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Turn, turn, turn: in addition to its daily spin and its annual trip around the Sun, the Earth wobbles—affecting the seasons, the "north star," and human history
Natural History, Dec, 2006 by Donald Goldsmith
Winter brings the year's longest nights--extra hours of darkness in which to watch the stars wheel their ways around our basic point of reference in the sky: a star named Polaris. Known today as the North Star (for its unique status as the star most closely aligned with the projection of the Earth's north pole on the sky), Polaris seems to stay steady no matter how long the dark night. The explorers who first sailed between continents clung to it for orientation in their travels. And yet, as surely as the nights will again grow shorter and the seasons will change, Polaris will lose its role as our north star. A slow cycle of the heavens will eventually bring Polaris back to its familiar role--but not for another 26,000 years, and not before other stars have taken their turn as celestial beacons for the stargazers of the distant future.
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Centuries of effort by the world's finest thinkers have led to a basic understanding of why the winter nights are cold and dark. Today most schoolchildren know that the Earth rotates on its axis once each day and revolves around the Sun once every year--discoveries that long ago shook the very core of human understanding and led to the abandonment of the idea that the Earth lay at the center of the cosmos. Those two rhythms rule our lives: the Earth's daily spin produces sunrise, sunset, and the alternation of night and day. Our planet's annual trip around the Sun takes us through the cycle of seasons, winter to spring to summer to fall.
And just how, exactly, does spring emerge from winter every year? Not, as many believe, because the Earth's elliptical path takes us closer to the Sun. Changes in the distance between Earth and Sun have only a modest effect on the seasonal cycle. Instead, seasonal variations arise because the Earth's axis of rotation, the imaginary line through the north and south poles, does not stand upright with respect to the plane of the Earth's orbit around the Sun. The rotation axis tilts by about 23.5 degrees from perpendicular. It also maintains a constant orientation in space--in other words, with respect to the stars--throughout the course of a year. So our annual motion alternately exposes each hemisphere, northern, then southern, then northern again, to more direct sunlight.
That direct sunlight, as the Sun rises higher and stays longer in the sky, causes summer in the hemisphere that tilts toward the Sun and winter in the hemisphere that tilts away. (The higher Sun and the longer days make roughly the same contribution to the seasonal differences, though of course the two effects are closely intertwined.) On two days of the year, the spring and fall equinoxes (which fall on or close to March 21 and September 22 each year), the Earth's rotation axis tilts neither toward nor away from the Sun. On those two days, day and night have equal lengths all over the world. In fact, if our planet's rotation axis were perpendicular to the plane of its orbit, rather than tilted, day and night would be equal throughout the year, and there would be no seasons to celebrate.
Lurking within the faithful cycles of day and night, winter and summer, is a third cyclical motion, which arises from the Earth's daily rotation and interacts with its annual revolution. That motion is called precession. It is, in essence, an almost imperceptibly slow wobble that creates a subtle and intriguing wrinkle in time. To visualize precession, imagine a top slowing down as it spins on the floor. Before it stops spinning entirely, it begins to wobble, its rotation axis rolling in various directions. As the axis changes direction, it sweeps out the shape of an upside-down cone, perpendicular to the floor; that motion is precession. There is, of course, one big difference between the top and the Earth: the precession of the top can take less than a second; the precession of the Earth takes almost 26,000 years.
Compared with twenty-six millennia, time scales measured in decades or even in centuries are so brief that for most practical purposes, the Earth's axis continues to point in the same direction. Today, north in any season can be determined by noting the position of Polaris. In the long run, however, the spatial orientation of the Earth's rotation axis does change. Every 26,000 years (more accurately, every 25,785 years), the two points on the sky directly above the Earth's North and South Poles--the extended ends of the axis of rotation--trace out complete circles on the background of stars. The radius of each circle is equal to the tilt of the rotation axis, 23.5 degrees. Thus the rotation axis changes only its orientation, while maintaining a constant angle to the Earth's orbital plane [see diagram on this page], as it sweeps out an inverted cone in space.
In spite of the, glacial rate of precession, you can't fully understand ancient history or archaeology without taking account of the fact that Polaris has not always pointed the way north. Four-and-a-half millennia ago, when the Egyptian pharaoh Khufu built the Great Pyramid, Polaris was nowhere near the "north celestial pole," the point that lies directly above the Earth's north pole at any particular time. In those days, astronomical observers relied on a much fainter star, Thuban, in the constellation Draco, the dragon, to serve as the north star; they may even have oriented the galleries of the pyramid on the basis of Thuban's position.
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