central meridian: seemeridian central.
comet: asmall solar system body that normally exhibits the ejection of volatile material for some part of its orbitalperiod.
conjunction: the phenomenon in which two bodies have the same apparentecliptic longitude orright ascension as viewed from a third body. Conjunctions are usually tabulated asgeocentric phenomena. For Mercury and Venus,geocentric inferior conjunctions occur when theplanet is between the Earth and Sun, and superior conjunctions occur when the Sun is between theplanet and Earth. (Seelongitude, ecliptic.)
constellation: 1. A grouping of stars, usually with pictorial or mythical associations, that serves to identify an area of thecelestial sphere. 2. One of the precisely defined areas of thecelestial sphere, associated with a grouping of stars, that theInternational Astronomical Union (IAU) has designated as a constellation.
Coordinated Universal Time (UTC): the time scale available from broadcast time signals. UTC differs fromInternational Atomic Time (TAI) by an integral number of seconds; it is maintained within \(\pm\) 0s.9 seconds ofUT1 by the introduction ofleap seconds. (SeeInternational Atomic Time (TAI);leap second;Universal Time (UT).)
culmination: the passage of a celestial object across the observer'smeridian; also called ''meridian passage''.
culmination, lower: (also called ''culmination below pole'' for circumpolar stars and the Moon) is the crossing farther from the observer'szenith.
culmination, upper: (also called ''culmination above pole'' for circumpolar stars and the Moon) ortransit is the crossing closer to the observer'szenith.
day: an interval of 86 400 SI seconds, unless otherwise indicated. (Seesecond, Système International (SI).)
declination: angular distance on thecelestial sphere north or south of thecelestial equator. It is measured along thehour circle passing through the celestial object. Declination is usually given in combination withright ascension orhour angle.
defect of illumination: (sometimes, greatest defect of illumination): the maximum angular width of the unilluminated portion of the apparent disk of a solar system body measured along a radius.
deflection of light: the angle by which the direction of a light ray is altered from a straight line by the gravitational field of the Sun or other massive object. As seen from the Earth, objects appear to be deflected radially away from the Sun by up to 1.″75 at the Sun'slimb. Correction for this effect, which is independent of wavelength, is included in the transformation frommean place to apparent place.
deflection of the vertical: the angle between the astronomicalvertical and the geodeticvertical. (Seeastronomical coordinates;geodetic coordinates;zenith.)
delta T: see \(\bf{\Delta {\rm T}}\).
delta UT1: see \(\bf{\Delta {\rm UT1}}\) (or \(\bf{\Delta {\rm UT}}\)).
direct motion: fororbital motion in the solar system, motion that is counterclockwise in theorbit as seen from the north pole of theecliptic; for an object observed on thecelestial 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.
diurnal parallax: seeparallax, geocentric.
dwarf planet: a celestial body that is inorbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared the neighbourhood around itsorbit, and is not a satellite. (Seeplanet.)
dynamical equinox: the ascendingnode of the Earth's meanorbit on the Earth's true equator; i.e., the intersection of theecliptic with thecelestial equator at which the Sun'sdeclination changes from south to north. (Seecatalog equinox;equinox;true equator and equinox.)
dynamical time: the family of time scales introduced in 1984 to replaceephemeris time (ET) as the independent argument of dynamical theories and ephemerides. (SeeBarycentric Dynamical Time (TDB);Terrestrial Time (TT).)
Earth Rotation Angle (ERA): the angle, \(\theta\), measured along theequator of theCelestial Intermediate Pole (CIP) between the direction of theCelestial Intermediate Origin (CIO) and theTerrestrial Intermediate Origin (TIO). It is a linear function ofUT1; its time derivative is the Earth's angular velocity.
eccentricity: 1. A parameter that specifies the shape of a conic section. 2. One of the standardorbital elements, usually denoted by \(e\), used to describe an elliptical orbit or ahyperbolic orbit. For anelliptical orbit, \(e = \sqrt{1 - (b^2/a^2)}\), where \(a\) and \(b\) are the lengths of the semi-major and semi-minor axes, respectively; for an parabolic orbit \(e = 1\); and for ahyperbolic orbit, the quantity \(e = \sqrt{1 + (b^2/a^2)}\). (Seeorbital elements.)
eclipse: the obscuration of a celestial body caused by its passage through the shadow cast by another body.
eclipse, annular: a solareclipse in which the solar disk is not completely covered but is seen as an annulus or ring at maximumeclipse. An annulareclipse occurs when the apparent disk of the Moon is smaller than that of the Sun. (Seeeclipse, solar.)
eclipse, lunar: aneclipse in which the Moon passes through the shadow cast by the Earth. Theeclipse may be total (the Moon passing completely through the Earth'sumbra), partial (the Moon passing partially through the Earth'sumbra at maximumeclipse), orpenumbral (the Moon passing only through the Earth'spenumbra).
eclipse, solar: actually anoccultation of the Sun by the Moon in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon'sumbra), partial (observer in the Moon'spenumbra), annular, or annular-total. (Seeeclipse, annular.)
ecliptic: 1. The mean plane of theorbit of the Earth-Moonbarycenter around the solar systembarycenter. 2. The apparent path of the Sun around thecelestial sphere.
ecliptic latitude: seelatitude, ecliptic.
ecliptic longitude: seelongitude, ecliptic.
elements:a set of parameters used to describe the position and motion or orientation (attitude) of an astronomical object.
elements, Besselian: seeBesselian elements.
elements, Keplerian: seeKeplerian elements.
elements, mean: seemean elements.
elements, orbital: seeorbital elements.
elements, osculating: seeosculating elements.
elements, rotational: seerotational elements.
ellipsoid: a quadratic surface defined by three mutually perpendicular semi-axes. If two of the semi-axes are equal then the figure is called a spheroid, and if all three are the same it is called a sphere.
elliptical orbit: seeorbit, elliptical.
elongation: thegeocentric angle between two celestial objects.
elongation, greatest: 1. For satellites, the maximum value of a satelliteelongation during anorbit about its primary. Often a general direction is given. For example, greatest easternelongation is the maximum value of a satelliteelongation that occurs on the eastern half of the apparentorbit. 2. For bodies thatorbit the Sun, the maximum value ofelongation during anorbit about the Sun.
elongation, planetary: the usuallygeocentric angle between aplanet and the Sun. Planetary elongations are measured from 0° to 180°, east or west of the Sun.
elongation, satellite: thegeocentric angle between a satellite and its primary. Theelongation is usually designated as being east or west of the primary, but on rare occasions could be designated north or south.
epact: 1. The age of the Moon. 2. The number of days since new moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (Seecalendar, Gregorian;lunar phases.)
ephemeris: a tabulation of the positions of a celestial object in an orderly sequence for a number of dates.
ephemeris hour angle: anhour angle referred to theephemeris meridian.
ephemeris longitude: longitude measured eastward from theephemeris meridian. (Seelongitude, terrestrial.)
ephemeris meridian: seemeridian, ephemeris.
