The point on the celestial sphere at which the Sun passes from south to north of the celestial equator each year. It is also known as the spring equinox or the first point of Aries. It currently occurs on March 20. At the vernal equinox, as with the autumnal equinox six months later, night and day are equal in length the world over. Right ascension is measured from the vernal equinox, which by definition has zero right ascension and declination.