The vernal equinox (or spring equinox) marks the beginning of astronomical spring. It occurs around 20 March in the Northern Hemisphere, and around 23 September in the Southern Hemisphere. In Chinese culture the vernal equinox marks the middle of spring. At the same moment that the vernal equinox occurs in one hemisphere, it is the time of the autumnal equinox in the other hemisphere.
In astronomy, the March equinox (or northward equinox) is the moment when the Sun appears to cross the celestial equator, heading northward. The March equinox is known as 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 20 March each year.
Nomenclature
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 phrase "the dawning of the Age of Aquarius").
Apparent movement of the Sun in relation to the 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 slightly above the horizon at the moment when its "true" middle is rising or setting. For viewers at the north or south poles, it moves 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 March 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.
