For thousands of years Polaris, the cold bright star at the end of the Little Dipper’s handle, has sat at a point in the sky just about due north. You can see it every cloud-free night.

If you were to set up a lawn chair facing north and watch above the pale poplar tops from dusk to dawn, you’d see that all the stars circle counterclockwise around Polaris. About 8 p.m. in October, the Little Dipper is upside down. By 5 the next morning, the dipper has circled beneath Polaris and turned nearly right side up. If you fast-forward it in your imagination, it looks like a giant wheel turning once a day.

If you were to keep your lawn chair in the same place all winter and into spring, you’d see that at 8 o’clock on a late-March evening the Little Dipper’s position was just about opposite what it was in apple-picking season. On March mornings, it’s upside down.

It’s as if two different wheels are grinding against each other up there. One turns once a day, and the other turns the whole thing slowly backwards once a year.

The daily wheel results from the Earth spinning. The yearly wheel results from the Earth moving around the sun. As if these two perfectly rhythmic wheels weren’t enough, a third wheel is also turning up there. It’s too slow to notice in one lifetime or several, or even in a millennium. Astronomers call it precession.

The Earth (we observed here a few weeks ago) is tilted to the sun, causing the seasons to change. But not only is it tilted, it’s also slowly wobbling along the axis of the tilt, like a gyroscope slightly off center. Pulled by the moon and sun – and by the planets – the Earth makes a slow twist every 25,800 years.

This wobble points the north pole toward different stars. Polaris will circle away from north until in about 12,000 years Vega will be the north star. More than 5,000 years ago the star Thuban, in the nearby constellation Draco, the dragon, was at roughly the point Polaris occupies now. In about 20,000 years, Thuban again will be the north star. And 25,800 years from now, Polaris will have returned to the same place it is tonight. People, poplars and apples will repeat their cycles again and again, and the stars will circle the same.

It all grows sort of scary because of the size of it in space, and time. We measure time by what happens in space. One day is the time from one sunrise to the next. One month is the time the moon takes to complete its phases from full to new; a week is measured from the roughly seven days of each phase. A year is the 3651/4 days it takes us to circle the sun once.

That 25,800-year period has been called the “Great Year.” There’s evidence that ancient astronomers, long before the Greeks, somehow knew this circuit existed – though they probably didn’t know the Earth wobbles.

They fast-forwarded the motions of the sky in their imaginations, and saw giant cosmic wheels grinding against each other in tandem. The planets were turning too. Time, they noticed, is like a convergence of colossal millstones, making flour out of us all.