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 bydynamical time.
ephemeris transit: the passage of a celestial body or point across theephemeris meridian.
epoch: an arbitrary fixed instant of time or date used as a chronological reference datum forcalendars, celestial reference systems, star catalogs, ororbital motions. (Seecalendar;orbit.)
equation of the equinoxes: the difference apparentsidereal time minus meansidereal time, due to the effect ofnutation in longitude on the location of theequinox. Equivalently, the difference between the right ascensions of the true and meanequinoxes, expressed in time units. (Seesidereal time.)
equation of the origins: the arc length, measured positively eastward, from theCelestial Intermediate Origin (CIO) to theequinox along the intermediateequator; alternatively the difference between theEarth Rotation Angle (ERA) andGreenwich Apparent Sidereal Time (GAST), namely, (ERA — GAST).
equation of time: the differenceapparent solar time minusmean solar time.
equator: the great circle on the surface of a body formed by the intersection of its surface with the plane passing through the center of the body perpendicular to the axis of rotation. (Seecelestial equator.)
equinox: 1. Either of the two points on thecelestial sphere at which theecliptic intersects thecelestial equator. 2. The time at which the center of the Sun croses the Earth's equator. At these times the apparentecliptic longitude of the Sun is approximately either 0° or 180°. 3. Thevernal equinox. (Seemean equator and equinox;true equator and equinox.)
equinox, autumnal: 1. The decendingnode of theecliptic on thecelestial sphere. 2. The time which the apparentecliptic longitude of the Sun is 180°.
equinox, catalog: the intersection of thehour angle of zeroright ascension of a star catalog with thecelestial equator. Obsolete.
equinox, dynamical: the ascendingnode of theecliptic on the Earth's true equator.
era: a system of chronological notation reckoned from a specific event.
ERA: seeEarth Rotation Angle (ERA).
flattening: a parameter that specifies the degree by which aplanet'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.
frame bias: the orientation of themean equator and equinox of J2000.0 with respect to theGeocentric Celestial Reference System (GCRS). It is defined by three small and constant angles, two of which describe the offset of the mean pole at J2000.0 and the other is the GCRSright ascension of the mean inertialequinox of J2000.0.
frequency: the number ofperiods of a regular, cyclic phenomenon in a given measure of time, such as a second or ayear. (Seeperiod;second, Système International (SI);year.)
frequency standard: a generator whose output is used as a precisefrequency reference; a primaryfrequency standard is one whosefrequency corresponds to the adopted definition of the second, with its specified accuracy achieved without calibration of the device. (Seesecond, Système International (SI).)
GAST: seeGreenwich Apparent Sidereal Time (GAST).
Gaussian gravitational constant (obsolete): (\(k\) = 0.017 202 098 95). The constant which by means of Kepler's third law defined the astronomical system of units of length [astronomical unit (au)], mass (solar mass) and time (day) prior to 2012. The dimensions of \(k^2\) are those of Newton's constant of gravitation: \({\rm L^{3}M^{-1}T^{-2}}\).
geocentric: with reference to, or pertaining to, the center of the Earth.
Geocentric Celestial Reference System (GCRS): a system ofgeocentric space-time coordinates within the framework of General Relativity. The metric tensor used in the system is specified by theIAU 2000 resolutions. The GCRS is defined such that its spatial coordinates are kinematically non-rotating with respect to those of theBarycentric Celestial Reference System (BCRS). (SeeGeocentric Coordinate Time (TCG).)
Geocentric Coordinate Time (TCG): the coordinate time of theGeocentric Celestial Reference System (GCRS), which advances by SI seconds within that system. TCG is related toBarycentric Coordinate Time (TCB) andTerrestrial Time (TT), by relativistic transformations that include a secular term. (Seesecond, Système International (SI).)
geocentric coordinates: 1. The latitude and longitude of a point on the Earth's surface relative to the center of the Earth. 2. Celestial coordinates given with respect to the center of the Earth. (Seelatitude, terrestrial;longitude, terrestrial;zenith.)
geocentric zenith: seezenith, geocentric.
geodetic coordinates: the latitude and longitude of a point on the Earth's surface determined from the geodeticvertical (normal to the reference ellipsoid). (Seelatitude, terrestrial;longitude, terrestrial;zenith.)
geodetic zenith: seezenith, geodetic.
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 position of an object defined by a straight line (vector) between the center of the Earth (or the observer) and the object at a given time, without any corrections forlight-time, aberration, etc.
GHA: seeGreenwich Hour Angle (GHA).
GMST: seeGreenwich Mean Sidereal Time (GMST).
greatest defect of illumination: seedefect of illumination.
Greenwich Apparent Sidereal Time (GAST): the Greenwich hour angle of the true equinox of date.
Greenwich Hour Angle (GHA): angular distance on thecelestial sphere measured westward along thecelestial equator from theGreenwich meridian to thehour circle that passes through a celestial object or point.
Greenwich Mean Sidereal Time (GMST): the Greenwich hour angle of the mean equinox of date.
Greenwich meridian: seemeridian, Greenwich.
Greenwich sidereal date (GSD): the number of siderealdays elapsed at Greenwich since the beginning of the Greenwich siderealday that was in progress at theJulian date (JD) 0.0.
Greenwich sidereal day number: the integral part of theGreenwich sidereal date (GSD).
