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GLOSSARY M
Δ T : the difference between Terrestrial Time (TT) and Universal Time (UT) : ΔT = TT − UT1. ΔUT1 (or Δ UT) : the value of the difference between Universal Time (UT) and Coor- dinated Universal Time (UTC) : ΔUT1 = UT1 − UTC. aberration (of light): the apparent shift in the position of a body from its geometric position, caused by the finite velocity of light in combination with the motion of the observer with respect to the body. aberration, annual: the component of stellar aberration resulting from the motion of the Earth about the Sun. (See aberration, stellar .) aberration, diurnal: the component of stellar aberration resulting from the observer’s diurnal motion about the center of the Earth due to Earth’s rotation. (See aberration, stellar .) aberration, E-terms of: terms of the annual aberration that depend on the eccentricity and longitude of perihelion of the Earth. (See perihelion ; aberration, annual .) aberration, elliptic: see aberration, E-terms of. aberration, planetary: the total 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 and light-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. (See aberration, 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 the diurnal, annual, and secular components. (See aberration, diurnal ; aberra- tion, 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 the zenith distance. anomaly: angular separation of a body in its orbit from its pericenter. anomaly, eccentric: in undisturbed elliptic motion, the angle measured at the center of the orbit 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 anomaly, mean ; anomaly, true. anomaly, mean: the product of the mean motion of an orbiting body and the interval of time since the body passed the pericenter. Thus, the mean anomaly is the angle from the pericenter of a hypothetical body moving with a constant angular speed that is equal to the mean motion. In realistic computations, with disturbances taken into account, the mean anomaly is equal to its initial value at an epoch plus an integral of the mean motion over the time elapsed since the epoch. (See anomaly, true ; anomaly, eccentric ; mean anomaly at epoch .) anomaly, mean at epoch: the value of the mean anomaly at a specific epoch , i.e., at some fiducial moment of time. It is one of the six Keplerian elements that specify an orbit. (See Keplerian elements ; orbital elements ; instantaneous orbit .) anomaly, true: 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 orbital elements ; anomaly, eccentric ; anomaly, mean .) aphelion: the most distant point from the Sun in a heliocentric orbit. apogee: the point at which a body in orbit around the Earth reaches its farthest distance from the Earth. Apogee is sometimes used in reference to the apparent orbit of the Sun around the Earth.
M2 GLOSSARY
apparent place: coordinates of a celestial object, referred to the true equator and equinox at a specific date, obtained by removing from the directly observed position of the object the effects that depend on the topocentric location of the observer, i.e., refraction , diur- nal aberration , and geocentric (diurnal) parallax. Thus, the position at which the object would actually be seen from the center of the Earth — if the Earth were transparent, non- refracting, and massless — referred to the true equator and equinox. (See aberration, diurnal .) apparent solar time: the measure of time based on the diurnal motion of the true Sun. The rate of diurnal motion undergoes seasonal variation caused by the obliquity of the ecliptic and by the eccentricity of the Earth’s orbit. Additional small variations result from irregularities in the rotation of the Earth on its axis. Aries, First point of: another name for the vernal equinox. aspect: the position of any of the planets or the Moon relative to the Sun, as seen from the Earth. astrometric ephemeris: an ephemeris of a solar system body in which the tabulated positions are astrometric places. Values in an astrometric ephemeris are essentially comparable to catalog mean places of stars after the star positions have been updated for proper motion and parallax. astrometric place: direction of a solar system body formed by applying the correction for light-time displacement to the geometric position. Such a position is directly compa- rable with the astrometric positions of stars after the star positions have been updated for proper motion and parallax. astronomical coordinates: the longitude and latitude of the point on Earth relative to the geoid. These coordinates are influenced by local gravity anomalies. (See zenith ; longitude, terrestrial ; latitude, terrestrial .) astronomical unit (AU): the radius of a circular orbit in which a body of negligible mass, and free of perturbations , would revolve around the Sun in 2 π/k days, k being the Gaussian gravitational constant. This is slightly less than the mean semimajor axis of the Earth’s orbit. astronomical zenith: see zenith, astronomical. 