The period of half light before the sun rises, or after it has set, is caused by sunlight scattered by the upper layers of the Earth's atmosphere. The disc of the sun can be seen when its center is at any altitude greater than minus 50 minutes of arc (that is, we see it when it is actually below the horizon). Of this 50 minutes, 16 minutes are due to the sun's not being a point like a star, but 34 are due to the bending of light rays by the Earth's atmosphere. At sunset, twilight begins when the disk of the sun is no longer visible, and at sunrise twilight ends when the disk becomes visible. The other end point of twilight is defined conventionally, in three different ways:
Astronomical twilight ends at sunset when the center of the sun reaches an altitude of minus 18°. At sunrise it begins when the center of the sun reaches minus 18°. At this time the amount of sunlight falling on a horizontal surface is less than that from starlight and airglow. Astronomers make their observations with optical telescopes between these twilights.
Nautical twilight begins at sunrise (and ends at sunset) when the center of the sun reaches an altitude of minus 12°. At this time, the horizon at sea is invisible–the water cannot be distinguished from the sky–making it impossible to find the altitudes of stars with instruments like a sextant. To fix a ship's position from the stars, the ship's navigator must take readings between these twilights.
In the morning, civil twilight begins when the center of the sun reaches an altitude of minus 6° and in the evening it ends when the center of the sun reaches an altitude of minus 6°. The brightest stars are visible but so is the sea horizon. Artificial illumination is required.
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Last revised: 8 March 2004.