Stefanie Kadin, Age 13, Of Cherry Hill, N. J., for her Question:

What causes twilight?

The dazzling daytime sky, of course, is lit with the rays of the sun. The sunbeams are disbursed and scattered, reflected and refracted by the atmosphere that reaches hundreds of miles above our heads. If we could see the sunny side of the earth from far out in space, it would seem to be circled with a filmy halo of hazy light. This halo is caused by sunbeams piercing the atmosphere that clothes the globe in a blanket perhaps 1000 miles thick.

We see the sun as it traverses the sky from horizon to horizon. But its dazzling rays also are lighting up the air far beyond our visible horizon. Vhen the sun itself sinks out of sight, we still can see the edge of the earth's daytime halo. The sky in the west is still bright with these twilight beams. They fade as we spin around farther from the sun, and the half light of twilight fades into the darkness of the shadowy night.