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 eastward along the **horizon** from a specified reference point (usually north). Azimuth is measured to the point where the great circle determining the **altitude** of an object meets the horizon. **barycenter:** the center of mass of a system of bodies; e.g., the center of mass of the solar system, or that of the Earth-Moon system. **barycentric:** with reference to, or pertaining to, the **barycenter** (usually of the solar system). **Barycentric Celestial Reference System (BCRS):** a system of **barycentric** 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 the IAU 2000 resolution B1.3. For all practical applications, unless otherwise stated, the BCRS is assumed to be oriented according to the ICRS axes. (See **Barycentric Coordinate Time (TCB)** .) **Barycentric Coordinate Time (TCB):** the coordinate time of the **Barycentric Celestial Reference System (BCRS)** , which advances by SI seconds within that system. TCB is related to **Geocentric Coordinate Time (TCG)** and **Terrestrial Time (TT)** by relativistic transformations that include a secular term. (See **second, Systeme International .) Barycentric Dynamical Time (TDB): A time scale defined by an IAU 1976 resolution for use as an independent argument of barycentric ephemerides and equations of motion.
M4 GLOSSARY
the Moon) is the crossing farther from the observer’s zenith. culmination, upper: (also called “culmination above pole” for circumpolar stars and the Moon) or transit is the crossing closer to the observer’s zenith. day: an interval of 86 400 SI seconds, unless otherwise indicated. (See second, Syst`eme International .) 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 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’s limb. Correction for this effect, which is independent of wavelength, is included in the transformation 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 : see Δ T. delta UT1: see Δ UT. 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. 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 changes 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 (ET) as the independent argument of dynamical theories and ephemerides. (See Barycen- tric Dynamical Time (TDB) ; Terrestrial Time (TT) .) Earth Rotation Angle (ERA): the angle, θ, measured along the equator of the Celestial Intermediate Pole (CIP) between the direction of the Celestial Intermediate Origin (CIO) and the Terrestrial Intermediate Origin (TIO). It is a linear function of UT1 ; 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 standard elements used to describe an elliptic or hyperbolic orbit. For an elliptical orbit, the quantity e =
1 − (b^2 /a^2 ), where a and b are the lengths of the semimajor and semiminor axes, respectively. (See orbital elements .) eclipse: the obscuration of a celestial body caused by its passage through the shadow cast by another body. eclipse, annular: a solar eclipse in which the solar disk is not 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. (See eclipse, solar .) 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 ), annular, or annular-total. (See eclipse, annular .)
GLOSSARY M
ecliptic: the mean plane of the Earth’s orbit around the Sun. ecliptic longitude: see longitude, ecliptic. 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, Keplerian: see Keplerian elements. elements, mean: see mean elements. elements, orbital: see orbital elements. elements, osculating: see osculating elements. elongation, greatest: the instant when the geocentric angular distance of Mercury or Venus from the Sun is at a maximum. elongation, planetary: the geocentric angle between a planet and the 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. Satellite elongations are measured from 0 ◦^ east or west of the planet. 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. (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 measured eastward from the ephemeris meridian. (See longitude, terrestrial .) ephemeris meridian: a fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by Terrestrial Time (TT). The ephemeris meridian is 1 .002 738 ΔT east of the Greenwich meridian, where ΔT = TT − 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, celestial reference systems, star catalogs, or orbital motions. (See calendar ; orbit ). equation of the equinoxes: the difference apparent sidereal time minus mean sidereal time, due to the effect of nutation in longitude on the location of the equinox. Equivalently, the difference between the right ascensions of the true and mean equinoxes, expressed in time units. (See sidereal time ). equation of the origins: the arc length, measured positively eastward, from the Celestial Intermediate Origin (CIO) to the equinox along the intermediate equator ; alternatively the difference between the Earth Rotation Angle (ERA) and Greenwich Apparent Sidereal Time (ERA − GAST). equation of time: the difference 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: 1. Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator. 2. The time at which the Sun passes through either of these intersection points; i.e., when the apparent ecliptic longitude of the Sun is 0 ◦^ or 180 ◦.