Gregorian calendar: seecalendar, Gregorian.
height: the distance above or below a reference surface such as mean sea level on the Earth or aplanetographic reference surface on another solar systemplanet.
heliocentric: with reference to, or pertaining to, the center of the Sun.
heliocentric parallax: seeparallax, heliocentric.
horizon: 1. A plane perpendicular to the line from an observer through thezenith. 2. The observed border between Earth and the sky.
horizon, astronomical: the plane perpendicular to the line from an observer to theastronomical zenith that passes through the point of observation.
horizon, geocentric: the plane perpendicular to the line from an observer to thegeocentric zenith that passes through the center of the Earth.
horizon, natural: the border between the sky and the Earth as seen from an observation point.
horizontal parallax: seeparallax, horizontal.
horizontal refraction: seerefraction, horizontal.
hour angle: angular distance on thecelestial sphere measured westward along thecelestial equator from themeridian to thehour circle that passes through a celestial object.
hour circle: a great circle on thecelestial sphere that passes through the celestial poles and is therefore perpendicular to thecelestial equator.
hyperbolic orbit: seeorbit, hyperbolic.
IAU: seeInternational Astronomical Union (IAU).
illuminated extent: the illuminated area of an apparent planetary disk, expressed as a solid angle.
inclination: 1. The angle between two planes or their poles. 2. Usually, the angle between anorbital plane and a reference plane. 3. One of the standardorbital elements that specifies the orientation of theorbit. (Seeorbital elements.)
instantaneous orbit: seeorbit, instantaneous.
intercalate: to insert an interval of time (e.g., aday or amonth) within acalendar, usually so that it is synchronized with some natural phenomenon such as the seasons orlunar phases.
intermediate place (or position): theproper place of an object expressed with respect to the true (intermediate)equator and CIO of date.
International Astronomical Union (IAU): an international non-governmental organization that promotes the science of astronomy. The IAU is composed of both national and individual members. In the field of positional astronomy, the IAU, among other activities, recommends standards for data analysis and modeling, usually in the form of resolutions passed at General Assemblies held every three years.
International Atomic Time (TAI): the continuous time scale resulting from analysis by the Bureau International des Poids et Mesures of atomic time standards in many countries. The fundamental unit of TAI is theSI second on thegeoid, and theepoch is 1958 January 1. (Seesecond, Système International (SI).)
International Celestial Reference Frame (ICRF): 1. A set of extragalactic objects whose adopted positions and uncertainties realize theInternational Celestial Reference System (ICRS) axes and give the uncertainties of those axes. 2. The name of the radio catalog whose defining sources serve as fiducial points to fix the axes of the ICRS, recommended by theInternational Astronomical Union (IAU). The first such catalog was adopted for use beginning in 1997. The second catalog, termed ICRF2, was adopted for use beginning in 2010.The third catalog, termed ICRF3, was adopted for use beginning in 2019.
International Celestial Reference System (ICRS): a time-independent, kinematically non-rotatingbarycentric reference system recommended by theInternational Astronomical Union (IAU) in 1997. Its axes are those of theInternational Celestial Reference Frame (ICRF).
international meridian: seemeridian, Greenwich.
International Terrestrial Reference Frame (ITRF): a set of reference points on the surface of the Earth whose adopted positions and velocities fix the rotating axes of theInternational Terrestrial Reference System (ITRS).
International Terrestrial Reference System (ITRS): a time-dependent, non-inertial reference system co-moving with the geocenter and rotating with the Earth. The ITRS is the recommended system in which to express positions on the Earth.
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: seecalendar, Julian.
Julian date (JD): the interval of time in days and fractions of a day, since 4713 B.C. January 1, Greenwich noon, Julianproleptic calendar. In precise work, the timescale, e.g.,Terrestrial Time (TT) orUniversal Time (UT), should be specified.
Julian date, modified (MJD): theJulian date (JD) minus 2400000.5.
Julian day number: the integral part of theJulian date (JD).
Julian year: seeyear, Julian.
Keplerian elements: a certain set of sixorbital elements, sometimes referred to as the Keplerian set. Historically, this set included the meananomaly at theepoch, the orbitalsemi-major axis, theeccentricity and three Euler angles: thelongitude of the ascending node, theinclination, and the argument ofpericenter. The time ofpericenter passage is often used as part of the Keplerian set instead of the meananomaly at theepoch. Sometimes the longitude ofpericenter (which is the sum of thelongitude of the ascending node and the argument ofpericenter) is used instead of the argument ofpericenter.
Laplacian plane: 1. For planets seeinvariable plane. 2. For a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component.
latitude, celestial: seelatitude, ecliptic.
latitude, ecliptic: angular distance on thecelestial sphere measured north or south of theecliptic along the great circle passing through the poles of theecliptic and the celestial object. Also referred to as celestial latitude.
latitude, terrestrial: angular distance on the Earth measured north or south of theequator along themeridian of a geographic location.
leap second: a second inserted as the 61st second of a minute at announced times to keepUTC within 0s. ofUT1. Generally, leap seconds are added at the end of June or December as necessary, but may be inserted at the end of anymonth. Although it has never been utilized, it is possible to have a negative leap second in which case the 60th second of a minute would be removed. (SeeCoordinated Universal Time (UTC);second, Système International (SI);Universal Time (UT).)
librations: the real or apparent oscillations of a body around a reference point. When referring to the Moon, librations are 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, theobliquity of the Moon'sequator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface.
light, deflection of: seedeflection of light.
light-time: the interval of time required for light to travel from a celestial body to the Earth.
light-time displacement: the difference between the geometric and astrometric place of a solar system body. It is caused by the motion of the body during the interval it takes light to travel from the body to Earth.
light-year: the distance that light traverses in a vacuum during oneyear. Since there are various ways to define ayear, there is an ambiguity in the exact distance; theIAU recommends using theJulian year as the time basis. A light-year is approximately 9.46 × 1012 km, 5.88 × 1012 statute miles, 6.32 × 104 au, and 3.07 × 10-1 parsecs. Often distances beyond the solar system are given in parsecs. (Seeparsec (pc).)
