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Astronomical twilight
Astronomical twilight starts in the morning when the geometric centre of the Sun is 18 degrees below the horizon and ends when the geometric centre of the Sun is 12 degrees below the horizon. It starts in the evening when the geometric centre of the Sun is 12 degrees below the horizon and ends when the geometric centre of the Sun is 18 degrees below the horizon.
The horizon is not visible under these conditions, but moderately bright stars can be seen.
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