3. The vernal equinox. (See mean equator and equinox ; true equator and equinox .) equinox, autumnal: 1. The descending node of the ecliptic on the celestial sphere. 2. The time at which the apparent ecliptic longitude of the Sun is 180 ◦. equinox, catalog: the intersection of the hour angle of zero right ascension of a star catalog with the celestial equator. Obsolete. equinox, dynamical: the ascending node of the ecliptic on the Earth’s true equator.
GLOSSARY M
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 the Julian date (JD) 0.0. Greenwich sidereal day number: the integral part of the Greenwich sidereal date (GSD). Gregorian calendar: The calendar introduced by Pope Gregory XIII in 1582 to replace the Julian calendar. This calendar is now used as the civil calendar in most countries. In the Gregorian calendar, 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: the distance above or below a reference surface such as mean sea level on the Earth or a planetographic reference surface on another solar system planet. heliocentric: with reference to, or pertaining to, the center of the Sun. horizon: 1. A plane perpendicular to the line from an observer through the zenith. 2. The observed border between Earth and the sky. horizon, astronomical: the plane perpendicular to the line from an observer to the astronomical zenith that passes through the point of observation. horizon, geocentric: the plane perpendicular to the line from an observer to the geo- centric 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: 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. IAU: see International 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 an orbital plane and a reference plane. 3. One of the standard orbital elements that specifies the orientation of the orbit. (See orbital elements .) instantaneous orbit: the unperturbed two-body orbit that a body would follow if pertur- bations were to cease instantaneously. Each orbit in the solar system (and, more generally, in the many-body setting) can be represented as a sequence of instantaneous ellipses or hyperbolae whose parameters are called orbital elements. If these elements are chosen to be osculating, each instantaneous orbit is tangential to the physical orbit. (See orbital elements ; osculating elements .) International Astronomical Union (IAU): an international non-governmental organization that promotes the science of astronomy in all its aspects. 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 the SI second on the geoid , and the epoch is 1958 January 1. (See second, Syst`eme International .) International Celestial Reference Frame (ICRF): 1. A set of extragalactic objects whose adopted positions and uncertainties realize the International Celestial Reference Sys- tem (ICRS) axes and give the uncertainties of those axes. 2. The name of the radio catalog whose 212 defining sources serve as fiducial points to fix the axes of the ICRS, recommended by the International Astronomical Union (IAU) in 1997.
