How to find the North Star (Polaris)

The North Star is one of the smaller pieces of practical knowledge worth carrying around. It costs nothing to learn, takes ten minutes the first time, and once you know how to find it you can find it again from anywhere in the Northern Hemisphere for the rest of your life. It also gives you the rough direction of north anywhere there’s a clear sky, which has been a useful thing for several thousand years.

The first thing to clear up is that Polaris is not particularly bright. This trips up almost every beginner. Looking up on a clear night, the brightest things you can see are usually a planet or two, and after that several stars considerably brighter than Polaris. Polaris is roughly magnitude 2, which puts it about fiftieth on the list of brightest stars. What makes Polaris useful is not how bright it is. It is that it sits, by happy coincidence of our planet’s rotation axis, almost exactly above the North Pole. That means it doesn’t move. Every other star wheels around the sky as the night goes on, but Polaris sits where it is, all night, every night.

The standard method

You need the Big Dipper, also known as the Plough, the Great Bear, Karlavagn, the Saucepan, or various other names depending on what part of the world your stargazing tradition comes from. It is seven stars arranged in a recognizable shape: a rectangle of four stars, with a curved handle of three more attached to one side.

Find the two stars on the front edge of the rectangle, opposite the handle. They are called Merak and Dubhe, in case you want their names, though it’s not necessary to remember them. Imagine a straight line that goes from Merak, through Dubhe, and continues out into the sky away from the rest of the Dipper. Now imagine extending that line about five times the distance between Merak and Dubhe.

You will land on Polaris.

That’s it. Once you’ve done this once, the geometry sticks, and after a few nights you’ll be picking out Polaris in under a second.

When the Dipper isn’t visible

In summer in much of Europe, the Big Dipper sinks low enough that trees and houses can block it. The backup is Cassiopeia, a constellation shaped like a W (or M, depending on the season, since it rotates around Polaris like everything else does). Cassiopeia sits on the opposite side of Polaris from the Big Dipper, which means whatever one of the two is blocked, the other is probably still up.

Polaris sits roughly halfway between the Big Dipper and Cassiopeia. If you’ve found one of them, you’ve narrowed down where to look for Polaris quite a lot.

A trick that used to matter a great deal

The altitude of Polaris above your horizon equals your latitude in degrees.

Stand at the North Pole and Polaris is straight overhead, ninety degrees up. Stand in Prague at fifty degrees north and Polaris is fifty degrees up, halfway between horizon and zenith. Stand at the equator and Polaris is right on the horizon. Stand in Sydney and you can’t see Polaris at all because it has dropped below the horizon — there’s no equivalent useful star above the south pole, which is one reason early navigation in the Southern Hemisphere was harder.

This relationship is the foundation of most pre-GPS celestial navigation. A sailor with a sextant or even just a quadrant could measure the angle of Polaris above the horizon, and they had their latitude to within a degree or two, which on a long voyage made the difference between landfall and disaster. Christopher Columbus could find latitude this way; longitude is what stumped him.

What it actually is

Polaris is around 433 light-years away, which is not particularly close as stars go. It’s a yellow supergiant, considerably bigger and brighter than the Sun, but distant enough that it looks dimmer than nearby stars that are intrinsically much less luminous. It is also part of a triple star system, though the companions are too faint and too close to see without a telescope.

The last useful thing to know is that Polaris won’t be the North Star forever. The Earth wobbles slowly on its axis, taking about 26,000 years to complete one wobble, and as it wobbles the celestial pole drifts among different stars. In 12,000 years the pole will be near Vega, the brightest star in the summer triangle. In ancient Egypt, when the pyramids were built, the pole star was Thuban in Draco. Right now, by happy timing, we live in an era when the closest reasonable star to the celestial pole is the easily-findable, moderately-bright Polaris. It’s a coincidence worth using while it lasts.