limb: the apparent edge of the Sun, Moon, or aplanet or any other celestial body with a detectable disk.
limb correction: generally, a small angle (positive or negative) that is added to the tabulated apparentsemidiameter of a body to compensate for local topography at a specific point along thelimb. Specifically for the Moon, the angle taken from the Watts lunarlimb data (Watts, C. B., APAE XVII, 1963) that is used to correct thesemidiameter of the Watts meanlimb. The correction is a function of position along thelimb and the apparentlibrations. The Watts meanlimb is a circle whose center is offset by about 0.″6 from the direction of the Moon's center of mass and whose radius is about 0.″4 greater than thesemidiameter of the Moon that is computed based on itsIAU adopted radius in kilometers.
local place: a topocentric place of an object expressed with respect to theGeocentric Celestial Reference System (GCRS) axes.
local sidereal time: thehour angle of thevernal equinox with respect to the localmeridian.
longitude of the ascending node: given anorbit and a reference plane through the primary body (or center of mass): the angle, \(\Omega\), at the primary, between a fiducial direction in the reference plane and the point at which theorbit crosses the reference plane from south to north. Equivalently, \(\Omega\) is one of the angles in the reference plane between the fiducial direction and the line ofnodes. It is one of the sixKeplerian elements that specify anorbit. For planetaryorbits, the primary is the Sun, the reference plane is usually theecliptic, and the fiducial direction is usually toward theequinox. (Seenode;orbital elements.)
longitude, celestial: seelongitude, ecliptic.
longitude, ecliptic: angular distance on thecelestial sphere measured eastward along theecliptic from thedynamical equinox to the great circle passing through the poles of theecliptic and the celestial object. Also referred to as celestial longitude.
longitude, terrestrial: angular distance measured along the Earth'sequator from theGreenwich meridian to themeridian of a geographic location.
luminosity class: distinctions in intrinsic brightness among stars of the same spectral type, typically given as a Roman numeral. It denotes if a star is a supergiant (Ia or Ib), giant (II or III), subgiant (IV), or main sequence --- also called dwarf (V). Sometimes subdwarfs (VI) and white dwarfs (VII) are regarded as luminosity classes. (Seespectral types or classes.)
lunar phases: cyclically recurring apparent forms of the Moon. The principal phases: new moon, first quarter, full moon and last quarter are defined as the times at which the excess of the apparentecliptic longitude of the Moon over that of the Sun is 0°, 90°, 180° and 270°, respectively. (Seelongitude, ecliptic.)
lunation: theperiod of time between two consecutive new moons.
magnitude of a lunar eclipse: the fraction of the lunar diameter obscured by the shadow of the Earth at the greatestphase of a lunar eclipse, measured along the common diameter. (Seeeclipse, lunar.)
magnitude of a solar eclipse: the fraction of the solar diameter obscured by the Moon at the greatestphase of a solar eclipse, measured along the common diameter. (Seeeclipse, solar.)
magnitude, stellar: a measure on a logarithmic scale of the brightness of a celestial object. Since brightness varies with wavelength, often a wavelength band is specified. A factor of 100 in brightness is equivalent to a change of 5 in stellar magnitude, and brighter sources have lower magnitudes. For example, the bright star Sirius has a visual-band magnitude of -1.46 whereas the faintest stars detectable with an unaided eye under ideal conditions have visual-band magnitudes of about 6.0.
mean distance: an average distance between the primary and the secondary gravitating body. The meaning of the mean distance depends upon the chosen method of averaging (i.e., averaging over the time, or over the true anomaly, or the mean anomaly. It is also important what power of the distance is subject to averaging.) In this volume the mean distance is defined as the inverse of the time-averaged reciprocal distance: \((\int r^{-1}\, \rm{d}t)^{-1}\). In the two body setting, when the disturbances are neglected and theorbit is elliptic, this formula yields theorbitalsemi-major axis, \(a\), which plays the role of mean distance.
mean elements: average values of theorbital elements over some section of theorbit or over some interval of time. They are interpreted as theelements of some reference (mean)orbit that approximates the actual one and, thus, may serve as the basis for calculatingorbitperturbations. The values of meanelements depend upon the chosen method of averaging and upon the length of time over which the averaging is made.
mean equator and equinox: the celestial coordinate system defined by the orientation of the Earth'sequatorial plane on some specified date together with the direction of thedynamical equinox on that date, neglectingnutation. Thus, the meanequator andequinox moves in response only toprecession. Positions in a star catalog have traditionally been referred to a catalogequator andequinox that approximate the meanequator andequinox of astandard epoch. (Seecatalog equinox;true equator and equinox.)
mean motion: defined for bound orbits only. 1. The rate of change of the mean anomaly. 2. The value \(\sqrt{Gm/a^3}\), where \(G\) is Newton's gravitational constant, \(m\) is the sum of the masses of the primary and secondary bodies, and \(a\) is the orbitalsemi-major axis of the relative orbit. For unperturbed elliptic or circular orbits, these definitions are equivalent; the mean motion is related to theperiod through \(nT=2\pi\) where \(n\) is the mean motion and \(T\) is theperiod. For perturbed bound orbits, the two definitions yield, in general, different values of \(n\), both of which are time dependent.
mean place: coordinates of a star or other celestial object (outside the solar system) at a specific date, in theBarycentric Celestial Reference System (BCRS). Conceptually, the coordinates represent the direction of the object as it would hypothetically be observed from the solar systembarycenter at the specified date, with respect to a fixed coordinate system (e.g., the axes of theInternational Celestial Reference Frame (ICRF)), if the masses of the Sun and other solar system bodies were negligible.
mean solar time: seesolar time, mean.
meridian: a great circle passing through the celestial poles and through thezenith of any location on Earth. Forplanetary observations a meridian is half the great circle passing through theplanet's poles and through any location on theplanet.
meridian, central (planetary): half of the great circle passing through theplanet's poles and through the sub-earth point). This is the same as the longitude of the sub-earth point. Do not confuse withplanetaryprime meridian. See diagram on page E4.
