ASTRONOMICAL GLOSSARY

This glossary gives a list of definitions and explanations. It is mainly based on glossary published in Introduction aux éphémérides astronomiques by Bureau des longitudes. The Explanatory supplement to the Astronomical Almanac by the Nautical Almanac Office has also been used. The words in blue refer to other entries in this glossary (click on the word).

Select letter or word in the below list

  Lettre:   Mots:





 

A

  Aberration:
In astrometry, the apparent displacement of the observed position of a celestial body caused by the finite velocity of light in combination with the motions of the observer and of the observed body. (See Aberration (stellar), Aberration (planetary)).

  Aberration (annual):
The component of the aberration (stellar) resulting from the motion of the centre of mass of the Earth about the centre of the solar system.

  Aberration of fixes:
See stellar aberration.

  Aberration (diurnal):
The component of the stellar aberration resulting from the observer's diurnal motion about the centre of mass of the Earth

  Aberration (elliptic):
Terms of the annual aberration depending on the excentricity and longitude of perihelion (see pericentre) of the Earth's orbit. In the observational catalogs before 1984, the terms in elliptic aberration corresponding to the epoch of the catalog were included in the places of the stars.

  Aberration (optical):
In optics, the effects of the defects of the optical instruments which damage the quality of the images.

  Aberration (planetary):
The apparent displacement of the observed position of a body of the solar system from its geometric position caused by the finite velocity of light in combination with the motions of the centre of mass of the Earth and of the observed body about the centre of mass of the solar system.

  Aberration (stellar):
The apparent displacement of the observed position of a star from the position given by a catalog caused by the finite velocity of light in combination with the motion of the observer. Stellar aberration is divided in annual aberration and diurnal aberration.

  Aphelion:
See Apocentre

  Apocentre:
The point in an elliptic orbit that is farthest to the focus corresponding to the centre of force. The apocentre is named apogee when the centre of force is the Earth and aphelion when the centre of force is the Sun.

  Apogee:
See Apocentre

  Azimuth:
One of the alt-azimuth coordinates. The angular distance measured clockwise along the horizon from the vertical circle of the celestial pole (south for the astronomers, north for the navigators) to the vertical circle of the direction.

  Astrometric direction:
The direction joining the position of the Earth at time t with the position of the body at time t - [DeltaT], [DeltaT] being the light-time. It is comparable to the direction of a star given by a catalog, after that the proper motion and annual parallax corrections have been made.

  Alt-azimuth of direction:
One of the coordinates alt-azimuth. Angle of the direction with the horizon of location.

  Astronomical unit (au):
The semi-major axis of an orbit that an unperturbed planet of negligible mass would revolve around the Sun with a mean motion equal to k radians by day, k being the Gaussian constant.


 

B

  Barycentric:
With reference to a coordinate system the centre of which is the barycentre of the solar system.

  Barycentric Coordinate Time (TCB):
The coordinate time timescale related to the barycentric space-time reference system replacing the Barycentric Dynamical Time TDB in the IAU system recommended in 1991. TCB differs from the Terrestrial Time TT by periodic terms, secular terms and Poisson terms.

  Barycentric Dynamical Time (TDB):
The coordinate time timescale recommended by the IAU in 1976 for the ephemerides and dynamical theories referred to the barycentre of the solar system. TDB differs from the Terrestrial Time by periodic terms and Poisson terms. In 1991, the IAU recommended to replace TDB by the Barycentric Coordinate Time TCB.


 

C

  Conjunction:
The phenomenon in which two or several celestial bodies have the same geocentric celestial longitudes (see longitudes (celestial)) or right ascensions. For a conjunction of the outer planets with the Sun the geocentric celestial longitudes of the planet and the Sun are the same. For a conjunction of Mercury or Venus with the Sun the geocentric celestial longitudes of the planet and the Sun are the same; the conjunction is superior when the Sun is between the Earth and the planet, inferior if the planet is between the Earth and the Sun. For a conjunction of a planet with another planet, the Moon or a star, the right ascensions of the planet and the other body are the same.

  Constant of the general precession:
The coefficient of time in the mathematical representation of the general precession in longitude. The value of this constant is determined from observation.

  Coordinates (apparent):
The coordinates giving the direction of the body such as it would be seen by an observer at the centre of the Earth for this date. The apparent coordinates are referred to the true equator and equinox of date or to the true equinox and the mean ecliptic.

  Coordinates (astrometric):
The right ascension and declination of the astrometric direction of the body for this date referred to the mean equator and equinox of a date of reference (J2000, for the ephemerides since 1984).

