SUMMARY / RELATED TOPICS

In astronomy, a celestial coordinate system is a system for specifying positions of satellites, stars and other celestial objects. Coordinate systems can specify an object's position in three-dimensional space or plot its direction on a celestial sphere, if the object's distance is unknown or trivial; the coordinate systems are implemented in either rectangular coordinates. Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth; these differ in their choice of fundamental plane, which divides the celestial sphere into two equal hemispheres along a great circle. Rectangular coordinates, in appropriate units, are the cartesian equivalent of the spherical coordinates, with the same fundamental plane and primary direction; each coordinate system is named after its choice of fundamental plane. The following table lists the common coordinate systems in use by the astronomical community; the fundamental plane divides the celestial sphere into two equal hemispheres and defines the baseline for the latitudinal coordinates, similar to the equator in the geographic coordinate system.

The poles are located at ±90° from the fundamental plane. The primary direction is the starting point of the longitudinal coordinates; the origin is the zero distance point, the "center of the celestial sphere", although the definition of celestial sphere is ambiguous about the definition of its center point. The horizontal, or altitude-azimuth, system is based on the position of the observer on Earth, which revolves around its own axis once per sidereal day in relation to the star background; the positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth. It is based on the position of stars relative to an observer's ideal horizon; the equatorial coordinate system is centered at Earth's center, but fixed relative to the celestial poles and the March equinox. The coordinates are based on the location of stars relative to Earth's equator if it were projected out to an infinite distance.

The equatorial describes the sky as seen from the Solar System, modern star maps exclusively use equatorial coordinates. The equatorial system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found by adjusting the telescope's or other instrument's scales so that they match the equatorial coordinates of the selected object to observe. Popular choices of pole and equator are the older B1950 and the modern J2000 systems, but a pole and equator "of date" can be used, meaning one appropriate to the date under consideration, such as when a measurement of the position of a planet or spacecraft is made. There are subdivisions into "mean of date" coordinates, which average out or ignore nutation, "true of date," which include nutation; the fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. There are two principal variants of the ecliptic coordinate system: geocentric ecliptic coordinates centered on the Earth and heliocentric ecliptic coordinates centered on the center of mass of the Solar System.

The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun and planets. The heliocentric ecliptic system describes the planets' orbital movement around the Sun, centers on the barycenter of the Solar System; the system is used for computing the positions of planets and other Solar System bodies, as well as defining their orbital elements. The galactic coordinate system uses the approximate plane of our galaxy as its fundamental plane; the Solar System is still the center of the coordinate system, the zero point is defined as the direction towards the galactic center. Galactic latitude resembles the elevation above the galactic plane and galactic longitude determines direction relative to the center of the galaxy; the supergalactic coordinate system corresponds to a fundamental plane that contains a higher than average number of local galaxies in the sky as seen from Earth. Conversions between the various coordinate systems are given.

See the notes before using these equations. Horizontal coordinates A, azimuth a, altitude Equatorial coordinates α, right ascension δ, declination h, hour angle Ecliptic coordinates λ, ecliptic longitude β, ecliptic latitude Galactic coordinates l, galactic longitude b, galactic latitude Miscellaneous λo, observer's longitude ϕo, observer's latitude ε, obliquity of the ecliptic θL, local sidereal time θG, Greenwich sidereal time h = θ L − α or h = θ G + λ o − α α = θ L − h or α = θ G + λ o − h The classical equations, derived from spherical trigonometry, for