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Autumn begins at 5:13 a.m. on Saturday, when the sun crosses the Earth’s Equator. This moment--the autumnal equinox--and the spring equinox are the two days of the year when the day and night are equal in length. At the equinoxes the sun rises due east and sets due west--to the great annoyance of commuters who travel on east-west streets at sunrise or sunset.

Source: John Mosley, Griffith Observatory