  Coordinates (astronomical):
The polar coordinates of the vertical of the location referred to the true equator of date and the direction of reference, intersection of this plane with the prime meridian.

  Coordinates (ecliptic):
The coordinates of the direction referred to the mean ecliptic and the direction of reference of this plane etermined by the equinox. These coordinates are true when they are referred to the mean ecliptic and the true equinox of date, mean of date when they are referred to the mean ecliptic and equinox of date and mean of a date of reference when they are referred to the mean ecliptic and equinox of this date of reference. See Coordinates (true) and Coordinates (mean). Two systems of ecliptic coordinates are used : the cartesian ecliptic coordinates and the polar ecliptic coordinates, celestial longitude and latitude.

  Coordinates (equatorial):
The coordinates of the direction referred to the celestial equator and the direction of reference of this plane determined by the equinox. These coordinates are true when they are referred to the true equator and equinox of date, mean of date when they are referred to the mean equator and equinox of the date and mean of a date of reference when they are referred to the mean equator and equinox of this date of reference. See Coordinates (true) and Coordinates (mean). Two systems of equatorial coordinates are used : the cartesian equatorial coordinates and the polar equatorial coordinates, right ascension and declination.

  Coordinates (geometrical):
The coordinates giving the geometric position of this body.

  Coordinates time:
Polars coordinates of the direction referred to the true equator of date and in the origin direction, intersection of this plan and the celestial meridian of the location.

  Coordinates (alt-azimuth):
The polar coordinates of the direction referred to the horizon of the location and the direction of reference, intersection of this plane with the vertical circle of the celestial pole south (for the astronomers) or north (for the navigators).

  Coordinates (mean):
Coordinates referred to the equinox and mean equator or ecliptic of date (mean coordinates of date) or of a date of reference (mean coordinates of a date of reference). See Coordinates (ecliptic), Coordinates (equatorial)

  Coordinates (planetocentric):
Coordinates used to locate a point of the surface of a planet or a satellite for dynamical or astrometric purposes. The planetocentric longitude of a point is the angular distance measured, along the planetary equator of the body from an arbitrary prime meridian to the meridian of the point. It is expressed in degrees from 0° to 360° in the counterclockwise direction. The planetocentric latitude of a point is the angle between the planetary equator and a line to the centre of mass. It is expressed in degrees from the equator, from 0° to +90° towards the north pole and from 0° to -90° towards the south pole.

  Coordinates (true):
Coordinates referred to the true equator and equinox of date or to the true equinox and the mean ecliptic of date. (See coordinates (ecliptic), coordinates (equatorial)).

  Celestial body equator:
Great circle of the surface of a celestial body, considered as an ellipsoïde of revolution, perpendicular in its axis of rotation. (See celestial equator).

  Celestial equator:
The great circle of the celestial sphere perpendicular to an axis close to the Earth's axis of rotation. In a wider sense, plane of this great circle. (See also Celestial equator (true), Mean equator).

  Celestial equator (true) or true equator of date:
The great circle of the celestial sphere perpendicular to the direction of the celestial ephemeris pole (CEP).

  Celestial ephemeris pole (CEP):
The north reference pole for polar motion and nutation. Its direction, close to the Earth axis of rotation, is defined in the manner it has no diurnal or quasi-diurnal motion neither in Earth nor in space.

  Celestial poles:
The two points of intersection (north celestial pole and south celestial pole) of the celestial sphere with a diameter whose the direction is close to the direction of the Earth's axis of rotation.

  Celestial sphere:
A sphere with arbitrary centre and radius whose the points permit a representation of directions in space: to every direction D we can associate the point of intersection of the celestial sphere and the radiating line parallel to D and issued from the centre of the sphere.

  Coordinate time:
In relativity, the first space-time coordinate divided by the speed of light. In a barycentric space-time reference system, the coordinate time may be interpreted as the time which would be measured by a clock at rest with respect to the solar system barycentre and infinitely far from the planets.


 

D

  Declination of a direction:
One of the polar equatorial coordinates. Angle between the celestial equator and the direction. The declination is expressed in degrees from the celestial equator, from -90° to +90°. The declination is also given in combination with the hour angle.

  Day (d):
The unit of time of the IAU system of astronomical units. The day is equal to 86 400 seconds SI


 

E

  Eccentric anomaly:
In the Keplerian elliptic motion, the angle (OP, OM') where O is the centre of the ellipse, P the pericentre and where M´ is the point of the circle of radius OP from which a perpendicular to OP would intersect the orbiting body at time t

  Eclipse:
The obscuration of a celestial body caused by the interposition of another body between this body and the source of illumination.

