In “How We Found North in a Power Cut”, Ms Kulkarni pointed out Polaris and called it “the only star in the sky that is not going anywhere tonight”. It was a good line, and it worked: an hour later, every other star had moved and Polaris hadn’t. But notice the last word. Tonight. Over a human lifetime, Polaris is a reliable marker of north. Over a few thousand years, it isn’t, because the Earth’s axis, the line the whole sky seems to turn around, is slowly wandering.
The reason is that the Earth is not a perfect sphere. Because it spins, it bulges slightly at the equator: the distance through the Earth from one side of the equator to the other is about forty-three kilometres more than the distance from pole to pole. And the Earth’s axis is tilted, by about twenty-three and a half degrees, so that bulge is tilted too. The Sun and the Moon pull on it. They pull slightly harder on the near side of the bulge than the far side, and the effect is a twisting force that tries to straighten the Earth up, to pull its tilted bulge into line with the plane of their orbits.
If the Earth weren’t spinning, that might work. But a spinning object responds to a twist in a famously odd way. You can see it in a spinning top, or a lattu. Spin one so it leans a little, and gravity tries to pull it over. It doesn’t fall. Instead, the top of its axis swings slowly round in a circle, while the top keeps spinning. That slow circling is called precession. The faster the spin, the slower and steadier the circle. The Earth is doing exactly this, under the pull of the Sun and the Moon: still tilted at the same angle, but with its axis slowly swinging round, tracing out a great circle in the sky. One full circle takes about twenty-six thousand years.
As the axis swings, the point in the sky it aims at, the place everything appears to turn around, drifts along that circle. Right now, it happens to be passing very close to Polaris, less than a degree away. It’s still getting closer: around the year 2100, it will come within half a degree, and then begin to move away again. In a few thousand years, Polaris will be wheeling around the sky like any other star, and the north of the sky will have no bright star to mark it.
Go back in time and you can see the job changing hands. Around 2700 BCE, when the great pyramids of Giza were being built, the pole of the sky sat near a fairly faint star called Thuban, in the constellation of the dragon, Draco. Some passages inside the pyramids point towards the northern sky in a way that some historians link to Thuban, though nobody is certain what the builders intended. In about twelve thousand years, the axis will have swung round to the other side of its circle, near Vega, one of the brightest stars in the sky. Our descendants will have a Pole Star far more dazzling than ours. And twenty-six thousand years from now, it will be back near Polaris again.
Precession also explains Ms Kulkarni’s trick with her fists. The height of the pole above your horizon equals your latitude, and that will stay true forever, because it depends on the direction of the Earth’s axis and where you’re standing on the Earth, not on which star happens to sit at the pole. Sailors in the year 14,000 will still be able to find their latitude from the pole of the sky. They’ll just measure up to Vega instead.
People noticed this long before they could explain it. Around 130 BCE, the Greek astronomer Hipparchus compared the positions of stars he had carefully measured with positions recorded by astronomers about a century and a half earlier. They had all shifted in the same direction, by about the same small amount. He had discovered precession, without knowing anything about bulges, gravity or spinning tops. It took Isaac Newton, nearly two thousand years later, to explain it.
So the next time you find Polaris, give it a little more respect, not less. It isn’t a permanent fixture. It’s a star that happens to be passing through exactly the right spot during the short slice of history when we are here to use it. It is, in the most literal sense, only visiting.