M8 GLOSSARY
International Celestial Reference System (ICRS): a time-independent, kinematically non- rotating barycentric reference system recommended by the International Astronomical Union (IAU) in 1997. Its axes are those of the International Celestial Reference Frame (ICRF). International Terrestrial Reference Frame (ITRF): a set of reference points on the sur- face of the Earth whose adopted positions and velocities fix the (rotating) axes of the International Terrestrial Reference System (ITRS). International Terrestrial Reference System (ITRS): a time-dependent, non-inertial ref- erence 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: 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 century: a period of 100 Julian years , 36525 days. In precise work the timescale should be specified. Julian date (JD): the interval of time, in days and fractions of a day , since 4713 B.C. January 1, Greenwich noon, Julian proleptic calendar. In precise work, the time scale, e.g., Terrestrial Time (TT) or Universal Time (UT) , should be specified. Julian date, modified (MJD): the Julian date (JD) minus 2400000.5. Julian day number: the integral part of the Julian date (JD). Julian proleptic calendar: the calendric system employing the rules of the Julian calen- dar , but extended and applied to dates preceding its introduction. Julian year: a period of 365.25 days. It served as the basis for the Julian calendar. Keplerian Elements: a certain set of six orbital elements , sometimes referred to as the Keplerian set. Historically, this set included the mean anomaly at epoch , the semimajor axis , the eccentricity and three Euler angles: the longitude of the ascending node , the inclination , and the argument of the pericenter. The time of pericenter passage is often used as a part of the Keplerian set instead of the mean anomaly at epoch. Sometimes the longitude of pericenter (which is the sum of the longitude of the ascending node and the argument of the pericenter) is used instead of either the longitude of the ascending node or the argument of the pericenter. Laplacian plane: 1. For planets see invariable 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: see latitude, ecliptic. latitude, ecliptic: 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. Also referred to as celestial latitude. 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 inserted as the 61 st^ second of a minute at announced times to keep UTC within 0 s. 9 of UT1. Generally, leap seconds are added at the end of June or December. (See second, Syst`eme International ; Universal Time (UT) ; Coordinated Universal Time (UTC) .) 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
M10 GLOSSARY
formula yields the semimajor axis , a, which plays the role of mean distance. mean elements: average values of the orbital elements over some section of the orbit or over some interval of time. They are interpreted as the elements of some reference (mean) orbit that approximates the actual one and, thus, may serve as the basis for calculating orbit perturbations. The values of mean elements 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 reference system defined by the orientation of the Earth’s equatorial plane on some specified date together with the direction of the dynamical equinox on that date, neglecting nutation. Thus, the mean equator and equinox are affected only by precession. Positions in a star catalog have traditionally been referred to a catalog equator and equinox that approximate the mean equator and equinox of a standard epoch. (See catalog equinox ; true equator and equinox .) 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: coordinates of a star or other celestial object (outside the solar system) at a specific date, in the Barycentric Celestial Reference System (BCRS). Conceptually, the coordinates represent the direction of the object as it would hypothetically be observed from the solar system barycenter at the specified date, with respect to a fixed coordinate system (e.g., the axes of the International Celestial Reference Frame (ICRF) ), if the masses of the Sun and other solar system bodies were negligible. mean solar time: a measure of time based conceptually on the diurnal motion of a fiducial point, called the fictitious mean Sun, with uniform motion along the celestial equator. Obsolete. 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. In The Astronomical Almanac , they are computed as the times when the true zenith distance , referred to the center of the Earth, of the central point of the Moon’s disk is 90 ◦ 34 ′^ + s − π, where s is the Moon’s semidiameter , π 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 one of the nodes (the longitude of the ascending node ) is traditionally used as one of the standard orbital 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 in obliquity and longitude. (See longitude, ecliptic .) 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: 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 as geocentric phenomena. orbit: the path in space followed by a celestial body as a function of time. (See orbital
GLOSSARY M
