The sky — astronomy

Polaris: Why the North Star Stays Put

Polaris, the North Star, appears motionless because it lies almost exactly above Earth’s north pole, the point the whole sky seems to turn around. It is not the brightest star, only a moderately bright one. You can find it using the Big Dipper or Cassiopeia, and it points the way north.

Close to the north celestial pole

Take a long-exposure photo of the night sky and the stars trace arcs. They all circle around one spot, and Polaris (Alpha Ursae Minoris) sits very near the centre of it.

The reason is Earth’s rotation. Our planet spins around an imaginary axis, and that axis points to a particular place in the sky. Extended northward, it meets the celestial sphere at the north celestial pole, the imaginary point around which the sky appears to turn. The stars are not circling us. It only looks that way because Earth completes one turn relative to the stars in about 23 hours and 56 minutes.

Polaris lies less than one degree from that pole. For comparison, the full Moon is about half a degree wide. So the star traces a tiny circle and, to the eye, seems to stand still while the rest of the northern sky wheels around it.

Polaris is not exactly at the pole. It is currently about three quarters of a degree away, and calculations show it will come closest around the year 2100 before slowly drifting away again.

Not the brightest star

Many people assume the North Star must be the brightest star in the sky. It is not. Polaris has an apparent magnitude of about 2 (a brightness scale where smaller numbers mean brighter), which makes it moderately bright. It is easy to see from a town, but it does not stand out dramatically. The brightest star in the night sky is Sirius, and many other stars outshine Polaris.

Polaris belongs to the constellation Ursa Minor, the Little Bear, marking the end of the handle of the Little Dipper. It lies a few hundred light-years away, commonly estimated at around 430. Astronomers know it is a Cepheid variable (a type of star whose brightness pulsates slightly) and part of a multiple-star system. Its changes in brightness are small and not noticeable to the naked eye.

How to find it

Once you know where to look, Polaris is easy to locate. There are two reliable routes.

Using the Big Dipper. Find the Big Dipper and the two stars at the end of its bowl, Dubhe and Merak, the ones farthest from the handle. Draw an imaginary line through them and extend it about five times the distance between the two stars. You will arrive at Polaris, which stands fairly alone in that part of the sky.

Using Cassiopeia. When the Big Dipper is low near the horizon, look for Cassiopeia, the W-shaped group of stars on the opposite side of the pole. An imaginary line from the middle of the W, pointing outward, leads toward Polaris.

When you face Polaris, you are facing north. East is on your right, west on your left, and south behind you.

The height of Polaris above the horizon is roughly equal to your latitude. At 40° N it is about 40° up. At the North Pole it is almost overhead, and at the equator it sits close to the horizon. From the Southern Hemisphere it is practically never visible, except from places very close to the equator.

A practical tip: step away from street lights and give your eyes a few minutes to adjust to the dark. More ideas can be found in how to observe the sky.

Pole stars change because of precession

Polaris is not the permanent North Star. Earth’s axis does not point to the same spot forever. It slowly traces a large circle, like a wobbling spinning top, an effect called precession. One full cycle takes about 26,000 years.

As a result, the north celestial pole drifts slowly among the stars. About 5,000 years ago the nearest bright star to it was Thuban in the constellation Draco, a time associated with the building of the Egyptian pyramids. In the distant future other stars will take over, and in roughly 12,000 years the pole will lie near brilliant Vega. Polaris is the pole star for our era, but it is part of a long cycle rather than an eternal fact.

The south celestial pole has no bright star like Polaris. Southern observers use other guides, such as the Southern Cross.

In short

• Polaris seems still because it lies almost on Earth’s rotation axis.
• It is moderately bright, not the brightest.
• You can find it with the Big Dipper or Cassiopeia.
• Its role as North Star is temporary because of precession.

Next time you look up, remember that one quiet point above the northern horizon anchors the whole turning night sky.

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