Twilight

Twilight is the period when the sky is illuminated during daytime, though the sun is below the horizon. This light is not direct; it comes from the atmosphere scattering (spreading) the sun's rays that fall from the upper layers of the air. For this reason, the sky remains lit even though the sun cannot be seen.

Twilight occurs twice daily: in the morning, before sunrise, and in the evening, after sunset. Its duration depends on latitude and time of year. It is shorter in areas near the equator, while in areas near the southern and northern poles it becomes considerably longer.

Scientific definition

Astronomers divide twilight into three phases based on how far below the horizon the sun has sunk. The first phase is civil twilight, occurring when the sun is up to six degrees below the horizon; outdoor objects can still be seen without artificial light. The second phase is nautical twilight, ending when the sun reaches twelve degrees below the horizon; at this point seafarers can see both the horizon and stars simultaneously. The third phase is astronomical twilight, ending when the sun reaches eighteen degrees below the horizon; after this the sky becomes completely dark.

Source of the light

Twilight light comes from two atmospheric processes: scattering and reflection of light. Molecules in the air strongly scatter short-wavelength colors, such as blue, which gives the sky its blue hue. As the sun sinks further, light travels through a longer path of air, the blue light is removed, and red and yellow tones remain. For this reason twilight typically has red and orange hues toward the horizon.

Variation by location

In locations near the equator, such as Somalia, the sun passes directly through the horizon, so twilight is brief, lasting only a few minutes to an hour. In high northern latitudes, especially in summer, the sun does not reach the depths that define complete darkness, causing twilight to last throughout the night—a phenomenon known as midnight sun.

Cultural and religious use

Twilight serves as a basis for determining Islamic prayer times: morning prayer begins when the light of dawn twilight appears on the eastern horizon, while evening prayer begins when the red light of dusk twilight disappears from the western horizon. Nomads and seafarers also traditionally used twilight as a time marker for planning travel and camps.