March equinox
The March equinox or Northward equinox is the equinox on the earth when the Sun appears to cross the celestial equator, heading northward. The March equinox is the vernal equinox in the northern hemisphere and the autumnal equinox in the southern hemisphere.
The equinox can be as early as March 19 or as late as March 21, the precise time being about 5 hours 49 minutes later in a common year, and about 18 hours 11 minutes earlier in a leap year, than in the previous year. It is the balance of common years and leap years that keeps the calendar date of the equinox from drifting more than a day from March 20 each year.
Northward equinox solar year
The March equinox is one point in time commonly used to determine the length of the tropical year. The mean tropical year is the average of all the tropical years measured from every point along the earth's orbit. In 1983 the mean March equinox fell at 1.48 AM GMT on 23 March. When tropical year measurements from several successive years are compared, variations are found which are due to nutation, and to the planetary perturbations acting on the Sun. Meeus and Savoie (1992, p. 41) provided the following examples of intervals between northward equinoxes:
