The right ascension is one of the two coordinates of the equatorial system of reference, used by astronomers to locate a celestial object on the sky. It is the coordinate measuring the angular distance along the celestial equator between that object and the “ Vernal point,” which is in first approximation a fixed point with respect to the stars. The Vernal point is the location on the sky where the Sun crosses the equatorial plane at the March equinox. Right ascension is measured in units of hours, minutes, and seconds, positively toward the east. Because of the precession of the equinoxes, the Vernal point moves slowly with time, so that the coordinates (right ascension and declination) must be associated with an indication of the date (the equinox) at which they are valid.