Equatorial coordinate system - a simple definition
The direction of the rotation axis of the Earth remains almost constant and
so does the equatorial plane perpendicular to this axis. Therefore the
equatorial plane is a suitable reference plane for a coordinate frame that
has to be independent of time and the position of the observer.
The zero point of the first coordinate is called vernal equinox. This
coordinate is the angle from the vernal equinox measured along the equator
counterclockwise. It is called right ascension, or R.A. The values
are between 0 and 24 hours in format (hours minutes seconds), where
1 hour = 15 degrees.
The angular separation of a star from the equatorial plane is not affected
by rotation of the Earth. This angle is called declination, or dec.
Declination gets values between -90 and +90 degrees in format
(degrees minutes seconds).