The pyramids at Gizah in Egypt stand against the ancient sky when Thuban in Draco was the polestar. Our era’s polestar, Polaris, is the yellow star at left. Photo: Ricardo Liberato; background from Chris Marriott’s SkyMap.com.
Back in the good old days, Thuban (THU-ben) in the constellation Draco the Dragon was the polestar. It occupied the spot where Polaris the North Star sits now. Ah, but that was almost 5000 years ago. Much has changed since then.
Keeping track of the stars is bad enough. Earth’s rotation and revolution around the sun are infamous for sowing confusion among beginning skywatchers about what’s up when. Over a longer span of time we also have to contend with precession.
While our axis maintains its tip of 23 1/2 degrees, over a period of 25,800 years it changes the direction of its rotation like the gyroscope does in the animation. Since the "north star" is the star that Earth’s north polar axis happens to point to in space, that means we see a series of different north stars during a full cycle. Think of the Earth’s axis as describing a slow circle in the sky like a children’s top running down. Whatever star lies on that circle will eventually be a pole star.
Right now, Polaris in the Little Dipper holds that title but not forever. The role will slide to the star Gamma in Cepheus the King around the year 4000 A.D. and finally return to Thuban in 23,000 A.D. That’s so much time to think about it hurts my head.
This map shows the sky around 9 o’clock in late March as you face north. Thuban is easy to find between the two Dippers. Created with Stellarium.
Thuban, while not bright, is very easy to find. Just look midway between the two end stars of the Little Dipper and the star Mizar, in the bend of the Big Dipper’s Handle. In the days of the Great Pyramids, all the other stars in the northern sky circled around Thuban, hub star of the north, just like they do around Polaris in our era. Since Thuban’s is on the dim side, you wonder if anyone really took notice of it. I mean, it’s not like our North Star, which is as bright as one of the Dipper stars.
This is the circle described in the sky by Earth’s wobbling axis. Thuban was the polestar around 3000 B.C. and closest to the pole in the year 2787 B.C. Polaris has replaced it in our era. The minus numbers refer to B.C. years, the plus to A.D. Credit: Tao’lunga
If you’ve studied pyramids at all, you’ll come across frequent references to the astronomical orientation of Khufu’s Great Pyramid at Ghizah, constructed around 2560 B.C. There’s a shaft built into one face that some say is aligned with the Thuban. Others think it has more to do with building construction than astronomy but no one knows for sure. One thing’s for certain: the shaft does point north, nearly in the star’s direction. Perhaps it was the escape route for the pharoah’s soul. Whatever the truth is, who can deny the poetry of a star shining all night down a stoney hole into the heart of an ancient tomb?
Precession (left) is caused by the gravitational attraction of the sun and moon on the Earth’s slightly bulgy equator, which makes our axis wobble in rhythm. Didn’t know Earth was a little heavy around the middle? Photos may imply otherwise, but the Earth’s not a perfect sphere. Our planet’s rotation makes it bulge slightly at the equator, increasing the diameter there by 27 miles compared to the poles. Good thing my equatorial bulge is considerably smaller. It keeps me out of trouble.
Two views of the northern sky on a March evening separated by nearly 5000 years. Illustration: Bob King with Chris Marriott’s SkyMap.