The 2012 Autumn Equinox occurs on Tuesday 20 March

The March equinox will occur at 05:14 (or 5:14am) Coordinated Universal Time (UTC) on March 20, 2012. This marks the beginning of spring in the northern hemisphere and fall (autumn) in the southern hemisphere from an astronomical viewpoint.

Twice a year, around March 20 or 21 and September 22 or 23, the sun shines directly on the equator and the length of day and night are nearly equal in all parts of the world. These two days are known as the March equinox (vernal in the northern hemisphere) and the September equinox.

The March equinox is the movement when the sun crosses the true celestial equator – or the line in the sky above the earth’s equator – from south to north, around March 20 (or March 21) of each year. At that time, day and night are balanced to nearly 12 hours each all over the world and the earth’s axis of rotation is perpendicular to the line connecting the centres of the earth and the sun.

In gyroscopic motion, the earth’s rotational axis migrates in a slow circle based as a consequence of the moon’s pull on a nonspherical earth. This nearly uniform motion causes the position of the equinoxes to move backwards along the ecliptic in a period of about 25,725 years.

Historical Fact: There is some debate about whether astronomers in ancient Mesopotamia were already aware of the precession of the equinoxes, the slow movement among the stars of the two opposite places where the sun crosses the celestial equator. However, most sources claim that a Greek astronomer and mathematician named Hipparchus (ca. 190-ca.120 BCE) discovered the phenomenon. Hipparchus made observations of the equinox and solstice. However, the difference between the sidereal and tropical years (the precession equivalent) was known to Aristarchus of Samos (around 280 BCE) prior to this. There is some debate about whether the precession of the equinoxes was already known in ancient Mesopotamia.

Astronomers use the spring equinoctial point to define their frame of reference, and the movement of this point implies that the measured position of a star varies with the date of measurement. Hipparchus also compiled a star catalogue, but this has been lost.

March Equinox across Cultures: In the northern hemisphere the March equinox marks the start of spring and has long been celebrated as a time of rebirth. Many cultures and religions celebrate or observe holidays and festivals around the time of the March equinox, such as the Easter holiday period.

The astronomical Persian calendar begins its New Year on the day when the March equinox occurs before apparent noon (the midpoint of the day, sundial time, not clock time) in Tehran. The start of the New Year is postponed to the next day if the equinox is after noon.

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