Janna Jordan, age 11, of Chattanooga, Tenn., for her question:

WHAT IS TWILIGHT?

Twilight is the short period of time 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 the molecules of the earth's atmosphere.

Morning twilight starts when the sun is about 18 degrees below the horizon and ends when it reaches the horizon. Evening twilight begins when the sun reaches the horizon and ends when it has sunk to about 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 poles, dawn and dusk last a month each.

There is a period during the 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.