Does the earth wobble?

The sun, the moon and the stars seem to march over our skies in an orderly parade. The early stargazers clocked this heavenly parade and used it to figure the passing of time to make a calendar and a clock. Through the ages we have learned a lot more about the stars, but we still use the parade over the heavens as a time keeper.

Our basic time keeper, of course, is the earth which spins around on its axis once every day and around its orbit once every year. This gives us first one view and then another of the star studded vastness of outer space. As the earth rotates, the heavenly bodies come into sight above the eastern horizon, march overhead, and set out of sight in the west. As it revolves around its orbit, the constellations of fixed stars return with the seasons.

But modern instruments show that our world is not perfectly accurate as a time keeper. In the spring, its rotation is a tiny bit slower. Through the year it runs a small fraction faster then slower than a man made clock. Its rotation is gradually speeding up and in 2,000 years it gains about three and a half hour. It also gains, then loses, about 30 seconds during a 200 year period.

We ^an, however, consider the overall motions of the earth without being bothered by these small lapses. The big globe rotates in four minutes short of 24 hours. it revolves the 600 million miles around its orbit at an average speed of 18 1/2 miles a second. It speeds with the Solar System through the Galaxy at about 170 miles a second.

With all this activity to keep in mind, we can forgive the old earth a few lapses. One variation, however, is more serious   serious enough to change the position of Polaris it the sky and upset one of our neat gimmicks for finding our way home.

The earth, it seems, wobbles like a spinning top when the top begins to slow down.  The northern end of the earths axis, the north pole, swings slowly around in a circle. It points out, of course, at the sky directly above it and during this slow wobble it describes a circle in the sky. This motion is called precession and each circular wobble takes about 26,000 years. Right now, and for many centuries to come, the earth's north pole points to the place in the sky occupied by the bright star Polaris. But 12,000 years from now, it will point to Vega in the constellation Lyra and this star will be the Pole Star.

Through the long centuries of precession, the role of the north pole star is played by several different stars. Some 3,000 years ago, the north star was Thuban, a bright eye in the constellation Draco, the Dragon. During the next 5,000 years, the role of the pole star will be played by several of the bright twinklers along the western side of the constellation Cepheus.