  Eclipse (lunar):
Eclipse in which the Earth stands between the Moon and the Sun. A lunar eclipse may be total when the Moon passes completely through the Earth's umbra, partial when the Moon passes partially through the Earth's umbra, penumbral when the Moon passes only through the Earth's penumbra

  Eclipse (solar):
The obscuration of the Sun due to its passage behind the Moon . It is really an occultation of the Sun by the Moon. A solar eclipse may be total when the observer is in the Moon's umbra, annular when the solar disk is never completely covered but is seen as an annulus or ring, partial when the observer is in the Moon's penumbra

  Elements (orbital):
In the Keplerian elliptic motion, parameters that specify the position of a body in its orbit. Five parameters are sufficient to define the orbit itself, for instance the semi-major axis and the eccentricity of the ellipse, the inclination of the ellipse to a reference plane, the longitude of the ascending node of the ellipse on a reference plane, the longitude of the pericentre. A sixth parameter is necessary to locate the body on the orbit, for instance the mean anomaly, the true anomaly or the mean longitude. The first five parameters are constants and the sixth parameter is a function of time (linear function for the mean anomaly or the mean longitude). In the perturbed elliptic motion six osculating elements are defined as functions of time. See planetary theory (classical), planetary theory (general).

  Elements (osculating):
The orbital elements describing the orbit that a celestial body would follow at an instant of time t if perturbations were to cease instantaneously at that instant. The real orbit is tangent to the osculating orbit at the instant t.

  Equation of the equinoxes:
The difference true sidereal time minus mean sidereal time.

  Equation of centre:
The part of the equation of time with period 1 yr, due to the eccentricity of the orbit of the Earth. In the elliptic motion of the Earth around the Sun, it is the difference true anomaly minus mean anomaly

  Equation of time:
The difference mean solar time minus true solar time.

  Equinox of catalog:
The origin of the right ascensions given by the catalog. The catalog equinox is close to the mean dynamical equinox of the date of reference of the catalog but is not necessarily the same.

  Equinox (dynamical) of date:
The ascending node of the mean ecliptic of date on the mean equator of date (mean dynamical equinox) or on the true equator of date (true dynamical equinox). There are two dynamical equinoxes, inertial or rotational, according to the mean ecliptic used (see Mean ecliptic). The transformation from the mean dynamical equinox of a date to the mean dynamical equinox of another date is given by the theory of the precession.

  Eccentricity:
A parameter that specifies the shape of a conic section. In an ellipse, the ratio distance centre-focus over semi-major axis. The eccentricity is one of the standard orbital elements.

  Ephemeris meridian:
A fictitious meridian which is at the instantaneous position that the origin terrestrial meridian would have if the angular velocity of the Earth would be constant. Its longitude with respect to the Prime meridian is: -1.002 7379[DeltaT] where [DeltaT] = TT - UT1. All the astronomical computations made using the terrestrial time TT timescale and referring to the ephemeris meridian are formally identical to those made using UT1 and referring to the Prime meridian.

  Ephemeris Time (TE or ET):
Timescale used from 1952 to 1976 in the dynamical theories and until 1984 in the ephemerides of the solar system objects. Its definition is deduced from the Newcomb theory of motion of the Earth around the Sun. This timescale is now replaced by the Terrestrial Time (TT), Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG) and Barycentric Dynamical Time (TDB) timescales.


 

F


 

G

  Gregorian calendar:
The calendar introduced by Pope Gregory XIII in 1582 to replace the Julian calendar. The Gregorian calendar differs from the Julian calendar by the repartition of the leap years and by the deletion of ten days, so that 1582 October 4 (julian) was followed by 1582 October 15 (Gregorian). The leap years are the same as in the Julian calendar except for centurial years which must be exactly divisible by 400 to be leap years. Thus 1700, 1800 and 1900 are common years but 2000 is a leap year like in the Julian calendar. The average length of the Gregorian year (365.2425 days ) is a close approximation of the tropical year. The Gregorian calendar is now commonly used throughout most countries of the world.

  Gaussian constant:
The constante defining, in the astronomical system of units, units of length (astronomical unit), time (day) and mass (solar mass), by means of Kepler's third law. The dimensions of k2 are those of the constant of the gravitation L3 M-1 T-2

  Geocentric:
With reference to a coordinate system the centre of which is the centre of the Earth.

  Geoid:
An equipotential surface that coincides with mean sea level and extends it under the continents.

