Jimmy R. Ross, age 12, of Markham, Ontario, Canada, for his question:

 Will you please explain how the Big Dipper changes its positions?

This polite question called for a "thank you" that takes only a moment. The answer spans at least a whole night. It should also extend to include a celestial routine of 26,000 years plus another change that extends more than 100,000 years into the future.
The Big Dipper shines bright in the starry skies, summer and winter. Any clear night of the year, the two pointer stars on the outside end of the bowl guide the eye to the dependable North Star. However, this dependable guidance service is not available all over the world. Polaris, the North Star, is not the prime lode star of the Southern Hemisphere. Above the North Pole, it hangs directly overhead. As we move south from this high point, ft inches lower and lower in the sky. Toronto stands about 43 1/2 degrees of latitude north of the equator. Summer and winter the North Star shines from a point 43 1/2 degrees above the northern horizon. These degrees of sky distance equal about 22 times the width of the full moon.
The position of Polaris as seen from any one spot on the earth does not change. But the stars of the Big Dipper do appear to change. The earth's rotation leaves Polaris stationed above its northern axis. But it causes all the ether stars to seem to move with each calendar day. Those nearer the equator rise and set. A number of far northern con¬stellations draw daily circles in the sky. The big and little Dipper are swinging partners in this heavenly hoedown. Every night, they make half a circle around Polaris. As they swing, the two pointer stars in the Big Dipper remain pointed toward Polaris at the end of Little Dipper's handle. If you watch all night, you can see these circumpolar constellations move around half a circle.
The earth's axis is tilted and its yearly orbit points your city toward different vistas of the heavens. On winter nights, the Big Dipper is on the right side of the Little Dipper. Its handle points down and the handle of the Little Dipper is tipped upward. As summer advances, this picture inches into the reverse position. The Big Dipper changes to the right side, with its handle pointing up, and the Little Dipper takes the left side, with its handle tipped down. It takes a whole year of star gazing to observe this seasonal change of positions.
Meantime, the pair of swinging constellations are taking part in a much larger dance. The north polar axis of the earth is swinging around in slow circles, somewhat like a spin¬ning top as it slows down. Each circle takes about 26,000 years and naturally it brings slow changes in the positions of the stars relative to the earth. During each precession, the north polar axis draws a circle in the sky. It points first to one and then another star on this circle. Polaris merely plays the role of the North Star for a few thousand years. In 12,000 years, the North Star will be Vega.
Still another celestial motion changes our Big Dipper. Stars move around the galaxy in faster or flower traffic lanes. This galactic motion gradually causes stars to change their positions in relation to each other. In relation to the earth, the stars of the Big Dipper are moving in different directions. In 100,000 years, this familiar constellation will be pulled into an unfamiliar shape.
Suppose you returned to Earth in the year 14,000 A.D. The central Pole Star would then be the star, Vega, and you might notice slight changes in the shape of the Big Dipper. The three nearly straight stars in the handle and the two in the bottom of the bowl are shifting in more or less the same direction. The star at the tip of the handle and the one at the opposite tip of the bowl are shifting in the opposite direction. A return visit in 100,000 A.D. would show that the present handle had become a bowl and the present bowl had become a handle.