Sky Watch by J. Alex Knoll

As the World Turns

In cosmic terms, even north changes

Over the course of the evening, the moon, planets and stars rotate counter-clockwise through the heavens. All, that is, but Polaris, the North Star, which appears to stand still. Even its fellow stars amid the constellation Ursa Minor, or the Little Dipper, pivot around Polaris in a tight circle. Season after season, Polaris stands its ground while the rest of the sky changes.

While Polaris is now less than one degree from true north, celestial north itself changes over millennia as the direction of Earth’s axis changes. Like the changing seasons, this phenomenon is due to our planet’s 231⁄2-degree tilt.

Imagine earth spinning like a wobbling top and its handle representing north. Rather than a tight, fixed spin, the handle, earth’s north pole, traces an imaginary circle. In this case, that circle completes one cycle every 26,000 years, with celestial north changing along the way.

Five thousand years ago, Earth’s axis pointed to the star Thuban of the constellation Draco. At fourth magnitude, Thuban is not even the brightest star in its constellation. Even so, there’s no denying its importance to our ancient forebears. Within the burial chamber of the Great Pyramid of Khufu, the largest of the Giza pyramids, are two shafts leading outward. Astronomers have calculated that during the reign of Khufu around 2550 B.C., one of the shafts pointed directly to Thuban.

Five thousand years from now, Alderamin, the brightest star in Cepheus, will mark celestial north. Should our descendants still look to the heavens from earth 12,000 years from now, Vega, the lead star in Lyra, will be their north star. Then, another 9,000 years later, Thuban will again be the north star.

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