equatorial coordinates

Equatorial coordinates A system of celestial coordinates based on the celestial equator. Equatorial coordinates are those most commonly used when giving the position of an object on the celestial sphere for location purposes. The coordinate equivalent to latitude is declination (Dec. or d (delta)), which is measured in degrees north and south of the celestial equator. Northerly declinations are positive and southerly ones negative. The other coordinate, right ascension (RA or a (alpha)), is the equivalent of longitude but is measured in hours, minutes and seconds of time, reflecting the rotation of the celestial sphere once in 24 hours of sidereal time. Thus, a telescope pointing at a fixed direction in the sky will "see" an angle of one hour of right ascension sweep by in one hour of sidereal time. The zero point for right ascension is taken as the northern vernal equinox. Because of precession, this point is very slowly moving along the equator. Equatorial coordinates, therefore, have to be specified with reference to a particular epoch.