The Astronomical twilight reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Astronomical twilight

Astronomical twilight is defined as the time during which the Sun is between zero and eighteen degrees below an ideal horizon.

At high northern or southern latitude, astronomical twilight may last from dusk until dawn for several months in the summer as the sun is never sufficiently far below the horizon.

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