Right ascension α or a (Latin ascensio recta ‘straight ascent’) is one of the two polar coordinates in the rotating equatorial coordinate system of spherical astronomy.

Right ascension is more often given in time measure than in degrees or radians. The indication in time measure refers to the fact that the apparent rotation of the stars around the earth is proportional to time and time differences can be determined more easily as angles. If a star e.g. rises at the equator one sidereal hour after the vernal equinox, it has the right ascension 1h, corresponding to 15° (in one sidereal day of 24 sidereal hours the stars move apparently on a full circle with 360°).

The zero point of the right ascension is the vernal equinox. It increases like the geographical longitude towards east.