Last month, I encouraged you to venture outside just before sunrise to see the planet Mercury. By way of apology for such an outrageous suggestion about how to spend your winter pre-dawn hours, I’d like to offer something easy to watch this time. It’s a star that can be found at any time of night, all through the year — Polaris, more commonly known as the North Star.
As its common name suggests, if you find the North Star, you’ll have found which way is north. If you were to stay outside stargazing all night, you would see other stars on the move; they rise in the east, reach their zeniths (their highest points in the sky), then set in the west. But not the North Star: it remains fixed in one position. How is that possible? A clue to the answer lies in the star’s fancier name Polaris, which comes from the Latin stella polaris: the polar star.
Polaris is almost perfectly aligned with the north celestial pole. That is the point in the northern sky around which the Earth appears to rotate — the north axis of Earth’s rotation, extended visually into space. (There is also a south celestial pole that we can’t see from the northern hemisphere.)
From our point of view, stars appear to rotate around this axis. The further a star is from the pole, the greater its apparent motion from one horizon to the other. And the closer a star lies to the pole, the more its motion appears as rotation around that pole, rather than a straight up and down movement. If you were to watch for a few hours you would see the stars and constellations closest to the north celestial pole rotate around it in the sky. And at the center of the rotation — at the north celestial pole — there is a star that seems nearly motionless to our unaided eye. It is Polaris, just one degree from the pole.
Polaris has done nothing to earn its special position. Everything is in motion, including Earth’s own axis of rotation. Earth actually wobbles like a top as it rotates, a phenomenon called axial precession. Our axial precession takes 25,900 years to complete; over the course of that cycle the poles appear to trace out a small circle in the sky. This means that as the centuries pass, the celestial poles move across the backdrop of stars. Sometimes the poles point directly at a star for a time, but they don’t point to the same star forever.
When the Egyptians built the pyramids around 3000 BC, the north celestial pole pointed at the star Thuban in the constellation Draco. Back then, it would have claimed the title of North Star. But the pole continued its slow circle. To ancient astronomers, Thuban appeared to drift away from the pole, and eventually rise and set like any other ordinary star. For a long time, there was no pole star. It wasn’t until the Middle Ages that our axial precession brought a previously unremarkable star called Alpha Ursae Minoris into close alignment with the celestial pole, and the star’s motion appeared to cease. That star then became the North Star and, like any good monarch, assumed a ruling title: Polaris.
Polaris is term-limited. In about 300 years it will move away from the pole, and we will be without a North Star until 4200, when our axial precession will bring the star Gamma Cephei into alignment with the north celestial pole.
Ready to find Polaris for yourself? The time of year doesn’t matter. All you need is a clear night. Go outside and look north for the Big Dipper. It’s familiar enough to most people that it’s recognizable whether it’s right side up, or sideways, or upside down. (The Big Dipper is actually part of the constellation Ursa Major, the Big Bear, but that’s a topic for another column.)
Note the two stars at the end of the dipper you’d pour from if you were holding it. Now imagine a line connecting those two stars. Then, with your eye, follow that line “up” from the dipper (if the Big Dipper is upside down, the line “up” from the dipper will actually lead you down in the sky). Continue along that line about thirty degrees, or three fists at arm’s length, stacked on top of each other. You’ll arrive at a star there, and it’s Polaris.
Now besides knowing why the North Star appears to hold still, you also know how to find north without a compass. The North Star will be a good reference point, until about the year 2300. After that you’re on your own for a while. Clear skies!