meridian, ephemeris: a fictitiousmeridian that rotates independently of the Earth at the uniform rate implicitly defined byTerrestrial Time (TT). Theephemerismeridian is 1.002 738 \(\Delta {\rm T}\) east of theGreenwich meridian, where \(\Delta {\rm T} = {\rm TT} - {\rm UT1}\).
meridian, Greenwich: (also called international orprime meridian) is a generic reference to one of several origins of the Earth's longitude coordinate (zero-longitude). In The Astronomical Almanac, it is the plane defining the astronomical zeromeridian; it contains the geocenter, the Celestial Intermediate Pole and the Terrestrial Intermediate Origin. Other definitions are: the x-z plane of theInternational Terrestrial Reference System (ITRS); the zero-longitudemeridian of the World Geodetic System 1984 (WGS-84); and themeridian that passes through thetransit circle at the Royal Observatory, Greenwich. Note that the lattermeridian is about 100 m west of the others.
meridian, international: seemeridian, Greenwich.
meridian, prime: on Earth, same asGreenwich meridian. On other solar system objects, the zero-longitudemeridian, typically defined via international convention by an observable surface feature orrotational elements.
minor planet: a loosely defined term generally meaning a small solar system body that isorbiting the Sun, does not show a comet-like appearance, and is not massive enough to be adwarf planet. The term is often used interchangeably with ''asteroid'', although there is no implicit constraint that a minorplanet be interior to Jupiter'sorbit.
month: a calendrical unit that approximates theperiod of revolution of the Moon. Also, theperiod of time between the same dates in successivecalendar months.
month, sidereal: theperiod of revolution of the Moon about the Earth (or Earth-Moonbarycenter) in a fixed reference frame. It is the meanperiod of revolution with respect to the background stars. The mean length of the siderealmonth is approximately 27.322 days.
month, synodic: theperiod between successive new moons (as seen from the geocenter). The mean length of the synodicmonth is approximately 29.531 days.
moonrise, moonset: the times at which the apparent upperlimb of the Moon is on the astronomical horizon. In The Astronomical Almanac, they are computed as the times when the truezenith distance, referred to the center of the Earth, of the central point of the Moon's disk is 90° 34′ \(+ s - \pi\), where \(s\) is the Moon'ssemidiameter, \(\pi\) is thehorizontal parallax, and 34′ is the adopted value ofhorizontal refraction.
nadir: the point on thecelestial sphere diametrically opposite to thezenith.
Near Earth Object (NEO): anysmall solar system body, including comets, whose orbit brings it near the Earth. Asmall solar system body is conventionally considered an NEO if itsperihelion is less than 1.3 au.
node: either of the points on thecelestial sphere at which the plane of anorbit intersects a reference plane. The position of one of the nodes (thelongitude of the ascending node) is traditionally used as one of the standardorbital elements.
nutation: oscillations in the motion of the rotation pole of a freely rotating body that is undergoing torque from external gravitational forces. Nutation of the Earth's pole is specified in terms of components inobliquity and longitude.
obliquity: in general, the angle between theequatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of theecliptic is the angle between the planes of theequator and theecliptic; its value is approximately 23°.44.
occultation: the obscuration of one celestial body by another of greater apparent diameter; especially the passage of the Moon in front of a star orplanet, 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 aneclipse. The occultation of the Sun by the Moon is a solareclipse. (Seeeclipse, solar.)
opposition: the phenomenon whereby two bodies have apparent ecliptic longitudes or right ascensions that differ by 180° as viewed by a third body. Oppositions are usually tabulated asgeocentric phenomena.
orbit: the path in space followed by a celestial body, as a function of time. (Seeorbital elements.)
orbit, elliptical: a closedorbit with aneccentricity less than 1.
orbit, hyperbolic: an openorbit with aneccentricity greater than 1.
orbit, instantaneous: the unperturbed two-bodyorbit that a body would follow ifperturbations were to cease instantaneously. Eachorbit in the solar system (and, more generally, in any perturbed two-body setting) can be represented as a sequence of instantaneous ellipses or hyperbolae whose parameters are calledorbital elements. If theseelements are chosen to be osculating, each instantaneousorbit is tangential to the physicalorbit. (Seeorbital elements;osculating elements.)
orbit, parabolic: an openorbit with aneccentricity of 1.
orbital elements: a set of six independent parameters that specifies aninstantaneous orbit. Every realorbit can be represented as a sequence of instantaneous ellipses or hyperbolae sharing one of their foci. At each instant of time, the position and velocity of the body is characterised by its place on one such instantaneous curve. The evolution of this representation is mathematically described by evolution of the values oforbitalelements. Different sets of geometric parameters may be chosen to play the role oforbitalelements. The set ofKeplerian elements is one of many such sets. When the Lagrange constraint (the requirement that theinstantaneous orbit is tangential to the actualorbit) is imposed upon theorbitalelements, they are calledosculating elements.
osculating elements: a set of parameters that specifies the instantaneous position and velocity of a celestial body in its perturbedorbit. Osculatingelements describe the unperturbed (two-body)orbit that the body would follow ifperturbations were to cease instantaneously. (Seeorbit, instantaneous;orbital elements.)
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.
parallax, annual: seeparallax, heliocentric.
parallax, diurnal: seeparallax, geocentric.
parallax, geocentric: the angular difference between thetopocentric andgeocentric directions toward an object. Also calleddiurnal parallax.
parallax, heliocentric: the angular difference between thegeocentric andheliocentric directions toward an object; it is the angle subtended at the observed object. Also calledannual parallax.
parallax, horizontal: the angular difference between thetopocentric and ageocentric direction toward an object when the object is on the astronomical horizon.
parallax, solar: the angular width subtended by the Earth's equatorial radius when the Earth is at a distance of 1astronomical unit (au). The value for the solarparallax is 8.794143 arcseconds.
parallax in altitude: the angular difference between thetopocentric andgeocentric direction toward an object when the object is at a givenaltitude.
parsec (pc): the distance at which oneastronomical unit (au) subtends an angle of one arcsecond; equivalently the distance to an object having anannual parallax of one arcsecond. One parsec is 1/sin(1″) = 206264.806 au, or about 3.26 light-years.
