This data service provides geocentric position data for the major solar system bodies and selected bright stars.
Apparent position: A calculated apparent position corresponds most closely to the observed position of an object on the celestial sphere. The aberration of light (due to the velocity of the observer) and the relativistic bending of light (due to the Sun's gravitational field) are taken into account. For solar system objects, light propagation time is also included.
Astrometric position: An astrometric position is formed simply by a vector difference of the instantaneous positions of the object and the observer, as obtained from catalog data or the planetary ephemeris. It is comparable to the positions of stars that are published in catalogs and is therefore useful in plotting the positions of solar system objects on star charts. For solar system objects, light propagation time is also included. (Light-time computations are never done for stars; it is assumed that the catalog positions and proper motions of stars implicitly include light-time and its derivative.)
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.
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.
Distance: The distance from the center of the Earth to the Solar System object, given in astronomical units (kilometers for the Moon). Not calculated for stars.
Mean Equator and Equinox of J2000.0: This coordinate system is oriented with its xy-plane parallel to the mean Earth equator at epoch J2000.0, and its z-axis pointing toward the mean north celestial pole of J2000.0. The x-axis points toward the mean equinox of J2000.0. This coordinate system is used for expressing the positions of stars in catalogs and planets in basic solar system ephemerides.
True Equator and Equinox of Date: This coordinate system is oriented with its xy-plane parallel to the true instantaneous Earth equator at the time of observation, and its z-axis pointing toward the true instantaneous north celestial pole. The x-axis points toward the true instantaneous equinox. This coordinate system is useful for expressing the positions of observed objects with respect to Earth-based equatorially-mounted instruments.