The period before sunrise and after sunset during which the sky is not completely dark. Three different periods of twilight are defined according to how far the Sun is below the horizon. During civil twilight, the centre of the Sun is less than 6° below the horizon; during nautical twilight, the centre of the Sun is between 6° and 12° below the horizon; and during astronomical twilight the centre of the Sun is between 12° and 18° below the horizon.