Autumnal Equinox - occurs at 21:18 Coordinated Universal Time (UTC) on September 22, 2009. It is also referred to as the September or fall equinox in the northern hemisphere, as well as the spring or vernal equinox in the southern hemisphere. This is due to the seasonal contrasts between both hemispheres throughout the year.
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.
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.