Equinox

An equinox is the instant the Sun crosses the celestial equator, when its declination is zero and day and night are nearly equal worldwide. The vernal (spring) equinox around 20 March marks the Sun moving from south to north declination; the autumnal equinox around 23 September marks the reverse. The vernal equinox defines the first point of Aries, the zero reference from which Greenwich Hour Angle of Aries and right ascension are measured. Precession of the equinoxes drifts this point westward along the ecliptic by about 50 arcseconds a year, which is why almanac star positions update.