Mary O'Reefe, age 12, of St. Paul, Minnesota, for her question:

Is it true that Thuban was once the North Star?

Oh, yes, the master builders of Egypt's great pyramids depended upon Thuban to point to the north direction: They needed to know the points of the compass in order to orient their stupendous buildings: This was more than 5000 years ago: At that tine, Polaris our North Star, was in another part of the sky and Thuban  hung over the earth's North Pole. Today, Thuban is a modest little star in the tail Draw the Dragon. It is one of several stars between the Big and Little Dippers.

One of the earth's strangest motions is called precession. Its north polar axis spins around somewhat like a top that is slowing down. Instead of pointing to one spot in the sky, it points around a circle. In this era, it happens to be pointing to Polaris but this will change. Each circle of precession takes about 26,000 years and during this period the North Pole swings around to point to several different stars. About 12,000 years in the future, the role of the North Star will be played by Vega. Later Thuban will play the role again. About 20,000 years in the future, our Polaris can expect to become the North Star again.