Equinox

From Freepedia

This article is about the astronomical phenomenon. For the Styx album, see Equinox (album). For the SUV by General Motors see Chevrolet Equinox.
Image:Earth-lighting-equinox EN.png

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

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.


Equinox Dates/Times
(Universal Time)
Year Spring starts
(March) 		Autumn starts
(September)
2000 20th Mar 07:35 22nd Sep 17:27
2001 20th Mar 13:31 22nd Sep 23:04
2002 20th Mar 19:16 23rd Sep 04:55
2003 21st Mar 01:00 23rd Sep 10:47
2004 20th Mar 06:49 22nd Sep 16:30
2005 20th Mar 12:33 22nd Sep 22:23
2006 20th Mar 18:26 23rd Sep 04:03
2007 21st Mar 00:07 23rd Sep 09:51
2008 20th Mar 05:48 22nd Sep 15:44
2009 20th Mar 11:44 22nd Sep 21:18
2010 20th Mar 17:32 23rd Sep 03:09
2011 20th Mar 23:21 23rd Sep 09:04
2012 20th Mar 05:14 22nd Sep 14:49
2013 20th Mar 11:02 22nd Sep 20:44
2014 20th Mar 16:57 23rd Sep 02:29
2015 20th Mar 22:45 23rd Sep 08:20

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.

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.

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.

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.

Contents

Apparent behaviour of the sun

On the equinoxes, everywhere over the globe, the sun rises true east (parallel to lines of latitude), sets at true west and the length of the day equals the length of the night.

March equinox

At the North pole the sun passes from a 6-month-long night to a 6-month-long day.

At the Arctic circle the sun reaches an altitude of 23° in the South.

At the Tropic of Cancer the sun reaches an altitude of 67° in the South.

At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.

At the Tropic of Capricorn the sun reaches an altitude of 67° in the North.

At the Antarctic circle the sun reaches an altitude of 23° in the North.

At the South pole the sun passes from a 6-month-long day to a 6-month-long night.

September equinox

At the North pole the sun passes from a 6-month-long day to a 6-month-long night.

At the Arctic circle the sun reaches an altitude of 23° in the South.

At the Tropic of Cancer the sun reaches an altitude of 67° in the South.

At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.

At the Tropic of Capricorn the sun reaches an altitude of 67° in the North.

At the Antarctic circle the sun reaches an altitude of 23° in the North.

At the South pole the sun passes from a 6-month-long night to a 6-month-long day.

Equinoctial point

The equinoctial point occurs twice a year (around March 21st and September 23rd), when the sun passes through the equinox. At this point (equinoctial point) the length of the day and night are the same. Hipparchus used the equinoctial point to divide the day into 24 equinoctial hours. Before this the day (dawn to dusk) was broken up in 12 'hours' whose length varied according to the season (longer in the summer, shorter in winter).

See also

External links

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