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In astronomy, the vernal equinox (spring equinox, March equinox, or northward equinox) is the moment when the Sun appears to cross the celestial equator, heading northward. It is the precise moment that spring begins in the Northern Hemisphere and autumn in the Southern Hemisphere. In Chinese culture the vernal equinox marks the middle of spring. The term can also be used to refer to the point on the sky defined as the first point of Aries.
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 vernal equinox from drifting more than a day from 20 March each year.
If the day is defined as the period during which the true position (ignoring atmopheric effects) of the middle of the Sun is above the horizon, the length of the day (and the night) is almost exactly 12 hours at an equinox. The Sun would rise due East and set due West all over the Earth except at the poles, where it would lie on the horizon. At the equator at noon the Sun is straight overhead.
More commonly the day is defined as the period that sunlight may reach the ground in the absence of local obstacles. Because the Sun is not a point but appears as a disc, and because sunlight is refracted downwards by the atmosphere, at the equinox daylight is increased by almost 7 minutes at the equator, decreasing nighttime by the same amount. This causes the day to be longer than the night by almost 14 minutes, making the practical equality of day and night happen a few days towards the winter side. The disparity is increased toward the poles because there the Sun sets at an angle to the vertical, increasing the time it takes to set.
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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").
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 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 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 solar term Chunfen in Chinese astronomy
Chunfen (春分 / 春分) is a solar term begins when Sun lies between the celestial longitude of 0° and 15°. It sometimes refers in particular to the day when Sun exactly at the celestial longitude of 0°. It usually begins around March 21, and ends around April 5.
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Holidays
The Iranian festival of Norouz is celebrated on the vernal equinox, as are the Bahá'í Naw-Rúz (which marks the beginning a new year in the Bahá'í calendar), and the Wiccan Sabbat of Ostara (or Eostar).
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 ecclesiastical full moon day on or after the ecclesiastical vernal equinox day 21 March (see computus).
Earth Day was initially celebrated on the vernal equinox, 21 March 1970. It is currently celebrated in America on 22 April.
Tamil and Bengali New Years are celebrated after the sidereal vernal equinox (14 April). The former is celebrated in the South-Indian state of Tamil Nadu, and the later in Bangladesh and the East-Indian state of West Bengal.
Egg-balancing myth
A common old wives' tale regarding the vernal equinox is that this is the one day of the year that eggs can be balanced on their end. Although this myth is untrue (eggs can be balanced on any date with enough patience) and unsound (would it be different in the southern hemisphere? Why not only the instant of vernal equinox? Why not autumnal equinox?) it is often perpetuated in the news. For a fuller treatment of the issue, see Snopes.com or BadAstronomy.com.