The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are:
It is the most closely related to the geographic coordinate system, because they use the same fundamental plane, and the same poles. The projection of the Earth's equator onto the celestial sphere is called the celestial equator. Similarly, projecting the geographic poles onto the celestial sphere defines the north and south celestial poles.
There are two varieties:
- the hour angle system is fixed to the Earth like the geographic coordinate system
- the right ascension system rotates as the Earth does; it is fixed to the stars (actually, not quite, see precession and nutation). Thus, during a night or a few nights, it appears to rotate across the sky with the stars, but of course it's really the Earth rotating under the fixed sky. Because of the precession and nutation just referred to, when considering long intervals between observations it is necessary to specify an epoch (frequently J2000.0, for older data B1950.0) when specifying coordinates of planets, stars, galaxies, etc.
The latitudinal (latitude-like) angle of the equatorial system is called declination (Dec for short). It measures the angle of an object above or below the celestial equator. The longitudinal angle is called the right ascension (RA for short). 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.
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml