Astronomical Glossary: Terms Related to Celestial Bodies' Movements and Coordinates, Study notes of Astronomy

Download Astronomical Glossary: Terms Related to Celestial Bodies' Movements and Coordinates and more Study notes Astronomy in PDF only on Docsity! GLOSSARY M1 ΔT: the difference between Terrestrial Time (TT) and Universal Time (UT): ΔT = TT − UT1. ΔUT1 (or ΔUT): 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 2π/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, Système 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 toGeocentric Coordinate Time (TCG) and Terrestrial Time (TT) by relativistic transformations that include a secular term. (See second, Système 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. GLOSSARY M5 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. M6 GLOSSARY equinox, vernal: 1. The ascending node of the ecliptic on the celestial equator. 2. The time at which the apparent ecliptic longitude of the Sun is 0◦. ERA: see Earth Rotation Angle. era: a system of chronological notation reckoned from a specific event. flattening: a parameter that specifies the degree by which a planet’s figure differs from that of a sphere; the ratio f = (a − b)/a, where a is the equatorial radius and b is the polar radius. frame bias: the orientation of the mean equator and equinox of J2000.0 with respect to the Geocentric 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 GCRS right ascension of the mean inertial equinox of J2000.0. frequency: the number of periods of a regular, cyclic phenomenon in a given measure of time, such as a second or a year (see period; second; year). frequency standard: a generator whose output is used as a precise frequency reference; a primary frequency standard is one whose frequency corresponds to the adopted definition of the second, with its specified accuracy achieved without calibration of the device. (See second, Système International.) GAST: see Greenwich Apparent Sidereal Time. Gaussian gravitational constant: (k = 0.017 202 098 95): the constant defining the astronomical system of units of length (astronomical unit), mass (solar mass) and time (day), by means of Kepler’s third law. The dimensions of k2 are equal to those of Newton’s constant of gravitation: L3M−1T−2. geocentric: with reference to, or pertaining to, the center of the Earth. Geocentric Celestial Reference System (GCRS): a system of geocentric space-time coor- dinates within the framework of General Relativity. The metric tensor used in the system is specified by the IAU 2000 resolutions. The GCRS is defined such that its spatial coor- dinates are kinematically non-rotating with respect to those of the Barycentric Celestial Reference System (BCRS). (See Geocentric Coordinate Time (TCG).) 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. (See zenith; latitude, terrestrial; longitude, terrestrial.) Geocentric Coordinate Time (TCG): the coordinate time of the Geocentric Celestial Reference System (GCRS), which advances by SI seconds within that system. TCG is related to Barycentric Coordinate Time (TCB) and Terrestrial Time (TT), by relativis- tic transformations that include a secular term. (See second, Système International). geocentric zenith: see zenith, geocentric. geodetic coordinates: the latitude and longitude of a point on the Earth’s surface deter- mined from the geodetic vertical (normal to the reference ellipsoid). (See zenith; latitude, terrestrial; longitude, terrestrial.) geodetic zenith: see zenith, 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 for light-time, aberration, etc. GMST: see Greenwich Mean Sidereal Time. Greenwich Apparent Sidereal Time (GAST): the Greenwich hour angle of the true equinox of date. Greenwich Mean Sidereal Time (GMST): the Greenwich hour angle of the mean equi- nox of date. GLOSSARY M7 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 theGreenwich 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, Système 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. 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 M11 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 M12 GLOSSARY of rotation, the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian, and the y-axis completes a right-hand coordinate system. Longitude of a point is measured positive to the prime meridian as defined by rotational elements. Latitude of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point. planetographic coordinates: coordinates for cartographic purposes dependent on an equi- potential 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 the prime meridian defined by a clearly observable surface feature. Latitude of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and the normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface. polar motion: the irregularly varying motion of the Earth’s pole of rotation with respect to the Earth’s crust. (See Celestial Intermediate Pole (CIP).) pole, celestial: either of the two points projected onto the celestial sphere by the Earth’s axis. Usually, this is the axis of the Celestial 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 the Celestial Interme- diate Pole (CIP) (described by the conventional theories of precession and nutation) are those of the Tisserand mean pole with periods greater than two days in a celestial reference system (specifically, the Geocentric Celestial Reference System (GCRS)). precession: the smoothly changing orientation of an orbital plane or the equator 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’s equatorial bulge, called precession of the equator (formerly known as lunisolar precession). The slow change in the orientation of the Earth’s orbital plane is called precession of the ecliptic (formerly known as planetary precession). The combination of both motions – that is, the motion of the equator with respect to the ecliptic – is called general precession. (see nutation). proper motion: the projection onto the celestial sphere of the space motion of a star relative to the solar system; thus, the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century. quadrature: a configuration in which two celestial bodies have apparent longitudes 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. (See longitude, ecliptic.) radial velocity: the rate of change of the distance, usually corrected for the Earth’s motion with respect to the solar system barycenter. 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 of refraction the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.