July 16, 1997

North Star changes over time

EDITORS: Click here to download a 200 dpi graphic showing a map of the North Polar sky as it appears over Michigan in mid-July.

ANN ARBOR—Polaris, or the North Star, is perhaps the most famous star in the northern sky, even though it is only 49th in brightness among them all.

"We think of Polaris as marking the exact position of the North Pole—the place among the stars toward which the Earth's axis of rotation points—and that it would be exactly overhead to someone standing at Earth's North Pole," said Richard Teske, University of Michigan professor emeritus of astronomy. "But that's not true. Polaris is actually a small distance away from the North Pole. Moreover, the separation between them is changing from one year to the next."

The star's current distance from the exact North Pole of the sky is the width of one-and-one-half full moons, according to Teske, who added that their separation is slowly diminishing as the position of the pole moves among the stars. The pole and Polaris will be closest in the year 2102 when the width of just one full moon will fit between them. After that, their separation will begin to steadily widen.

The cause is a gradual displacement of the orientation of Earth's axis of rotation, Teske explained. "Like the axis of a spinning gyroscope, the axis of the rotating Earth would tend to point always in the same direction in space, if our planet were left to itself," he said. "But the gravitational pull of both the moon and sun on Earth's equatorial bulge disturbs the alignment of the axis, causing a gradual shift in the place on the sky toward which it points. As a result, the pole of the Earth's rotation smoothly traces a small circle on the sky once every 25,800 years. It is convenient for us that the pole is now passing fairly close to a bright star, Polaris, although that has not always been so."

The circle traced among the stars by the changing alignment of Earth's rotational axis can be visualized by observing July's late evening sky. "Let's take a tour of the polar sky in mid-summer and try to see where the pole was thousands of years ago and where it is going to be thousands of years from now," Teske said.

To join the tour, face North at around 11 p.m. while looking up at the North Star. The brilliant star Vega is almost directly overhead. Deneb in Cygnus, the Swan, lies to the east of Vega. The Big Dipper is halfway down the Northwestern sky to the left, its handle pointing upward. At 11 p.m. in mid-July over Michigan, the exact North Pole is located above Polaris one moon's width and to its left by the same distance. The pole slowly sweeps to the observer's right, almost directly toward the star gamma Cephei lying at the tip of King Cepheus's triangular crown. Cepheus's square face lies on its side at this hour, the crown in danger of falling off.

Look to the left of Polaris and slightly upward to see the star Thuban. It is located halfway between the bend of the Big Dipper's handle and the bowl of the Little Dipper. "Thuban was the North Star 4,800 years ago at the time of the 'Old Kingdom' in Egypt—a period when most of the pyramids were built," Teske said. "The exact pole has crept from Thuban to its present position near Polaris and now is headed toward gamma Cephei, which will be the North Star in about 2,000 years.

"It's important to remember that the location of the North Pole on Earth's surface is not changing, nor is the South Pole," Teske added. "But the stars and constellations that hang directly above the Earth's poles shift slowly over the years."

After the North Pole in the sky passes gamma Cephei, it will swing on through Cepheus toward Cygnus, the Swan, where the brightest star Deneb will become the Pole Star, although a fairly distant one, 8,000 years from now. By A.D. 17,000, the pole's circular path on the sky will bring it into the northern parts of the constellations Lyra and Hercules, which are located overhead for Michigan observers at 11 p.m. in mid-July. And by A.D. 23,000, Thuban in the constellation Draco, the Dragon, will once again claim the title of North Star.

Contact: Sally Pobojewski
Phone: (313) 647-1844
E-mail: pobo@umich.edu