Wednesday, September 23, 2015 will mark the beginning of Fall over the Northern Hemisphere, otherwise known as the Autumnal Equinox. More precisely, Fall will begin at 2:22 A.M. Mountain Daylight Time.
Since the beginning of Summer back in late June, the days have been shortening as the track of the Sun across the sky has been progressing further southward. On September 23, the apparent overhead position of the Sun will cross the Equator on its annual journey south, at a point about 1100 miles east of Nairobi, Kenya. That day marks one of two days in the year in which the length of day and night are nearly equal, about 12 hours each. For Cheyenne, sunrise on September 23 is at 6:48 A.M. with sunset at 6:55 P.M. MDT.
Perhaps you noticed that there are about seven minutes more daytime than night for that day even though it is the equinox when supposedly there should be about 12 hours of each. Where does that extra daylight come from? The answer is that there is an effect by our atmosphere called refraction which acts to bend the sun's light rays such that you see the Sun appear to rise a little before it actually does, and likewise set a little later than it actually does. This refractive effect is what produces most of that extra amount of light. If the atmosphere was not present, then the amount of time the Sun would appear to be up would be almost 12 hours on that day.
The days will continue to get shorter as Fall progresses, with Winter arriving over the Northern Hemisphere on December 21 at 9:49 P.M. Mountain Standard Time. The Fall season will last 89 days, 20 hours and 27 minutes (Don't forget the extra hour gained by the switch to Daylight Standard Time on November 1).
At the same time Fall begins across the Northern Hemisphere, Spring begins across the Southern Hemisphere.
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