  Greenwich meridian:
The terrestrial meridian passing through the Greenwich observatory. As the origin meridian, the Greenwich meridian is nowadays replaced by the Prime meridian.

  Geometric position of a celestial body:
The position where the object is really at time t, not taking into account the light-time.

  Geocentric Coordinate Time (TCG):
The coordinate time timescale related to the geocentric space-time reference system. TCG differs from the Terrestrial Time only by a secular term.


 

H

  Hour angle:
The angular distance on the celestial sphere measured westward along the celestial equator from the meridian of the location to the hour circle of the direction.

  Hour circle:
The semi-great circle of the celestial sphere determined by the celestial poles and the point of the celestial sphere corresponding to the direction. So the hour circle is perpendicular to the celestial equator.

  Half-major axis:
Parameter representing half of ellipse major axis. The half-major axis is one of the usual elliptic elements.

  Heliocentric:
With reference to a coordinate system the centre of which is the centre of the Sun.

  Horizon of a location:
Plane perpendicular to the vertical of a location and passing through the centre of the celestial sphere.


 

I

  Inclination:
The angle between an orbital plane and a reference plane. The inclination is one of the standard orbital elements

  International Atomic Time (TAI):
The continuous timescale whose the fundamental unit is the Second SI and resulting from the analysis by the Bureau international des poids et mesures of individual measurements by atomic time standards in many laboratories and countries.


 

J

  Julian year:
An auxiliary unit of time defined as equal to 365.25 days

  Julian calendar:
The calendar introduced by Julius Caesar in -45 to replace the roman calendar. In the Julian calendar three common years of 365 days are followed by a leap year of 366 days. In a leap year February has 29 days . The average length of the Julian calendar (365.25 jours) is a poor approximation of the tropical year and, for this reason, the Julian calendar was superseded by the Gregorian calendar.

  Julian date (JD):
The interval of time from - 4712 January 1 12 h, origin of the julian period. The julian date is expressed in days and fraction of days. In precise work, the timescale must be specified (UT, TT, ET, etc.).

  Julian day number:
The integral part of the julian date

  Julian period:
A chronological system to count the days without discontinuity from -4712 January 1 at 12h.


 

K

  Keplerian ellipse:
The orbit described by a celestial body in a Keplerian elliptic motion.

  Keplerian elliptic motion:
A Keplerian motion where the orbit of the object is an ellipse. This is for example the orbit of a planet around the Sun, the both considered as point masses, if only the solar attraction would be considered.

  Keplerian motion:
The relative motion of a point body M around a central point body O, the mass of M being small in comparison to the mass of O, only the Newtonian attractions between O and M being considered. In a Keplerian motion the orbit of M is a conic section with O at the focus.


 

L

  Latitude (astronomical) of a location on Earth:
One of the astronomical coordinates. Angle of the vertical of the location with the true equator. The astronomical latitude is expressed in degrees, from -90° to +90°.

  Latitude (celestial) of a direction:
One of the polar ecliptic coordinates. Angle of the direction with the mean ecliptic. The celestial latitude is expressed in degrees, from -90° to +90°.

  Latitude (planetocentric):
See coordinates (planetocentric).

  Latitude (planetographic):
See coordinates (planetographic)

  Libration of the Moon:
Oscillations in the orientation of the Moon's surface. Because of these librations an observer on the Earth can actually see a little more than one-half of the Moon's surface. The optical librations are due to variations in the rate of the Moon's orbital motion (libration in longitude), to the inclination of the Moon's equator to its orbital plane (libration in latitude) and to the motion of the observer due to the rotation of the Earth (diurnal libration). The much smaller physical librations are due to the variations of the rotation of the Moon around its axis.

  Longitude (astronomical) of a location:
One of the astronomical coordinates. Angle between the celestial meridian of the location and the celestial meridian passing through the crossing point of the Prime meridian and the true equator of the date. The astronomical longitude is generally measured in degrees, either from -180° to +180° westward like in France, or from 0° to 180° East or West as recommended by the IAU.

  Longitude (celestial) of a direction:
One of the polar ecliptic coordinates. Angular distance between the two half-great circles of the celestial sphere passing through the ecliptic poles and containing, respectively, the point representing the celestial object and the equinox. The celestial longitude is measured in degrees, eastward from 0° to 360°.

  Longitude (planetocentric):
See Coordinates (planetocentric)

  Longitude (planetographic):
See Coordinates (planetographic).

  Light-time:
The time delay for the light in emission or reflexion to reach the observer on the Earth. This time delay may be considered as constant for a given star but not for a solar system body.