penumbra: 1. The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body. 2. The area of partial shadow surrounding theumbra.
pericenter: the point in anorbit that is nearest to the origin of the reference system. (Seeperigee;perihelion.)
pericenter, argument of: one of theKeplerian elements. It is the angle measured in theorbit plane from the ascendingnode of a reference plane (usually theecliptic) to thepericenter.
perigee: the point in anorbit that is nearest to the Earth. Perigee is sometimes used with reference to the apparentorbit of the Sun around the Earth.
perihelion: the point in anorbit that is nearest to the Sun.
period: the interval of time required to complete one revolution in anorbit or one cycle of a periodic phenomenon, such as a cycle ofphases. (Seephase.)
perturbations: 1. Deviations between the actualorbit of a celestial body and an assumed referenceorbit. 2. The forces that cause deviations between the actual and referenceorbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the referenceorbit to obtain the precise coordinates.
phase: 1. The name applied to the apparent degree of illumination of the disk of the Moon or aplanet as seen from Earth (crescent, gibbous, full, etc.). 2. The ratio of the illuminated area of the apparent disk of a celestial body to the entire area of the apparent disk; i.e., the fraction illuminated. 3. Used loosely to refer to oneaspect of aneclipse (partial phase, annular phase, etc.). (Seelunar phases.)
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 wavelength range.
planet: a celestial body that is inorbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around itsorbit. (Seedwarf planet.)
planetocentric coordinates: coordinates for general use, where the \(z\)-axis is the mean axis of rotation, the \(x\)-axis is the intersection of the planetaryequator (normal to the \(z\)-axis through the center of mass) and an arbitraryprime meridian, and the \(y\)-axis completes a right-hand coordinate system. Longitude of a point is measured positive to theprime meridian as defined byrotational elements. Latitude of a point is the angle between the planetaryequator 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 of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of theprime meridian defined by a clearly observable surface feature. Latitude of a point is the angle between the planetaryequator (normal to the \(z\)-axis and through the center of mass) and normal to the reference surface at the point. Theheight of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.
polar motion: the quasi-periodic motion of the Earth's pole of rotation with respect to the Earth's solid body. More precisely, the angular excursion of the CIP from the ITRS \(z\)-axis. (SeeCelestial Intermediate Pole (CIP);International Terrestrial Reference System (ITRS).)
polar wobble: seewobble, polar.
pole, celestial: either of the two points projected onto thecelestial sphere by the Earth's axis. Usually, this is the axis of theCelestial Intermediate Pole (CIP), but it may also refer to the instantaneous axis of rotation, or the angular momentum vector. All of these axes are within 0.″1 of each other. If greater accuracy is desired, the specific axis should be designated.
pole, Tisserand mean: the angular momentum pole for the Earth about which the total internal angular momentum of the Earth is zero. The motions of theCelestial Intermediate Pole (CIP) (described by the conventional theories ofprecession andnutation) are those of the Tisserand mean pole with periods greater than two days in a celestial reference system (specifically, theGeocentric Celestial Reference System (GCRS)).
precession: the smoothly changing orientation (secular motion) of an orbital plane or theequator of a rotating body. Applied to rotational dynamics, precession may be excited by a singular event, such as a collision, a progenitor's disruption, or a tidal interaction at a close approach (free precession); or caused by continuous torques from other solar system bodies, or jetting, in the case of comets (forced precession). For the Earth's rotation, the main sources of forced precession are the torques caused by the attraction of the Sun and Moon on the Earth'sequatorial bulge, called precession of theequator (formerly known as lunisolar precession). The slow change in the orientation of the Earth's orbital plane is called precession of theecliptic (formerly known as planetary precession). The combination of both motions --- that is, the motion of theequator with respect to theecliptic --- is called general precession.
prime meridian: seemeridian, prime.
proleptic calendar: seecalendar, proleptic.
proper motion: the projection onto thecelestial 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 inright ascension anddeclination peryear or century.
proper place: direction of an object in theGeocentric Celestial Reference System (GCRS) that takes into account orbital or space motion andlight-time (as applicable), light deflection, and annual aberration. Thus, the position (geocentricright ascension anddeclination) at which the object would actually be seen from the center of the Earth if the Earth were transparent, non-refracting, and massless. Unless otherwise stated, the coordinates are expressed with respect to the GCRS axes, which are derived from those of the ICRS.
quadrature: a configuration in which two celestial bodies have apparent longitudes 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. (Seelongitude, ecliptic.)
radial velocity: the rate of change of the distance to an object, usually corrected for the Earth's motion with respect to the solar systembarycenter.
radius vector: an imaginary line from the center of one body to another, often from the heliocenter. Sometimes only the length of the vector is given.
refraction: the change in direction of travel (bending) of a light ray as it passes obliquely from a medium of lesser/greater density to a medium of greater/lesser density.
refraction, astronomical: the change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result ofrefraction the observedaltitude of a celestial object is greater than its geometricaltitude. The amount ofrefraction depends on thealtitude of the object and on atmospheric conditions.
refraction, horizontal: theastronomical refraction at the astronomical horizon; often, an adopted value of 34′ is used in computations for sea level observations.
retrograde motion: fororbital motion in the solar system, motion that is clockwise in theorbit as seen from the north pole of theecliptic; for an object observed on thecelestial sphere, motion that is from east to west, resulting from the relative motion of the object and the Earth. (Seedirect motion.)
right ascension: angular distance on thecelestial sphere measured eastward along thecelestial equator from theequinox to thehour circle passing through the celestial object. Right ascension is usually given in combination withdeclination.
rotational elements: typically, a set of six time-dependent parameters used to describe the instantaneous orientation (attitude) and the instantaneous spin (angular velocity) of a celestial body. When the orientation and spin are described in inertial space, the set of rotationalelements is often chosen to comprise the two angular coordinates of the direction of the north (or positive) pole and the location of theprime meridian at astandard epoch, and the time derivatives of each of those three angles. Additional parameters may be required when the object is a non-rigid body.
second, Système 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: half a diameter, radius. Semidiameter is often used in place of radius when the object is mildly elliptical to refer to a semi-axis of interest (e.g. the vertical semidiameter of the Sun or the semidiameter of the greatestdefect of illumination of Saturn).
semi-major axis: 1. Half the length of the major axis of an ellipse. 2. A standardorbital element used to describe anelliptical orbit or ahyperbolic orbit. (Theorbital semi-major axis is negative for ahyperbolic orbit.)) 3. Half the length of the longest axis of anellipsoid.
