In astronomy, the vernal equinox (spring equinox, March equinox, or northward equinox) is the equinox at the beginning of spring in the northern hemisphere: the moment when the sun appears to cross the celestial equator, heading northward. The term can also be used to refer to the point on the sky defined as the first point of Aries. Considered as a time, rather than a point on the sky, equinox occurs on or very close to March 21, the precise time being about 5 hours 49 minutes later in a common year, and about 17 hours 26 minutes earlier in a leap year, than in the previous year. (Refer to the 400-year cycle of leap years in the Gregorian Calendar).
The point where the sun crosses the celestial equator northwards is called the first point of Aries. However, due to the precession of the equinoxes, this point is no longer in the constellation Aries, but rather in Pisces. By the year 2600 it will be in Aquarius (hence the term "the dawning of the Age of Aquarius").
In the Southern Hemisphere, the equinox occurs at the same moment, but at the beginning of autumn. There are two conventions for dealing with this: either the name of the equinox can be changed to the autumnal equinox, or (apparently more commonly) the name is unchanged and it is accepted that it is out of sync with the season. The alternative terms "March equinox" or "northward equinox" (or even "vernal equinox" for people prepared to ignore the etymology) avoid any such ambiguity.
Apparent movement of sun in relation to horizon
At the equinox, the sun rises directly in the east and sets directly in the west. However, because of refraction it will usually appear all slightly above the horizon at the moment when its "true" middle is rising or setting. And for viewers at the north or south poles it is moving virtually horizontally on or above the horizon, not obviously rising or setting apart from the movement in "declination" (and hence altitude) of a little under half a degree per day - about 365.2/360 times the sine of 23.5 degrees.
For observers in either hemisphere not at the poles, the further one goes in time away from the vernal equinox in the 3 months before that equinox, the more to the south the sun has been rising and setting, and for the 3 months afterwards it rises and sets more and more to the north.
The Iranian festival of Norouz is celebrated on the vernal equinox, as is the Neopagan Sabbat of Ostara (or Eostar).
Also, in Japan Vernal Equinox Day (春分の日) is an official national holiday and is spent visiting family graves and holding family reunions.
Easter is celebrated on the Sunday after the first day of the ecclesiastical full moon on or after the day of the ecclesiastical vernal equinox 21 March (see computus).
Earth Day has been celebrated on the vernal equinox each year since the first Earth Day on 21 March 1970.