astronomical twilight

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The period before sunrise and after sunset when the centre of the Sun's disk is between 12° and 18° below the horizon. It is regarded as the time during which 6th-magnitude stars are visible at the zenith in a clear sky. In summer, astronomical twilight can last all night at latitudes higher than 48½° because the Sun is never more than 18° below the horizon. See also Twilight.

Subjects: Astronomy and Astrophysics.

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