Equinox

: This article is about the astronomical phenomenon. For the Styx album, see Equinox (album). For the SUV by General Motors see Chevrolet Equinox.

Related Topics:
Styx - Equinox (album) - Chevrolet Equinox

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In astronomy, an equinox is defined as the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator.

Related Topics:
Astronomy - Ecliptic - Celestial equator

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The word "equinox" comes from the Latin for "equal night". The equinoxes in March and September are the two occasions each year when the day and the night are of equal duration. For measuring the length of a day, sunrise is the moment when the sun is half-above the horizon and sunset is the moment when the sun is half-under the horizon. Using this definition, the length of the day (and the night) is precisely 12 hours at an equinox. More commonly the day is defined as the period that sunlight may reach the ground in 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, the day by that definition is almost 14 minutes longer than the night at the equator, and more towards the poles, making the practical equality of day and night happen a few days towards the winter side.

Related Topics:
Latin - Sunrise - Sun - Horizon - Sunset

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Equinox Dates/Times(Universal Time)

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Year

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Spring starts(March)

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Autumn starts(September)

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2000

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20th Mar 07:35

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22nd Sep 17:27

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2001

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20th Mar 13:31

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22nd Sep 23:04

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2002

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20th Mar 19:16

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23rd Sep 04:55

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2003

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21st Mar 01:00

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23rd Sep 10:47

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2004

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20th Mar 06:49

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22nd Sep 16:30

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2005

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20th Mar 12:33

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22nd Sep 22:23

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2006

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20th Mar 18:26

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23rd Sep 04:03

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2007

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21st Mar 00:07

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23rd Sep 09:51

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2008

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20th Mar 05:48

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22nd Sep 15:44

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2009

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20th Mar 11:44

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22nd Sep 21:18

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2010

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20th Mar 17:32

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23rd Sep 03:09

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2011

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20th Mar 23:21

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23rd Sep 09:04

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2012

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20th Mar 05:14

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22nd Sep 14:49

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2013

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20th Mar 11:02

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22nd Sep 20:44

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2014

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20th Mar 16:57

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23rd Sep 02:29

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2015

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20th Mar 22:45

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23rd Sep 08:20

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In the northern hemisphere, the March equinox is known as the vernal (spring) equinox and the September equinox is the autumnal (autumn) equinox. In the southern hemisphere, these names may be transposed.

Related Topics:
Northern hemisphere - Vernal - Autumnal - Southern hemisphere

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The northern vernal equinox typically falls on either 20 or March 21 and the autumnal equinox on 22 or September 23 – the dates vary because the tropical year is not a whole number of days. Because the Earth's orbit is elliptical, the dates on which the equinoxes fall do not divide the year into equal halves.

Related Topics:
20 - March 21 - 22 - September 23 - Tropical year

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The equinoxes can also be interpreted as virtual points in the sky. Although, during full daylight, stars other than the sun are overwhelmed by sunlight, making it hard to see where the sun is compared to other celestial bodies, the sun does have a position (as seen from the Earth) relative to the other stars. As the Earth moves around the sun, the apparent position of the sun relative to the other stars moves in a full circle over the period of a year. This circle is called the ecliptic, and is also the plane of the Earth's orbit projected against the whole sky. The other bright planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.

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Another virtual circle in the sky is the celestial equator, or the projection of the plane of the Earth's equator against the whole sky. Because the Earth's axis of rotation is tilted relative to the plane of Earth's orbit around the sun, the celestial equator is inclined to the ecliptic. Twice a year, the sun, making its progress around the ecliptic, crosses the plane of the Earth's equator. These two points are the equinoxes. The time at which the sun passes through each equinox point can be calculated precisely—so the equinox is actually a particular moment, rather than a whole day.

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