Equinox

The word equinox comes from the Latin for "equal night". The equinoxes in March and September are the two times each year when day and night are of equal duration. The exact date of the two equinoxes vary slightly from year to year, falling on a date between the 19th - 21st for the March equinox, and a date between the 21st - 24th for the September equinox. 

For terms of measuring the length of day on the equinox, sunrise starts when the sun is half way above the horizon and sunset is when the sun is halfway under the horizon. Using this definition, the length of the day on the equinox is precisely 12 hours. At the equinoxes, the sun rises due east and sets due west.

Astronomical Equinox

Although, during full daylight, other stars are overwhelmed by sunlight, making it hard to see where the sun is compared to other celestial bodies, the sun does have a definite position 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 naked-eye planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.

The other 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. 

In astronomy, an equinox is defined as the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator:  Thus the equinoxes can also be regarded as points in the sky.  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 astronomical equinox is actually a particular moment, rather than a whole day.

Ecclesiastical March Equinox

In the very early year, Christians tried to celebrate Easter in relation to the March Equinox.  The problem was that the difficulty of calculating the date of the Equinox led to the celebration of Easter on different dates in different countries.  Eventually, it was agreed that Easter would be celebrated according to the 'ecclesiastical March equinox' on March 21.

Behaviour of the sun during the equinoxes

On the equinoxes the sun rises exactly at East and sets exactly at West everywhere, and the length of the day equals the length of the night.

March equinox (19 - 21 March)

• 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 (21 - 24 September)

• 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.

Exact Dates of the Equinoxes, 2000 - 2010

This article is licensed under the GNU Free Documentation License. It uses material from  Wikipedia and from ShiningRise.com

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