Twilight is the period just before sunrise and the period just after sunset when the light in the sky is soft and mellow. Although the sun is below the horizon, light can be seen because the rays are scattered by molecules of the earth’s atmosphere. Morning twilight begins when the center of the sun is 18 degrees below the horizon and ends when the sun reaches the horizon. Evening twilight begins when the sun drops below the horizon and ends when its center has sunk to 18 degrees below the horizon.
Twilight lasts the longest time at the North and South poles and the shortest time at the equator. During the six sunless months at the North and South poles, dawn and dusk last a month each. But there is a period during Arctic and Antarctic summers when the sun never sinks below the horizon, and twilight does not occur. Just south of the Arctic, the summer sun never reaches 18 degrees below the horizon, and twilight lasts from sunset to sunrise. At the equator, twilight lasts about an hour, with some seasonal variations.
