celestial coordinate system

A celestial coordinate system is a coordinate system used to specify the position of an object in the sky (celestial sphere). Spherical polar coordinates are commonly used, but systems differ in their orientation, i.e., the direction of their axes. Some systems:

• horizontal coordinate system - from the horizon and from north (or south).
• equatorial coordinate system - from the celestial equator and the vernal equinox.
• ecliptic coordinate system - from the ecliptic and the vernal equinox.
• galactic coordinate system - from the galactic plane.
• supergalactic coordinate system - from the supergalactic plane.

(Among these, the horizontal coordinate system indicates a point in the viewed sky relative to the observer, while the others indicate such a point relative to the stars.) Beyond these general ideas, certain detailed specifications are necessary, including exactly where the axes will fall within the celestial sphere, and whether and how the changes in the position and location of Earth will be accommodated. The International Celestial Reference System (ICRS) is a system standardized by the International Astronomical Union and can be considered a particular detailed-implementation of equatorial coordinates.