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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.
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.
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 vernal equinox typically falls on either 20 or March 21 and the autumnal equinox on 22 or September 23 – the dates vary because some years are leap years, shifting the calendar by a day or so relative to the seasons. 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.
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 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 Cancer the sun passes from South, where it reaches its maximum altitude of that day, that is 66°33'.
At the Tropic of Capricorn the sun passes from North, where it reaches its maximum altitude of that day, that is 66°33'.
At the North pole the sun passes from a 6-month-long night to a 6-month-long day.
At the South pole the sun passes from a 6-month-long day to a 6-month-long night.
September equinox 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 Cancer the sun passes from South, where it reaches its maximum altitude of that day, that is 66°33'.
At the Tropic of Capricorn the sun passes from North, where it reaches its maximum altitude of that day, that is 66°33'.
At the North pole the sun passes from a 6-month-long day to a 6-month-long night.
At the South pole the sun passes from a 6-month-long night to a 6-month-long day.
See also
External links - Details about the Length of Day and Night at the Equinoxes (http://aa.usno.navy.mil/faq/docs/equinoxes.html)
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