In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere. There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. For example, the fundamental plane of the geographic system is the Earth's equator. Each coordinate system is named for its choice of fundamental plane.
The horizontal, or altazimutal, 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 is therefore ephemeral, and used mainly for computation of times of rising or setting, f.ex. sunrise and sunset. Of old it was also used for navigation, such as determining a planets altazimutal position, with accurate timing, and therefrom determining the latitude and longitude of the traveling ship. Many telescopes also use altazimutal mounts, but then computing the horizontal position from time, geographic position and the objects' position is typically performed by computer.
The equatorial coordinate system is centered at Earth's center and fixes the sky around us, so that it appears fixed while earth, and we on its surface, revolves around its own axis. The equatorial describes the sky, as seen from the solar system, and modern star maps are almost exclusively perusing equatorial coordinates. Ancient Eastern astronomers used this for their star charts.
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.
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