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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 Earth's equator).
- vernal equinox (analogous to the Greenwich Meridian, for this system).

Note that the relationship between right ascension and longitude shifts with time, its most obvious shift occurring over a daily cycle. In contrast, the relationship between declination and latitude is basically constant.

The *equatorial coordinate system* (and the closely related International Celestial Reference System)
is commonly used for the position of planets in the celestial
sphere and its coordinates are commonly used for identifying and
naming objects beyond the solar system (e.g., J designators).

The Earth's precession of the equinoxes slowly changes the coordinates of points on the celestial sphere on the order of an arcsecond per month. This is a primary reason for specifying an epoch.

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

celestial coordinate system

celestial pole

celestial reference frame

celestial sphere

declination (dec)

ephemeris

epoch

equinox

Catalogues of Fundamental Stars

International Celestial Reference System (ICRS)

epoch J2000.0

J2000.0 equinox

J designator

precession of the equinoxes

right ascension (RA)

supergalactic coordinate system

zenith