Alternate Definition
More conveniently, atmospheric refraction is ignored and the center of the sun is often used in place of the upper limb for computing a day's duration. When sunrise and sunset do occur, the day duration can be computed as 2ωo/15°, where ωo is the sunset hour angle in degrees (°) given by the sunset equation. When sunrise and sunset do not occur during the course of a day, the day duration is either 0 or 24 hours.
Read more about this topic: Day Length
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