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The **equatorial coordinate system** is a
spherical coordinate system used for directions into the
celestial sphere,
fixed to the stars/constellations as seen from Earth.
Elements:

- right ascension (analogous to longitude).
- declination (analogous to latitude).
- celestial equator (analogous to the equator).
- vernal equinox (analogous to the Greenwich Meridian, for this system).

The *equatorial coordinate system* (and the closely related International Celestial Reference System)
is commonly used for planetary orbit calculation and
its coordinates are commonly used in identifying and naming
objects (e.g., J designators).

The Earth's precession of the equinoxes results in a slow but never-ending shift in the equatorial coordinates that indicate a point on the celestial sphere: on the order of an arcsecond per month. This is a primary reason for specifying an epoch.

http://en.wikipedia.org/wiki/Equatorial_coordinate_system

celestial coordinate system

celestial pole

celestial sphere

ephemeris

epoch

equinox

International Celestial Reference System (ICRS)

epoch J2000.0

J designator

supergalactic coordinate system