SI second: seesecond, Système International (SI).
sidereal day: theperiod between successive transits of theequinox. The mean siderealday is approximately 23 hours, 56 minutes, 4 seconds. (Seesidereal time.)
sidereal hour angle: angular distance on thecelestial sphere measured westward along thecelestial equator from theequinox to thehour circle passing through the celestial object. It is equal to 360° minusright ascension in degrees.
sidereal month: seemonth, sidereal.
sidereal time: thehour angle of theequinox. If the meanequinox is used, the result is mean sidereal time; if the trueequinox is used, the result is apparent sidereal time. Thehour angle can be measured with respect to the localmeridian or theGreenwich meridian, yielding, respectively, local or Greenwich (mean or apparent) sidereal times.
small solar system body: a body orbiting the Sun that is not massive enough to be adwarf planet and is not acomet.
solar parallax: seeparallax, solar.
solar time: the measure of time based on thediurnal motion of the Sun.
solar time, apparent: the measure of time based on thediurnal motion of the true Sun. The rate ofdiurnal motion undergoes seasonal variation caused by theobliquity of theecliptic and by theeccentricity of the Earth'sorbit. Additional small variations result from irregularities in the rotation of the Earth on its axis.
solar time, mean: a measure of time based conceptually on thediurnal motion of a fiducial point, called the fictitious mean Sun, with uniform motion along thecelestial equator.
solstice: either of the two points on theecliptic at which the apparent longitude of the Sun is 90° or 270°; also the time at which the Sun is at either point. (Seelongitude, ecliptic.)
spectral types or classes: categorization of stars according to their spectra, primarily due to differing temperatures of the stellar atmosphere. From hottest to coolest, the commonly used Morgan-Keenan spectral types are O, B, A, F, G, K and M. Some other extended spectral types include W, L, T, S, D and C.
standard epoch: a date and time that specifies the reference system to which celestial coordinates are referred. (Seemean equator and equinox.)
stationary point: the time or position at which the rate of change of the apparentright ascension of aplanet is momentarily zero. (Seeapparent place (or position).)
sub-earth point: the point on a body's surface that lies directly beneath the Earth on the line (geodesic) connecting the body's center to the geocenter. For spherical bodies, the Earth would be at the zenith for an observer at the sub-earth point. As viewed from the Earth, a body's sub-earth point appears at the center of the body's disk. In The Astronomical Almanac, the sub-earth point is typically described by a planetographic longitude and latitude. See diagram on page E4.
sub-solar point: the point on a body's surface that lies directly beneath the Sun on the line (geodesic) connecting the body's center to the heliocenter. For spherical bodies, the Sun would be at the zenith for an observer at the sub-solar point. In The Astronomical Almanac, the sub-solar point of aplanet is typically described by aplanetographic longitude and latitude, its distance from thesub-earth point (center of disk), and its position angle (north through east). See diagram on page E4.
sunrise, sunset: the times at which the apparent upperlimb of the Sun is on the astronomical horizon. In The Astronomical Almanac they are computed as the times when the truezenith distance, referred to the center of the Earth, of the central point of the disk is 90° 50′, based on adopted values of 34′ forhorizontal refraction and 16′ for the Sun'ssemidiameter.
surface brightness: the visual magnitude of an average square arcsecond area of the illuminated portion of the apparent disk of the Moon or aplanet.
synodic month: seemonth, synodic.
synodic period: the mean interval of time between successive conjunctions of a pair of planets, as observed from the Sun; or 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 aplanet to the sameaspect as determined by Earth.
syzygy: 1. A configuration where three or more celestial bodies are positioned approximately in a straight line in space. Often the bodies involved are the Earth, Sun and either the Moon or aplanet. 2. The times of the new moon and full moon.
Teph: the independent argument of the JPL planetary and lunar ephemerides DE405/LE405; in the terminology of General Relativity, abarycentric coordinate time scale. \(T_{\rm eph}\) is a linear function ofBarycentric Coordinate Time (TCB) and has the same rate asTerrestrial Time (TT) over the time span of the ephemeris. \(T_{\rm eph}\) is regarded as functionally equivalent toBarycentric Dynamical Time (TDB). (SeeBarycentric Coordinate Time (TCB);Barycentric Dynamical Time (TDB);Terrestrial Time (TT).)
TAI: seeInternational Atomic Time (TAI).
TCB: seeBarycentric Coordinate Time (TCB).
TCG: seeGeocentric Coordinate Time (TCG).
TDB: seeBarycentric Dynamical Time (TDB).
TDT: seeTerrestrial Dynamical Time (TDT).
TNO: seetrans-Neptunian Object (TNO).
terminator: the boundary between the illuminated and dark areas of a celestial body.
Terrestrial Dynamical Time (TDT): the time scale for apparentgeocentric ephemerides defined by a 1979IAU resolution. In 1991, it was replaced byTerrestrial Time (TT). Obsolete.
Terrestrial Ephemeris Origin (TEO): the original name for theTerrestrial Intermediate Origin (TIO). Obsolete.
Terrestrial Intermediate Origin (TIO): the non-rotating origin of theTerrestrial Intermediate Reference System (TIRS), established by theInternational Astronomical Union (IAU) in 2000. The TIO was originally set at theInternational Terrestrial Reference Frame (ITRF) origin of longitude and throughout 1900-2100 stays within 0.1 mas of the ITRF zero-meridian. Formerly referred to as theTerrestrial Ephemeris Origin (TEO).
Terrestrial Intermediate Reference System (TIRS): ageocentric reference system defined by the intermediateequator of theCelestial Intermediate Pole (CIP) and theTerrestrial Intermediate Origin (TIO) on a specific date. It is related to theCelestial Intermediate Reference System by a rotation of the Earth Rotation Angle, \(\theta\), around the Celestial Intermediate Pole.