 

M

  Mean anomaly:
In the Keplerian elliptic motion, the product of the mean motion of the orbiting body and the interval of time since the body passed pericentre

  Mean ecliptic of date:
The plane perpendicular to the mean angular momentum of the Earth-Moon barycentre in its heliocentric motion. The ecliptic is inertial when the angular velocity vector is computed in a non-rotating reference system and rotational when it is computed in a rotating reference system. The mean angular momentum is computed from the true angular momentum given by a classical planetary theory by removing the terms depending on the mean longitudes of the planets and the arguments of the Moon.

  Mean elements:
The secular terms of the mathematical expression of the orbital elements of a celestial body given by a classical planetary theory of the motion of the body. Mean elements may be referred to the mean ecliptic and dynamical equinox of a date of reference (for instance J2000) or to the mean ecliptic and dynamical equinox of date. They correspond to the development with respect to time of the long period terms of the general planetary theories. Mean elements are used to determine the starting integration constants of classical and general planetary theories and improve the long period terms of the general planetary theories.

  Mean equator of date:
The equator determined from the true equator of date by a transformation given by the theory of the nutation. The transformation from the mean equator of a date to the mean equator of another date is given by the theory of the precession. See Precession-nutation

  Meteor shower:
A swarm of particles associated with comets that disintegrated and whose material was spread out over their entire orbits.

  Mean longitude:
In the Keplerian elliptic motion, the parameter [lambda] defined as [lambda] where M represents the mean anomaly and [varpi], the longitude of the pericentre.

  Mean longitude of a planet:
The linear function depending on the time t and defined as [lnt] where n is the mean motion of the planet and [lambda]0 the integration constant of the mean longitude of the planet. The mean mean longitudes are usual arguments of the classical planetary theories.

  Meridian (celestial) of a location:
The semi-great circle of the celestial sphere containing the true celestial poles and the zenith of a location (see Vertical). In a wider sense the semi-plane containing this semi-great circle.

  Meridian of a celestial body:
The semi-great circle of the celestial sphere containing the poles of the celestial body.

  Meridian (terrestrial) of a location:
The semi-great circle of the geocentric celestial sphere containing the terrestrial poles and whose the semi-plane passes through the point representing this location.

  Meteoroids:
Objects of the solar system which the size is typically included between 100 microns and 10 m. By entering the Earth's atmosphere, these particles are the cause of meteors (or shooting stars in the popular language).

  Mean motion:
In the Keplerian elliptic motion, the mean angular velocity of a body which completes revolution in an orbit with a given semi-major axis. The mean motion n and the semi-major axis a are related by means of Kepler's third law n2a3 = constant.

  Maximum elongation of an inner planet:
The times of the maxima of elongation are those at which the differences of the geocentric celestial longitudes of the planet and the Sun are maximum.

  Main problem of the lunar theory:
The study of the motion of the Moon, assuming that the Sun is the only perturbing body, the Earth-Moon barycentre moving on a Keplerian ellipse.

  Mean solar time:
The true solar time with corrections of the inequalities in right ascension of the Sun: this is the linear part of the true solar time.


 

N

  Nadir:
See Vertical of a location.

  Node:
One of the two points on the celestial sphere associated to the intersection of the plane of the orbit and a reference plane. The position of the node is one of the usual orbital elements.

  Nutation:
See Precession-nutation.

  Nutation (lunisolar):
See Precession-nutation (lunisolar).


 

O

  Obliquity of the ecliptic:
The inclination of the mean ecliptic on the mean equator at a given date.

  Occultation:
The obscuration of a celestial object which cannot be observed because another object is located between itself and the observer.

  Opposition of an outer planet with the Sun:
Configuration where the difference between the geocentric celestial longitudes of the planet and the Sun is 180°.

  Orbit:
The path of a celestial body in space.


 

P

  Phase angle:
A fundamental data for the observation of the surface of a celestial body which is the angle between the heliocentric and the geocentric direction of the centre of the body.

  Prime meridian:
A terrestrial meridian close to the Greenwich meridian and conventionally defined by the coordinates of points on the terrestrial surface.

  Perturbed elliptic motion:
A motion close to the Keplerian elliptic motion where the object is not only under the attraction of the central primary object but also of other perturbing objects with masses small with respect to those of this central object. This is for example the motion of the planets around the Sun (the Sun and the planets being considered as point masses).

  Proper motion of a star:
The motion in right ascension and declination of a star which makes its position varying in function of the time.

  Parallax:
The difference between the apparent directions of a celestial body when the observer goes from one point of space to another. Angular separation of the two points as seen from the celestial body. See Parallax (annual), Parallax (diurnal).

