Brian Files, age 9, of Derby, Kansas, for his question:

Will the,Big Dipper ever change its shape?

We depend on the Big Dipper to point to Polaris, the North Star, which guides our directions here on Earth. Several of its bright stars also point to key constellations and guide the eye around the sky. It would be very annoying if we could not count on the Big Dipper to keep up this good work. Nevertheless, it is gradually changing its shape. It has been changing slowly since the earth began, and will keep on changing as long as the earth endures.

Those seven bright stars in the Big Dipper really are not related to each other. There are vast oceans of space between them, and all of them are very many light years away from us. What's more, each one is traveling its own way at its own speed. We see them arranged as the Big Dipper shape only because that is how they appear to be grouped    when seen from our small, faraway planet.

Though the seven big stars are moving at high speeds in different directions, the picture does not change much in a lifetime, or even in 100 lifetimes. This is because outer space is so enormous that the distances Up There tend to fool us. From here, the spaces between the stars look very small. Actually it may take a beam of light many centuries to travel from one to the next. And light always zips along at 186,000 miles per second.

From the Earth, the changes in the Big Dipper appear very slow and very slight. But we would notice a big difference if we returned in the year 100,000 A.D. Then the old handle will be the bowl, the old bowl will be the handle    and the whole thing will be standing on its head. You can figure out how this remodeling is done when you know the directions in which the stars are moving.

The easy way is to draw the Big Dipper, placing its seven major stars where we now see them. Get set to add a small arrow to show which direction each star is going. Count the four stars in the handle and ignore the one at the tip. The first three are marching away from the bowl, one behind the other. The two stars at the bottom of the bowl are moving in more or less the same direction. The last star in the handle and the one on the far side of the bowl are moving in opposite directions from the other five.

In 100,000 years or so, the star at the end of the handle will swing around and reshape the handle into an upside down bowl. The stars in the present bowl will swerve around to add a new handle.

Meantime another startling surprise is occurring in this part of the sky. Polaris is losing its place as the Pole Star. During the next 26,000 years, several other stars will take turns in this role. However, this has nothing to do with the real motions of the stars. It happens because the Earth wobbles like a top running doom. Its North Pole swings slowly around, pointing to first one and then another Pole Star in the sky.