elements .) orbit, instantaneous: see instantaneous orbit. orbital elements: a set of six independent parameters that specifies an instantaneous orbit. Every real orbit 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 characterized by its place on one such instantaneous curve. The evolution of this representation is mathematically described by evolution of the values of orbital elements. Different sets of geometric parameters may be chosen to play the role of orbital elements. The set of Keplerian elements is one of many such sets. When the Lagrange constraint (the requirement that the instantaneous orbit is tangential to the actual orbit) is imposed upon the orbital elements, they are called osculating elements. osculating elements: a set of parameters 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. (See orbital elements ; instantaneous orbit .) 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 (AU) subtends an angle of one second of arc; equivalently, the distance to an object having an annual parallax of one second of arc. 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 par- tially but not completely cut off by an intervening body. 2. 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 .) pericenter, argument of: one of the Keplerian elements. It is the angle measured in the orbit plane from the ascending node of a reference plane (usually the ecliptic ) to the pericenter. perigee: the point at which a body in orbit around the Earth is closest to 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 is closest to 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: 1. Deviations between the actual orbit of a celestial body and an assumed reference orbit. 2. 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: 1. The name applied to the apparent degree of illumination of the disk of the Moon or a planet 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 one aspect of an eclipse (partial phase, annular phase, etc.). (Also see lunar 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 wave- length range. planetocentric coordinates: coordinates for general use, where the z-axis is the mean axis
GLOSSARY M
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. second, Systeme International (SI second):** 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: 1.** Half the length of the major axis of an ellipse. **2.** A standard element used to describe an elliptical **orbit**. (See **orbital elements** .) **SI second:** see **second, Systeme International. 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 of the Sun is 90 ◦^ or 270 ◦; also, the time at which the Sun is at either point. (See longitude, 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 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. (See mean equator and equinox .) stationary point: the time or position at which the rate of change of the apparent right ascension of a planet is momentarily zero. (See apparent place .) sunrise, sunset: the times at which the apparent upper limb of the Sun is on the astro- nomical horizon. In The Astronomical Almanac they are computed as the times when the true zenith distance , referred to the center of the Earth, of the central point of the Sun’s disk is 90 ◦ 50 ′, based on adopted values of 34 ′^ for horizontal refraction and 16 ′^ for the Sun’s semidiameter. surface brightness: the visual magnitude of an average square arcsecond area of the illuminated portion of the apparent disk of the Moon or a planet. 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 a planet to the same aspect as determined by Earth. TAI: see International Atomic Time (TAI). TCB: see Barycentric Coordinate Time (TCB). TCG: see Geocentric Coordinate Time (TCG). TDB: see Barycentric Coordinate Time (TCB). T eph : the independent argument of the JPL planetary and lunar ephemerides DE405/LE405; in the terminology of General Relativity, a barycentric coordinate time scale. Teph
M14 GLOSSARY
is a linear function of Barycentric Coordinate Time (TCB) and has the same rate as Terrestrial Time (TT) over the time span of the ephemeris. In this volume, Teph is regarded as functionally equivalent to Barycentric Dynamical Time (TDB). (See Barycentric Coordinate Time (TCB) ; Terrestrial Time (TT) ; Barycentric Dynamical Time (TDB) ). terminator: the boundary between the illuminated and dark areas of a celestial body. Terrestrial Ephemeris Origin (TEO): the original name for the Terrestrial Intermediate Origin (TIO). Obsolete. Terrestrial Intermediate Origin (TIO): the non-rotating origin of the Terrestrial In- termediate Reference System (TIRS) , established by the International Astronomical Union (IAU) in 2000. The TIO was originally set at the International Terrestrial Ref- erence Frame (ITRF) origin of longitude and throughout 1900-2100 stays within 0.1 mas of the ITRF zero-meridian. Formerly referred to as the Terrestrial Ephemeris Origin (TEO). Terrestrial Intermediate Reference System (TIRS): a geocentric reference system defined by the intermediate equator of the Celestial Intermediate Pole (CIP) and the Terrestrial Intermediate Origin (TIO) on a specific date. It is related to the Celestial Intermediate Reference System by a rotation of the Earth Rotation Angle , θ, around the Celestial Intermediate Pole. Terrestrial Time (TT): an idealized form of International Atomic Time (TAI) with an epoch offset; in practice, TT = TAI + 32s .184. TT thus advances by SI seconds on the geoid. Used as the independent argument for apparent geocentric ephemerides. (See second, Syst`eme International .) TT: see Terrestrial Time (TT). topocentric: with reference to, or pertaining to, a point on the surface of the Earth. transit: 1. The passage of the apparent center of the disk of a celestial object across a meridian. 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. (See eclipse, 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. 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 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 ◦. (See sunrise , sunset .) 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 generic reference to one of several time scales that approximate the mean diurnal motion of the Sun; loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC) ; in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it to sidereal time. Thus, UT1 is observationally determined by the apparent diurnal 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 0 s. 9 of UT1 by the introduction of 1-second steps when necessary. (See leap second .)