  Parallax (annual):
The difference between the apparent directions of a celestial body as seen by an observer located at the barycentre of the solar system and by an observer located at the centre of the Earth. For a star, angular separation corresponding to the semi-major axis of the Earth's orbit as seen from the star.

  Parallax (diurnal):
The difference between the apparent directions of a celestial body as seen by an observer located at the centre of the Earth and by an observer located on the surface of the Earth. For a star the diurnal parallax is negligible.

  Penumbra of the Earth, of a planet or a natural satellite:
The portion of space from which the celestial object partly occults the Sun.

  Pericentre:
In an elliptic orbit, the point that is nearest to the centre of force, focus of the ellipse. The position of the pericentre is one of the usual orbital elements. The pericentre is named perigee when the centre of force is the Earth, perihelion when it is the Sun.

  Perigee:
See Pericentre.

  Perihelion:
See Pericentre.

  Planetary perturbations (direct) of the lunar theory:
Perturbations on the Earth-Moon radius vector of the lunar motion due to the Newtonian attraction of the planets.

  Planetary perturbations (indirect) of the lunar theory:
Perturbations on the Earth-Moon radius vector of the lunar motion due to the deviation of the heliocentric motion of the Earth-Moon barycentre from the Keplerian elliptic motion due to the attraction of the planets.

  Phases of the Moon:
The successive configurations occurring when the geocentric celestial longitudes of the Moon and the Sun are identical (New Moon), when they differ with 90° (First Quarter), with 180° (Full Moon), or 270° Last Quarter).

  Poles of celestial body:
The two points of intersection (North pole and South pole) of the surface of this object with its axis of rotation.

  Precession:
See Precession-nutation

  Precession (general):
The sum of the lunisolar precession and planetary precession.

  Precession in longitude (general):
The secular displacement of the equinox along the moving ecliptic. This effect is a combination of the lunisolar precession in the retrograd direction along the ecliptic of the reference epoch and the planetary precession in the direct direction along the moving equator, due to the displacement of the ecliptic.

  Precession (lunisolar):
See Precession-nutation (lunisolar).

  Precession-nutation:
The displacement of the equator and ecliptic in function of time, with respect to an inertial reference system, due to the gravitational attractions of the Moon, the Sun and the planets. The mathematical representation of this displacement shows secular terms, periodic series and Poisson series. Conventionally, precession denotes the sum of the secular terms and nutation denotes the sum of the periodic series and the Poisson series.

  Precession-nutation (lunisolar):
The displacement of the equator mainly due to the attraction of the Moon and the Sun. Nevertheless, the planetary attractions are not negligible and are taken into account in modern theories. Like the precession-nutation, the lunisolar precession-nutation is conventionally dispatched into lunisolar precession and lunisolar nutation.

  Precession (planetary):
The slow displacement of the ecliptic due to the gravitational attractions of the Earth by the planets.

  Pulsar:
A neutron star (star at the end of life, very dense and with very small diameter) which rotates very fastly and emits radio pulsations with a noticeable constant period from 1 millisecond up to a few seconds. Pulsars studies and observations are useful for the definition of time scales.

  Poisson series of order p:
Developments in power of time t under the form S0 + tS1 + t2S2 + ... + tpSp where the functions Si are Fourier series.

  Proper time:
In relativistic framework, the time given by a clock in a laboratory. It differs from the coordinate time.

  Planetary theory (general):
A mathematical model of the perturbed elliptic motion of a planet where the coordinates are under the form of Fourier series. The arguments of these series are combinations of linear functions of time. These functions can be arguments with periods of the order of the period of revolution of the planets, such as for example the mean mean longitudes (short period arguments) or arguments with periods of the order of the longitudes of nodes and pericentre (long period arguments). The interval of validity of these theories is very large (of the order of million or ten million years) but the precision of these models are generally too low to produce ephemerides. They are used to study the evolution of the solar system.

  Planetary theory (classical):
A mathematical model of the perturbed elliptic motion of a planet where the coordinates are under the form of secular terms and Poisson series. The arguments of these series are combinations of linear functions of time having only periods of the order of the period of revolution of the planets, such as for example the mean mean longitudes. The interval of validity of these theories is several thousands of years, their precision is good enough to produce ephemerides.


 

Q

  Quadrature of a outer planet with the Sun:
The configuration where the difference between the geocentric celestial longitudes of the planet and the Sun is 90°.


