The autumnal equinox occurs at 9:09 PM, MDT, on Wednesday, September 22, 2010. Also known as the fall equinox or September equinox, this event marks the beginning of Fall in the northern hemisphere. At this time the Sun is crossing the celestial equator from the northern half of the sky to the southern half of the sky. The celestial equator is simply the Earth’s equator stretched out into space, and splits the celestial sphere into a northern half and a southern half. On this day the Sun will rise directly east and set directly west. Day and night are both 12 hours long–equniox means ‘equal night.’ From this point on, days are shorter than nights, and we note that the Sun is a little lower in the sky at mid-day.
In the southern hemisphere the seasons are reversed. The Sun is still moving from the northern half of the sky to the southern half, but south of the equator this marks the beginning of spring, as the sun is climbing higher in the sky, when viewed at mid-day.