Terrestrial Time (TT): an idealized form ofInternational Atomic Time (TAI) with anepoch offset; in practice \(TT = TAI + 32^{\rm s}.184\). TT thus advances by SI seconds on thegeoid. Used as an independent argument for apparentgeocentric ephemerides. (Seesecond, Système International (SI).)
Tisserand mean axis: the axis of a rotating deformable body chosen such that the contribution to angular momentum arising from its deformation integrated over its volume is 0.
topocentric: with reference to, or pertaining to, a point on the surface of the Earth.
topocentric place (or position): theproper place of an object computed for a specific location on or near the surface of the Earth (ignoring atmosphericrefraction) and expressed with respect to either the true (intermediate)equator and equinox of date or the trueequator and CIO of date. In other words, it is similar to an apparent or intermediate place, but with corrections for geocentric parallax and diurnal aberration. (Seeaberration, diurnal;parallax, geocentric.)
transit: 1. The passage of the apparent center of the disk of a celestial object across ameridian. 2. 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. (Seeeclipse, annular;eclipse, solar.)
transit, shadow: The passage of a body's shadow across another body; however, the passage of the Moon's shadow across the Earth is called a solar eclipse.
trans-Neptunian Object (TNO): a solar system body with a semi-major axis greater than Neptune's.
true equator and equinox: the celestial coordinate system defined by the orientation of the Earth'sequatorial plane on some specified date together with the direction of thedynamical equinox on that date. The trueequator andequinox are affected by bothprecession andnutation. (Seemean equator and equinox;nutation;precession.)
TT: seeTerrestrial Time (TT).
twilight: the interval before sunrise and after sunset during which the scattering of sunlight by the Earth's atmosphere provides significant illumination. The qualitative descriptions of astronomical, civil and nautical twilight will match the computed beginning and ending times for an observer near sea level, with good weather conditions, and a levelhorizon. (Seesunrise, sunset.)
twilight, astronomical: the illumination level at which scattered light from the Sun exceeds that from starlight and other natural sources before sunrise and after sunset. Astronomicaltwilight is defined to begin or end when the geometriczenith distance of the central point of the Sun, referred to the center of the Earth, is 108°.
twilight, civil: the illumination level sufficient that most ordinary outdoor activities can be done without artificial lighting before sunrise or after sunset. Civiltwilight is defined to begin or end when the geometriczenith distance of the central point of the Sun, referred to the center of the Earth, is 96°.
twilight, nautical: the illumination level at which thehorizon is still visible even on a moonless night allowing mariners to take reliable star sights for navigational purposes before sunrise or after sunset. Nauticaltwilight is defined to begin or end when the geometriczenith distance of the central point of the Sun, referred to the center of the Earth, is 102°.
umbra: the portion of a shadow cone in which none of the light from an extended light source (ignoringrefraction) can be observed.
Universal Time (UT): a generic reference to one of several time scales that approximate the meandiurnal motion of the Sun; loosely,mean solar time on theGreenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or toCoordinated Universal Time (UTC); in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it tosidereal time. Thus, UT1 is observationally determined by the apparentdiurnal motions of celestial bodies, and is affected by irregularities in the Earth's rate of rotation. UTC is an atomic time scale but is maintained within 0s.9 of UT1 by the introduction of 1- second steps when necessary. (Seeleap second.)
UT0: a rarely used local approximation to Universal Time; not corrected forpolar motion.
UT1: seeUniversal Time (UT).
UTC: seeCoordinated Universal Time (UTC).
vernal equinox: see equinox, vernal.
vertical: the apparent direction of gravity at the point of observation (normal to the plane of a free level surface).
week: an arbitraryperiod of days, usually seven days; approximately equal to the number of days counted between the four principal phases of the Moon. (Seelunar phases.)
wobble, polar: 1. In current practice including the phraseology used in The Astronomical Almanac, it is identical topolar motion. 2. In certain contexts it can refer to specific components ofpolar motion, e.g. Chandler wobble or annual wobble. (Seepolar motion.)
year: aperiod of time based on the revolution of the Earth around the Sun, or theperiod of the Sun's apparent motion around thecelestial sphere. The length of a given year depends on the choice of the reference point used to measure this motion.
year, anomalistic: theperiod between successive passages of the Earth throughperihelion. The anomalisticyear is approximately 25 minutes longer than the tropicalyear.
year, Besselian: theperiod of one complete revolution inright ascension of the fictitious mean Sun, as defined by Newcomb. Its length is shorter than a tropicalyear by 0.148 × \(T\) seconds, where \(T\) is centuries since 1900.0. The beginning of the Besselianyear occurs when the fictitious mean Sun is at meanright ascension 18h 40m. Now obsolete.
year, calendar: theperiod between two dates with the same name in acalendar, either 365 or 366 days. TheGregorian calendar, now universally used for civil purposes, is based on the tropical year.
year, eclipse: theperiod between successive passages of the Sun (as seen from the geocenter) through the same lunarnode (one of two points where the Moon'sorbit intersects theecliptic). It is approximately 346.62 days.
year, Julian: aperiod of 365.25 days. It served as the basis for theJulian calendar.
year, sidereal: theperiod of revolution of the Earth around the Sun in a fixed reference frame. It is the meanperiod of the Earth's revolution with respect to the background stars. The siderealyear is approximately 20 minutes longer than the tropicalyear.
year, tropical: theperiod of time for theecliptic longitude of the Sun to increase 360 degrees. Since the Sun'secliptic longitude is measured with respect to theequinox, the tropicalyear comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. The mean tropicalyear is approximately 365 days, 5 hours, 48 minutes, 45 seconds.
zenith: in general, the point directly overhead on thecelestial sphere.
zenith, astronomical: the extension to infinity of a plumb line from an observer's location.
zenith, geocentric: The point projected onto thecelestial sphere by a line that passes through the geocenter and an observer.
zenith, geodetic: the point projected onto thecelestial sphere by the line normal to the Earth's geodetic ellipsoid at an observer's location.
zenith distance: angular distance on thecelestial sphere measured along the great circle from thezenith to the celestial object. Zenith distance is 90° minusaltitude.
