For those keeping track, the autumnal equinox will occur at 1758 Z (or 1:58 PM EDT, 12:58 PM CDT, and so forth) this coming Sunday, 22 September. At this instant the sun will appear to be directly over the equator and it will mark one of the two times during the year when essentially all places on earth experience equal intervals of daylight and equal night during a 24 hour period. This event is popularly known as the beginning of autumn or the fall season in the Northern Hemisphere, as the length of daylight rapidly diminishes toward a minimum on the winter solstice (21 December).