Janet Hartzler, age 11, of Rochester, NY.. For her question:

How can they tell time by the stars?

It took our stargazing ancestors ages to chart the motions of the wheeling heavens. Countless details were fitted together to get a moving picture of the changing skies. Some stars rise and set and some swing in endless circles above the poles. The starry picture changes every minute, every hour through the night, every week and eveyv month through the seasons. And the orderly parade repeats itself with each yearly orbit of the earth around the sun.

The stars provide a reliable clock because they move on schedules. When a pilot has a chart of their regular motions, he can figure the time at any point on earth fro ,. Their positions in the sky. The height of a certain star above the horizon can tell him the exact time. The stars also can tell him his exact position on the surface of the earth.