 

R

  Right ascension:
One of the polar equatorial coordinates. The angular distance measured eastward along the celestial equator from the equinox to the hour circle of the direction. Right ascension is sometimes expressed in degrees, from 0° to 360°, but more normally in hours from 0h to 24h (1h = 15°).

  Radiant:
The point of the celestial sphere from which the meteors of a meteor shower seem to be issued.

  Reduction to equator:
The part of the equation of time with a period of six monthes, due to the obliquity of the ecliptic.

  Refraction (astronomical) or refraction:
The deviation of the light rays coming from a celestial body when they cross the terrestrial atmosphere (or more generally a planetary atmosphere). The refraction induces that the observed zenith distance of the body is smaller than the zenith distance it would have without atmosphere. Its amount depends on the zenith distance of the body, on the atmospheric conditions and on the spectral band of the light.

  Reference system (inertial) or Galilean:
A reference system in space associated to an uniform timescale and commonly used in Newtonian mechanics. Two inertial reference systems can be deduced from each other thanks to a translation with a constant velocity. The fundamental laws of the general mechanics are valid in this type of reference systems.

  Reference system (space-time):
A reference system used in relativity in which space coordinates and time coordinates are not strictly separated. See coordinate time. In general relativity, there is no more universal reference system but reference systems are local systems. In the solar system we have the following hierarchy of the reference systems: barycentric reference system centred at the solar system barycentre, heliocentric reference system centred at the Sun, Earth-Moon local reference sytem centred at the Earth-Moon barycentre, geocentric reference system centred at the Earth centre of masses, and topocentric reference system centred at a point of the Earth surface.


 

S

  Standard epoch:
See Origin times

  Sunrise and sunset moment of celestial body in a location on Earth:
The times referring either to the upper limb or to the centre of the Sun. Therefore, the sunrises and sunsets of the upper limb of the Sun are the times at which the zenith distance of the centre of the Sun outside the Earth's atmosphere is : z 90° + R(90°) is value of refraction for a 90° zenith distance (refraction at hoziron). Value of the refraction on the horizon were badly known, the moments of the sunset and sunrise of celestial bodies can be calculated in a precision better no than the minute.

  Sunset and sunrine of Moon at given moment:
Relate either to the superior edge of the Moon, or to its center, and are calculated by taking into account the parallax. The moments of sunset and sunrise of the superior edge of the Moon are thus the moments when the zenithal distance z of the center of the Moon except the atmosphere is z = 90° + R (90°) + - where R (90°) is the refraction on the horizon, s the apparent radius of Moon and parallax.

  Sunset and sunrine of Sun at given moment:
Relate either to the superior edge of the Sun, or to its center, and are calculated by taking into account the parallax. The moments of sunset and sunrise of the superior edge of the Sun are thus the moments when the zenith distance z of the center of the Sun except the atmosphere is z = 90° + R (90°) + - where R (90°) is the refraction on the horizon, s the apparent radius of Sun and parallax.

  Subsolar point:
The point of the surface of a celestial body located at the intersection with the direction from the centre of this object to the centre of the Sun.

  Subterrestrial point:
The point of the surface of a celestial body located at the intersection with the direction from the centre of this object to the centre of he Earth.

  Second SI (s):
The unit of time in the International System since 1967. The SI second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.

  Standard time (or legal time):
Time used in a country and determined through that country's internal political process. Generally, Standard time differs by an exact number of hours from the Universal Time Coordinated UTC. For instance, in France, for 2000, Standard time is UTC + 1 hour in winter but UTC + 2 hours in summer (summer time or advanced time).

  Sidereal time for a location at a given date:
The hour angle of the equinox. The true sidereal time is related to the true equinox and the mean sidereal time is related to the mean equinox of the date. For a given location and at a given date the sum of the true right ascension of a body and of its hour angle is equal to the true sidereal time. At the upper transit of a body to the meridian, its right ascension is equal to the true sidereal time.

  Secular terms:
The time polynomials present in the mathematical model of different astronomical phenomena, such as for example the theory of motion of celestial bodies or the precession-nutation theory.


 

T

  Tropical year:
The interval of time between two successive coincidences of the Sun with the mean equinox. At the present time, the tropical year is about 365.2422 days long.

  True anomaly:
In the Keplerian elliptic motion, the angle (FP, FM) where F is the focus of the ellipse corresponding to the centre of force, P the pericentre and M the position of the orbiting body at time t.

  True equator of date:
See True celestial equator.

  Time origin (or standard epoch):
In 1984 the origin of time was fixed to 2000 January 1 at 12h in the timescale considered. It corresponds to the Julian date 2 451 545.0 and it is denoted J2000.0 or J2000. By definition, the beginning of a Julian year> and the standard epoch differ with an exact number of Julian years.

