Carter Beattie, age 9, of Richmond, Virginia, for his question:

How can scientists tell when certain stars will be visible?

Everybody loves to watch a parade go by and the best of them all is the circus parade. The funny clowns frisk and frolic past you and then comes the merry calliope playing a jolly tune. Then comes a tiger in a cage, a dwarf and his friendly giant, and then a cage of lions. At last, the impossible elephants lumber by, almost close enough to touch. Suppose the clowns ahead make a U turn and lead the parade down the next street. You can run a block across town and watch it all pass by you again. The people and animals will follow each other in the same order as the first time.

The stars cross the night sky in an orderly parade. They are arranged in pat¬terns called constellations. Each constellation keeps its place in line with the same one ahead and the game one behind. The starry parade rises and sets each night. It moves at a set speed, exactly on time. But each night it rises a little later. It lags behind and causes different stars to appear at different seasons of the year.

This yearly parade has been going on for ages and experts have its time schedule down to the second. They know just which stars in the parade will be visible on any night of the year and exactly when they will rise and set. They know because the starry parade has been repeating its schedule for millions of years.