Sunset -- from Eric Weisstein's World of Astronomy

Sunset is the time at which the trailing limb of the Sun first sets below the horizon. The effect of refraction in the Earth's atmosphere lifts the image of the Sun about half a degree at the horizon, making sunset about two minutes later than would be expected from the actual position of the Sun in space. Refraction and the fact that sunrise and sunset are calculated from the limb (and not the center) slightly lengthen "day" relative to "night."

The above plot shows the time of sunset from Chicago in U.S. Central Standard Time, so actual times must be shifted to account for daylight saving time. In 1999, the earliest and latest sunsets occur on December 7-8 at 17:27 CST and June 27-28 at 20:26 CST, respectively.

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