Autumn begins astronomically in the Northern Hemisphere at 12:05 GMT on September 23, while spring begins in the Southern Hemisphere at the same moment.
At that moment, the sun will rise vertically over a specific point in the tropical Pacific Ocean, located 400 kilometers southeast of Tarawa, the capital of Kiribati.
This date is called the "Equinox," derived from Latin and meaning "equal night," referring to the idea that day and night are of equal length all over the world. But in reality, this is not entirely accurate.
Why aren't day and night actually equal in length?
Defining the equinox as the time when day and night are equal is an oversimplification, because it treats night simply as a period when the sun is below the horizon, ignoring twilight. If the sun were merely a point of light and the Earth had no atmosphere, then at the equinox, the sun would indeed spend half its path above the horizon and half below it. But in reality, atmospheric refraction raises the sun's disk by more than its apparent diameter at sunrise or sunset. When we see the sun as a reddish-orange sphere on the horizon, we are seeing an optical illusion: it is, in fact, just below the horizon.
In addition to refraction, which hastens sunrise and delays sunset, another factor makes the day longer than the night at the equinox: sunrise and sunset are defined as the moments when the first or last part of the sun's upper edge appears above the horizon, not the center of the disk. Therefore, if you check your local sunrise and sunset times on the equinox, you'll find that the day still lasts slightly more than 12 hours, not exactly 12 as the term "equinox" suggests.
In Pittsburgh, for example, in the northeastern United States, the sun rises at 7:07 a.m. and sets at 7:16 p.m., giving the city 12 hours and 9 minutes of daylight. Day and night are not truly equal there until September 25, when the sun rises at 7:10 a.m. and sets 12 hours later.
At the North Pole, the sun now traces a complete circle across the sky, appearing to just touch the horizon. At the moment of the autumnal equinox this year, the sun should theoretically disappear from view, but its disk will remain suspended above the horizon. After only 50 hours and 44 minutes, the last point of the sun's upper edge will finally fall out of view. This powerful refraction effect also causes the sun's disk to appear elliptical when near the horizon, as the refraction increases so rapidly that the lower edge of the sun rises higher than the upper, significantly distorting the disk.
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