March equinox

The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vernal equinox (spring equinox) in the Northern Hemisphere and as the autumnal equinox (autumn equinox) or fall equinox in the Southern Hemisphere.

On the Gregorian calendar, the northward equinox can occur as early as 19 March or as late as 21 March at 0° longitude. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from 20 March each year.

The March equinox may be taken to mark the beginning of astronomical spring and the end of astronomical winter in the Northern Hemisphere but marks the beginning of astronomical autumn and the end of astronomical summer in the Southern Hemisphere.

In astronomy, the March equinox is the zero point of sidereal time and, consequently, the right ascension and ecliptic longitude. It also serves as a reference for calendars and celebrations in many cultures and religions.