Ursa Minor

Ursa Minor, the Little Bear, is a fainter cousin of the famous Great Bear (Ursa Major). What it lacks in brightness it more than makes up for in importance, because at the tip of its tail sits Polaris, the North Star. Polaris is the one star in the sky that does not seem to move as the Earth turns, so it has been used by travellers, sailors and pilots for thousands of years to find direction at night.

  • Best seen fromNorthern hemisphereAlways visible north of the equator
  • Famous starPolaris (the North Star)Sits almost directly above the North Pole
  • Distance to Polarisapprox. 433 light yearsLooks bright because it is a yellow supergiant
  • Size rank56th of 88A small, faint constellation
  • Famous asterismLittle DipperSeven stars shaped like a smaller ladle
  • Guardians of the poleKochab and PherkadThe two brightest stars after Polaris

Polaris: the North Star

The most important star in Ursa Minor is Polaris, also called the North Star or the Pole Star. Polaris sits almost directly above the Earth's North Pole. That is why, as the Earth turns on its axis, every other star in the sky appears to wheel around Polaris, while Polaris itself stays put. From the Northern Hemisphere, if you can see Polaris, you know where north is.

Polaris is not actually one star: it is a triple-star system. The main star is a yellow supergiant about 433 light years from Earth, and it shines around 1,500 times brighter than the Sun. Two smaller companion stars orbit it. Polaris is also a variable star, getting slightly brighter and dimmer over about 4 days, but the change is too small to see without a telescope.

How to find Polaris

Polaris is not the brightest star in the sky (despite what many people think). It is only the 48th brightest. The trick to finding it is to use the famous Big Dipper in Ursa Major as a sign-post.

  • Find the Big Dipper (the saucepan-shape of seven bright stars).
  • Identify the two stars at the outer edge of the "bowl": Merak and Dubhe.
  • Draw an imaginary straight line from Merak through Dubhe.
  • Keep going in that direction for about 5 times the distance between those two stars.
  • The first bright star you arrive at is Polaris.

Polaris also marks the end of the handle of the Little Dipper, the asterism that gives Ursa Minor its other name. The Little Dipper looks like a smaller, fainter and flipped version of the Big Dipper.

How sailors used the Little Bear

For at least 3,000 years, sailors in the northern hemisphere have used Polaris to find their way. Because Polaris always points north, you only need to find it in the sky to know which direction you are facing. Even better, the height of Polaris above the horizon tells you your latitude: how far north of the equator you are. Stand at the North Pole and Polaris sits directly overhead. Stand at the equator and Polaris sits right on the horizon.

Sailors used this trick long before they had compasses. The Greek geographer Pytheas of Massalia, who sailed to Britain around 320 BC, used Polaris's height to estimate how far north he had travelled. Polynesian wayfinders crossing the Pacific used it too. So did the Vikings as they reached Iceland and Greenland in the 800s. It is one of the oldest navigation tools in the world.

Fact Polaris has not always been the North Star and it will not be forever. The Earth's axis wobbles slowly in a circle that takes about 26,000 years to complete, a motion called precession. As the axis wobbles, it points at different stars at different times. About 14,000 years ago the North Star was Vega (the brightest star in Lyra). In about 12,000 years from now Vega will be the pole star again.

The guardians of the pole

Two of the brightest stars in Ursa Minor sit at the front edge of the Little Dipper's bowl, opposite Polaris at the end of the handle. They are called Kochab and Pherkad, and together they have a poetic old nickname: the Guardians of the Pole, because they appear to march around Polaris all night long without ever fading from view.

Kochab is an orange giant about 130 light years away. Interestingly, Kochab was actually the North Star itself for a few hundred years between roughly 2000 BC and 500 AD, before precession nudged the Earth's axis onto Polaris.

Did you know? Because Polaris sits almost exactly above the North Pole, it never rises or sets when seen from the Northern Hemisphere. It just sits there. People in the Southern Hemisphere, on the other hand, never see Polaris at all: the Earth itself is in the way. There is no equally convenient "South Star" near the South Pole, which made navigating the southern oceans much harder for early sailors.
Deeper dive: precession and why the pole star changes

The Earth spins on its axis once every day, like a top. But like any top, the axis itself slowly traces out a wider circle, a motion called precession. The cause is the gravitational pull of the Sun and the Moon on the Earth's equatorial bulge, gently trying to tip Earth's axis upright. Earth resists, but rotates in response, just as a spinning top does. One full precession takes about 26,000 years.

As the axis traces this slow circle, it sweeps past different stars. Right now it points at Polaris, but it will not stay there. By around 3000 AD, the axis will have moved to point at Errai (Gamma Cephei), then on through Cepheus and Cygnus over the next 10,000 years. In about 14,000 AD, the bright star Vega will be the new North Star. After that, the axis keeps drifting until it eventually comes back to Polaris around 27,800 AD.

The same effect happens in the Southern Hemisphere: there is no bright South Star today, but in about 7,500 years a star called Delta Velorum will sit fairly close to the south celestial pole and may serve as a future southern pole star. Precession is also the reason the dates for the astrological "sun signs" no longer match the actual constellations in front of the Sun: the constellations have stayed put, but the position of the equinoxes in the sky has gradually drifted by almost one whole zodiac sign in 2,000 years.

Once you can find Polaris, you can find your way around the rest of the sky. See Ursa Major for the Big Dipper, or Cassiopeia on the other side of Polaris.