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Vernal equinox

From Academic Kids

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Illumination of Earth by Sun on the day of equinox

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. At this moment, night and day are equal all over the world, the sun sets at the South Pole and rises at the North Pole and anyone standing on the equator at noon will not cast a shadow. The term can also be used to refer to the point on the sky defined as the first point of Aries.

The equinox occurs from March 19 to 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.

A common misconception drawn from the etymology of "equinox" or "equal night" is that daylight and night time are exactly equal on an equinox. In fact, sunrise and sunset are 12 hours apart a few days before the vernal equinox and a few days after the autumnal equinox. This is due to the fact that sun-rise designates the time that the leading edge of the sun slips up over the horizon and sun-set is when the trailing edge of the sun slips down below the horizon. So on an equinox there are a few more minutes of daylight than nighttime. The site http://www.sunrisesunset.com gives the following for London on 21 March 2005 (the vernal equinox): sunrise: 6:00 a.m. sunset: 6:13 p.m.

Contents

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.

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 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 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. While 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 (http://www.snopes.com/science/equinox.htm) or BadAstronomy.com (http://www.badastronomy.com/bad/misc/egg_spin.html).

See also

ko:춘분 ja:春分 no:Vårjevndøgn pl:Równonoc wiosenna sv:Vårdagjämningen vi:Xuân phân zh:春分

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