In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on. Coordinate systems can specify a position in 3-dimensional space, or merely the direction of the object on the celestial sphere, if its distance is not known or not important.
The coordinate systems are implemented in either spherical coordinates or rectangular coordinates. Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of the 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 simply the cartesian equivalent of the spherical coordinates, with the same fundamental (x,y) plane and primary (x-axis) direction. Each coordinate system is named for its choice of fundamental plane.
A star's galactic (yellow), ecliptic (red) and equatorial (blue) coordinates, as projected on the celestial sphere. Ecliptic and equatorial coordinates share the vernal equinox (magenta) as the primary direction, and galactic coordinates are referred to the galactic center (yellow). The origin of coordinates (the "center of the sphere") is ambiguous; see celestial sphere for more information.
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 vertical coordinates, analogous 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 horizontal 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.
|Coordinate system ||Center point
(also called Alt/Az or El/Az)
|observer||horizon||zenith / nadir||altitude (a) or elevation||azimuth (A)||north or south point of horizon|
|Equatorial||center of the Earth (geocentric) / center of the Sun (heliocentric)||celestial equator||celestial poles||declination (δ)||right ascension (α) or hour angle (h)||vernal equinox|
|Ecliptic||ecliptic||ecliptic poles||ecliptic latitude (β)||ecliptic longitude (λ)|
|Galactic||center of the Sun||galactic plane||galactic poles||galactic latitude (b)||galactic longitude (l)||galactic center|
|Supergalactic||supergalactic plane||supergalactic poles||supergalactic latitude (SGB)||supergalactic longitude (SGL)||intersection of supergalactic plane and galactic plane|
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 (23 hours, 56 minutes and 4.091 seconds) in relation to the "fixed" 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 distant stars and galaxies. 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, and modern star maps almost 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 also 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 also subdivisions into "mean of date" coordinates, which average out or ignore nutation, and "true of date," which include nutation.
The ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.
The ecliptic system describes the planets' orbital movement around the sun, and centers on the barycenter of the solar system (i.e. very close to the sun). The fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. The system is primarily 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, and 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.
The classical equations, derived from spherical trigonometry, for the longitudinal coordinate are presented to the right of a bracket; simply dividing the first equation by the second gives the convenient tangent equation seen on the left. The rotation matrix equivalent is given beneath each case. (This division is lossy because the tan has a period of 180° whereas the cos and sin have periods of 360°.)
Note that Azimuth (A) is measured from the South point, turning positive to the West. Zenith distance, the angular distance along the great circle from the zenith to a celestial object, is simply the complementary angle of the altitude: 90° − a.
These equations are for converting equatorial coordinates referred to B1950.0. If the equatorial coordinates are referred to another equinox, they must be precessed to their place at B1950.0 before applying these formulae.
These equations convert to equatorial coordinates referred to B1950.0.
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