equatorial coordinate system

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. It is the most widely used type of coordinate system used in astronomy. Equatorial coordinates (EQ) are aligned with Earth's equator and poles. 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).

For example, a point in the celestial sphere which is at the zenith (i.e., straight up) over some point on Earth's equator (i.e., with latitude 0°) has a declination of 0% and points similarly over the north pole (latitude 90° north) has a declination of +90°. However 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, IRCS) is commonly used to describe the positions of planets in the celestial sphere at some particular point in time, and its coordinates are commonly used for identifying and naming objects beyond the solar system (e.g., J designators), which change coordinates far more slowly.

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