apparent solar time

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Time as given by the daily movement of the Sun across the sky; strictly, the hour angle of the Sun plus 12 hours, which is added so that the solar day begins at midnight. Apparent solar time is the time shown on a sundial. The Sun's hour angle increases due to the Earth's rotation, but slightly more slowly than that of the stars because the Sun moves against the star background as the Earth orbits it. However, this movement is not entirely uniform because the Earth's orbit is elliptical, and consequently apparent solar time can be anything up to a quarter of an hour ahead of or behind mean solar time. The difference between apparent solar time and mean solar time is known as the equation of time.

Subjects: Astronomy and Astrophysics.

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