CBS NEWS - On Monday, Sept. 22, the Earth will have nearly equal amounts of light and darkness, as summer ends and a new fall season begins. Depending where you live, the changing colors of leaves or a sudden briskness in the air may have made it seem like the seasons had already shifted, but the equinox on Monday signals the official start of autumn.
The autumnal equinox will occur at 10:29 p.m. EDT on Monday (0229 Tuesday, Sept. 23 GMT), according to the National Weather Service (NWS) Forecast Office. Sometimes the autumnal equinox falls on Sept. 23 or 24 because of irregularities in the calendar and Earth's orbit.
In addition to the autumnal equinox in late September, the Earth undergoes the vernal, or spring, equinox in March; in both cases, the Earth's axis is not tilted toward or away from the sun, NWS officials said. At those times, the lengths of day and night are almost equal across the world. [Turning Leaves: The Rich Colors of Fall Foliage]
During each equinox, the sun is exactly over the equator at noon. As the world turns, the sun's rays are refracted, or bent to make it look like the sun is above the horizon for a longer period of time, even though it is not, NWS officials said in a statement. Though this happens with every sunset, it's longer during the equinox. Also, the days last longer at places farther from the equator because the sun takes longer to rise and set.
Though the word "equinox" translates to "equal" (aequus) and "night" (nox) in Latin, the days and nights aren't precisely equal. On the equinox and the days just before and after it, the day will last roughly 12 hours and six minutes, as the sun passes directly over the equator.
