The September Equinox Explained...

The September equinox occurs at 21:19 (or 9:19pm) Coordinated Universal
Time (UTC) on September 22, 2009. It is also referred to as the autumnal or
 fall equinox in the northern hemisphere, as well as the spring or vernal equinox
in the southern hemisphere (not to be confused with the March equinox).
This is due to the seasonal contrasts between both hemispheres throughout
the year.

What happens during the September equinox?
The sun crosses the celestial equator and moves southward in the northern
hemisphere during the September equinox. The location on the earth where
the sun is directly overhead at solar noon is known as the subsolar point. The
subsolar point occurs on the equator during the September equinox and March equinox.
At that time, the earth's axis of rotation is perpendicular to the line connecting
the centers of the earth and the sun. This is the time when many people believe
that the earth experiences 12 hours of day and night. However, this is not exactly
the case.

Dispelling the "exactly 12 hours of daylight" myth
It is important to note that day and night during the September equinox is not
exactly equal length. During the time of the September and March equinoxes
many regions around the equator have a daylight length of about 12 hours and
six-and-a-half minutes. Moreover, the day is slightly longer in places that are
further away from the equator and the sun takes longer to rise and set in these
locations.

According to the US Naval Observatory the dates of "equal" day and night occur
about February 25 and October 15 at a latitude of five degrees in the northern
hemisphere. They occur around March 17 and September 26 at a latitude of 40
degrees. On the dates of the equinoxes, the day is about seven minutes longer
than the night at latitudes up to about 25 degrees, increasing to 10 minutes or
more at a latitude of 50 degrees.

Vernal Equinox vs. Autumnal Equinox
The vernal equinox occurs in the spring while the autumnal equinox occurs
during fall (autumn). These terms are derivatives of Latin. It is important to
note that the northern hemisphere's vernal equinox is in March while its
autumnal equinox is in September. In contrast, the southern hemisphere's
vernal equinox is in September and its autumnal equinox is in March.

This distinction reflects the seasonal differences when comparing the two
hemispheres. Timeanddate.com refers to the two equinoxes simply as the
March and September equinoxes to avoid false assumptions that spring is
in March and fall (autumn) is in September worldwide. This is simply not the
case.

Historical Fact
A Greek astronomer and mathematician named Hipparchus (ca. 190-ca.120 BCE)
was known for discovering the precession of the equinoxes, the slow movement
among the stars of the two opposite places where the sun crosses the celestial
equator. Hipparchus made observations of the equinox and solstice. 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.

The word "equinox" derives from the Latin words meaning "equal night" and refers to
the time when the sun crosses the equator. In modern times this word is used to refer
not only to the positions on the ecliptic but also the times of the year when the sun
has reached them. The September equinox has been used as a reference point in
many calendars in the past, including the French Revolutionary Calendar. Although
very little is known about the ancient Macedonian calendar, some believe that the
first month began after the autumnal equinox.

More from This article:  http://is.gd/3yuiz