The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:
declination
right ascension
The projection of the Earth's equator on the celestial sphere is called the celestial equator. Projecting the geographic poles on the celestial sphere defines the north and south celestial poles.
There are two systems to specify the longitudinal coordinate:
the hour angle system is fixed to the Earth like the geographic coordinate system
the right ascension system is fixed to the stars, during a night or a few nights, it appears to move across the sky as the Earth spins and orbits under the fixed stars. Over long periods of time, precession and nutation effects alter the earth's orbit and the apparent location of the stars.
The latitudinal angle of the equatorial system is called declination (Dec). It measures the angle of an object above or below the celestial equator. The longitudinal angle is called the right ascension (RA). It measures the angle of an object east of the vernal equinox point. Unlike longitude, right ascension is usually measured in hours instead of degrees, because the apparent rotation of the equatorial coordinate system is closely related to sidereal time and hour angle. Since a full rotation of the sky takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in one hour of right ascension.