  True solar time for a location at a given date:
The hour angle of the centre of the Sun for a location at this date.

  Terrestrial Time (TT):
The timescale used for apparent geocentric ephemerides whose unit of time is the SI second. On 1977 January 1 at 0h TAI, the value of TT is 1977 January 1, 0h 0m 32.184s. This is an ideal timescale whose the practical realization is related to the International Atomic Time TAI, with TT = TAI + 32.184 s.

  Terminator:
On the surface of a body of the solar system, the curve of contact of the umbra cone tangent to the surface of the Sun. This curve is the boundary between the illuminated and the dark areas of the surface of a body of the solar system .

  Topocentric:
With reference to a coordinate system the centre of which is a point on the surface of the Earth.

  Twilight:
In astronomy, one calls twilight the gleam, increasing before the rising of the Sun, decreasing after its seeting which comes from the illumination of the roadbases of the atmosphere by the rays of the celestial body located under the horizon, but very close to this one. In the ordinary language the morning twilight is called dawn or daybreak.

  Twilight (civil):
It begins the evening with sunset and finishes at the time when the center of this celestial body is lowered of 6° below the horizon. At this moment if time is clear, start to appear planets and stars of first size. At the morning the phenomena are opposite. The Sun does not drop to 6° below the horizon when the algebraic sum of its variation and the latitude of the place is at least equal to 84° in bsolute value.

  Twilight (nautical):
It begins the evening with sunset and finishes at the time when the center of this celestial body is lowered of 12° below the horizon. At this moment if time is clear, start to appear in the sextant stars of second size, whereas the horizon is still visible. At the morning the phenomena are opposite. The Sun does not drop to 12° below the horizon when the algebraic sum of its variation and the latitude of the place is at least equal to 78° in absolute value.

  Twilight (astronomical):
It begins the evening with sunset finishes at the time when the center of this celestial body is lowered of 18° below the horizon. At this moment if time is clear, appear with the naked eye stars of sixth size; it's dark. At the morning the phenomena are opposite. The Sun does not drop to 18° below the horizon when the algebraic sum of its variation and the latitude of the place is at least equal to 72° in absolute value.


 

U

  Umbra of the Earth, of a planet or a natural satellite:
The portion of the space from which the object occults the Sun.

  Universal Time (TU ou UT):
A timescale closely related to the diurnal rotation of the Earth. For a long time it has been the base of the standard times. TU is defined by a relationship giving the sidereal time in function of the Universal Time. It is then possible to deduce the Universal Time from stellar observations (meridian transits for example). The Universal Time deduced by this method is referred to a fixed pole on Earth and is denoted UT0. The Universal Time referred to the Celestial Ephemeris Pole (CEP) can be get by deleting the polar motion and is then denoted UT1. Since 1984 the standard timescale is no more based on the Universal Time but on the Universal Time Coordinated UTC.

  Universal Time Coordinated (UTC):
A timescale distributed by broadcast time services and used to build the standard times. This is the International Atomic Time TAI from which an exact number of seconds is substracted. This number is regularly defined such as the difference between UTC and the Universal Time UT1 is less than 0.9 s in absolute value.


 

V

  Vertical of a direction, in a location:
The semi-great circle of the celestial sphere associated to the direction and containing the vertical of the location. In a wider sense the semi-plane containing this semi-great circle.

  Vertical of a location:
The direction opposite to the direction of the gravity at this location. The point of the celestial sphere associated to this direction is the zenith of the location, the point diametrically opposite is the nadir.

  VLBI:
An observational method in radio astronomy in which a signal and a very high accurate timing are recorded in two locations whose the separation may be as large as intercontinental distances. These records are then sent in a unique place to produce interferometric signal in a correlator. This method permits to access high astrometric precisions at the order of a few ten microseconds of degree.


 

W


 

X


 

Y


 

Z

  Zenith distance of a direction:
The angle between the direction and the direction of the zenith (see Vertical). The zenith distance is 90° minus altitude.

  Zenith:
See Vertical of a location

  ZHR:
ZHR measures the number of falling stars (meteoroïds) which we would observe if crossing off it with the swarm from which they result was in the zenith of the observation place (on a perfectly black night and clear time). It supplies a number which is generally much higher among meteor shower which we shall observe in practice, because the radiant can be sometimes very low, and same be situated under the horizon. But as many factors, like the position of the observer, this way of measuring the activity of a swarm allows to characterize it in a more objective way.

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