An equatorial coordinate system locates directions on the celestial sphere using right ascension and declination, referred to a specified equator, equinox and epoch.
The parallactic angle is the angle between the great circles from an observed direction toward the celestial pole and toward the zenith. Its change gives sky rotation in pupil-stabilized observations.
Declination is angular latitude north of the celestial equator, ranging from minus to plus 90 degrees.
Right ascension is the eastward angular coordinate measured around the celestial equator from the reference equinox. It is often quoted in hours, with 24 hours corresponding to a full turn.
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