The 2012 September equinox arrived on September 22, at 7:49 a.m. PDT (14:49 Universal Time).
The word equinox comes from the Latin words for "equal night." The fall and spring equinoxes are the only days of the year in which the Sun crosses the celestial equator. In the USA and some other areas in the northern hemisphere, the autumnal equinox marks the first day of fall (autumn).
On the two equinoxes every year the sun shines directly on the equator and the length of day and night is nearly equal – but not exactly.
Equinoxes are opposite on either side of the equator, so the autumnal (fall) equinox in the northern hemisphere is the spring (vernal) equinox in the southern hemisphere and vice versa.
The Autumnal Equinox signals the end of the summer months and the beginning of winter. At this time of year, days have been shortening since the Summer Solstice some three months earlier, and the Equinox is the point where nights reach the same length as days. After this point, the Sun will shine lower and lower on the horizon until the Winter Solstice in about three months' time.
Equinoxes occur because the Earth's axis of rotation isn't aligned with the plane of its orbit around the Sun: it tilts over by about 23½°. The direction of this tilt is effectively constant, relative to the stars, so that the Earth's north pole always points towards Polaris, the Pole Star, and the south pole always points at the constellation of Octans. (In fact, this direction is not completely constant, and the poles move against the stars by about a fifth of a degree every century).