An equinox is an astronomical event in which the plane of Earth's equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September. On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light and darkness are equal. See #Length of equinoctial day and night for further discussion.The March equinox or Northward equinox is the equinox on the earth when the Sun appears to leave the southern hemisphere and cross the celestial equator, heading northward as seen from earth. In the northern hemisphere the March equinox is known as the vernal equinox, and in the southern hemisphere as the autumnal equinox.
On the Gregorian calendar the Northward equinox can occur as early as March 19 or as late as March 21. 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 March 20 each year.
