AMATEUR NATURALIST

The once and future pole star

Posted Jan. 01, 2012, at 7:42 p.m.
Thuban
Courtesy of Space Telescope Science Institute/Digitized Sky Survey
Thuban

The great weight of winter is bearing down on us again. For younger readers this will be little more than an old guy’s grouchy personal mythology stinging their ears. But the more years you’ve lived through the apparently interminable stretch from December to March, the more the cold seems to peer with dragon eyes over the northern horizon. I mean, for months it will be too much trouble to clamber over snowdrifts to get to the Shed and its creaking floorboards only to forget what brought me there in the first place.

In a way the iciness to come lives in the idea of “north.” It’s where winter comes from, and certain stars reflect it all year round, especially Polaris, which marks almost exactly true north. Even in summer it looks bluish-white and icy itself up there. Who knows which came first, the cold white star or the cold white apprehension of inevitable winter.

Anyway, it doesn’t matter because a lot of northern stars will do for the idea of cold. Ursa Major — the Great Bear, or the Big Dipper — wheels around the pole all year, never setting, in tandem with Ursa Minor, or the Little Dipper, where Polaris is the tip of the handle. They shine brightest on cold nights, and have done so for millennia and longer. Our word “arctic” derives from the ancient Greek word arktos, meaning bear and associated, at some forgotten time in antiquity, with Polaris, the central axis around which the whole sky turns.

The central axis for now, I mean. Similar to the way the seasons eternally turn, the stars also turn, not just nightly from east to west as the Earth rotates, and yearly, as the Earth revolves around the sun, but also in a much longer cycle where the northern (and southern) polar stars travel a great circle. Polaris, in other words, was not always the pole star. About 2,500 years ago, Kochab, one of the bright stars in the Little Dipper’s bowl, was nearer true north than Polaris.

“True north,” in my parlance, is at the top of an imaginary line or axis running through the Earth perpendicular to the sun. The Earth’s spin, however, is not exactly along that axis. It’s tilted about 23.5 degrees. In other words, the North Pole does not point at true north. It’s 23.5 degrees off the perpendicular to the sun. It’s this tilt that gives rise to the seasons, as our hemisphere tips into the sun’s warming rays in summer, and away from them in winter.

And not only is the Earth tilted, but it’s slowly wobbling in its spin, like a gyroscope knocked off its up-and-down balance. The wobble is called precession. One full wobble takes a bit less than 26,000 years. This means the northern stars appear, over these thousands of years, to make a great circle. So every few thousand years a different star marks true north.

In our time, Polaris is within a degree or so of north. But around 500 B.C. (2,500 years ago), Kochab was nearer north. Around 3000 B.C. (5,000 years ago) Thuban, in the constellation Draco, the Dragon, was the pole star. About a thousand years from now, the star Alrai in the constellation Cepheus will mark true north. In 14000 A.D., Vega will be within about 5 degrees of north. In 27800 A.D., after one full circuit of the wobble, Polaris will return to be the North Star.

This has of course been going on for countless millions of years, the result of torque from the sun’s and moon’s gravitation on Earth’s bulging equator. How long ago humans first noticed the great slow star-circle, forgot it, then relearned it, no one knows. The entryway of the Egyptian Great Pyramid, which seems to have been built around 2800 B.C., aims exactly north, which in those days would have been marked by Thuban. This is not evidence the Egyptians knew about the great year, necessarily. But records or handed-down memories from that time indicating the positions of stars would, after a few hundred years, reveal the stars were changing position in slow circular motion. Plato in about 370 B.C. referred to the great year, and some scholars think mythological stories contain coded information about the stars and the great year and more. Stories that could be 50,000 years old, some of which are still remembered today but whose secret codes are forgotten. It may have been foreseen that Thuban ages hence would return to north.

Thuban is a strange star, to my mind. It’s designated as the alpha star of Draco even though it’s not the brightest in that constellation. Its name is Arabic for serpent, which makes sense, but it came by it through repeated medieval copying mistakes from the Arabic phrase “r’as al-tinnin,” the serpent’s head. It’s classified as a white giant, a type of star rarely seen. It’s cold-looking. Distant-seeming even beyond its relative faintness near the bowl of the Little Dipper, like an echo of something, a cycle of time so slow it was forgotten. It’s lean and hungry, and projects a boreal chill even in summer.

Or am I projecting my own chill? I don’t know. I can’t even remember how I got here. I only know it’s starting to be cold again, the trees and branches will be cracking and moonlight soon will refract through icicles and frost, while Thuban and Polaris watch.

Dana Wilde’s collection of Amateur Naturalist and other writing, “The Other End of the Driveway,” is available electronically and in paperback from Booklocker.com.

SEE COMMENTS →

ADVERTISEMENT | Grow your business
ADVERTISEMENT | Grow your business