Mary Lou Hebda, age 12, of Everson, Pennsylvania, for her question:

Why does light linger after the sun sets?

In temperate zones we expect a longish period of gentle twilight after the glaring summer sun sets below the horizon. For a while the world is neither light nor dark and everything glimmers with a glamorous radiance. This twilight comes indirectly from the sun and is relayed down to us by the filmy atmosphere that reaches hundreds of miles above our heads. The upper air is still bright with sun¬beams. The reflection off tiny particles in the atmosphere sheds some of the sun's brightness downward to bathe the earth in twilight.

If you stand in a valley, you can watch the sun set behind a western hill. But if you climb to the top of the hill, you can see more of the western sky and watch the sun sink down this extra piece of sky. It sets a little later than it did in the valley. The higher you go, the farther you can see around the western horizon and the longer you can see the sun. Miles above the earth, the visible sun shines through the atmosphere long after people on the earth have seen it dip below the horizon. This delayed sunset aloft causes the period of twilight below.