Twilight, the light diffused over the sky from sunset to darkness and from darkness to sunrise. Twilight occurs when upper portions of the atmosphere are in full sunlight, while for an observer on the earth's surface the sun is below the horizon. Twilight is produced by sunlight that is scattered by airborne dust particles and by the air itself. The twilight-producing atmosphere extends up to about 50 miles (80 km). Above that altitude, there is too little dust and air to scatter the sunlight and provide indirect illumination to the earth below.
There are several limits between twilight and night, each limit indicating a different stage of darkness. The limit of civil twilight occurs when the sun is 6 below the horizon; the limit of nautical twilight occurs when the sun is 12 below the horizon; and the limit of astronomical twilight occurs when the sun is 18 below the horizon. At the limit of civil twilight, some artificial illumination is needed for most kinds of outdoor activities. At the limit of nautical twilight, the horizon at sea becomes indistinct, and a marine sextant can no longer be used. At the limit of astronomical twilight, there is no solar illumination at all.
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The duration of twilight varies with latitude and with the season because it depends on the sun's angle of motion across the horizon at sunset and sunrise. The shortest twilight, 1 hour and 10 minutes (astronomical twilight), occurs at the Equator, where the sun rises and sets at right angles to the horizon throughout the year. The longest twilight occurs at the poles; it lasts for about six weeks before the annual sunrise and after the annual sunset.
