A coordinate on the celestial sphere, the equivalent of longitude on Earth. Right ascension is measured anticlockwise around the celestial equator, usually in hours, minutes, and seconds of sidereal time (although sometimes in degrees), starting from 0 h at the vernal equinox. The 0 h line of right ascension is the celestial equivalent of the Greenwich meridian on Earth. An hour of right ascension is equivalent to 15 degrees of arc, so that 24 h of RA is equivalent to 360°. Right ascension is an equatorial coordinate.