A Field, a Chair, and a Patient Sky
Picture a clear evening in late autumn. You have carried a folding chair into a dark garden, wrapped yourself in a blanket, and tipped your head back. At first the stars look fixed, like frost on black glass. Give them an hour, though, and the picture changes. Stars that sat just above the eastern rooftops have climbed. Stars that hung low in the west have slipped behind the trees. The whole sky has moved, quietly, while you sat still.
Or so it seems. You did not sit still at all. The ground under your chair is turning, and it carries you along with it.
The Spinning Stage
The Earth spins once a day, turning from west to east. Think of a child on a roundabout watching the playground. As the roundabout turns, the swings, the benches and the parents all seem to slide past in the other direction. Nothing out there is moving. The child is. In the same way, as our planet rolls eastward, the Sun, the Moon and every star appear to rise in the east, arc across the sky and set in the west.
The turning is slow but steady. A full circle of 360 degrees in roughly a day works out to about 15 degrees an hour, which is enough for you to notice a star creeping past a chimney during a single cup of tea.
Now comes the elegant part. Every spinning ball has an axis, and the Earth's points north and south through the poles. Extend that axis out into space, far beyond the atmosphere, and it pierces the sky at two spots: the north celestial pole and the south celestial pole. These are the hubs of the great wheel. Everything else turns around them, but they stay put. Stars near a hub trace small, tight circles. Stars far from it swing in huge arcs that dip below the horizon, and those are the ones we see rise and set.
From a northern garden, some stars sit so close to the north celestial pole that their circles never touch the horizon. They wheel round all night, every night of the year, and never set. Astronomers call them circumpolar stars, from the Latin for "around the pole".
The Plough Points the Way
By remarkable luck, a star sits almost exactly on the northern hub. Its name is Polaris, the Pole Star or North Star, and it lies less than one degree from the celestial pole, which is close enough that it barely seems to move at all. Here is a surprise for beginners: Polaris is not especially bright. It is a moderately bright star, easy to see but easy to overlook. Its fame comes from where it sits, not from how it shines.
To find it, look for the Plough, the seven-star pattern that North Americans call the Big Dipper. Four stars make a box-shaped bowl and three make a curving handle. The two stars on the outer edge of the bowl, the side farthest from the handle, are called the Pointers. The lower one is Merak and the upper one is Dubhe. Draw an imaginary line from Merak up through Dubhe and keep going, about five times the gap between them. You will land on a lonely, steady star with no bright neighbours. That is Polaris.
Watch the Plough over a few hours and you will see something lovely. Like the hand of a giant clock, it swings round Polaris anticlockwise, completing a full circle in just under a day. Whether it hangs upside down above Polaris or stands on its handle beside it, the Pointers always aim at the Pole Star.
The Southern Hub Without a Star
Travel south of the equator and Polaris sinks below the northern horizon, out of sight. The southern sky turns around its own hub, the south celestial pole, but nature was less generous there. No bright star marks it. The nearest naked-eye star, Sigma Octantis, is so faint that you need a truly dark sky to glimpse it at all.
Southern observers use a clever trick instead. First they find two brilliant stars in the constellation Centaurus, Alpha Centauri and Beta Centauri, which point toward the small, kite-shaped Southern Cross. Then they take the cross's long axis, running from Gacrux at its top down through Acrux at its foot, and extend that line about four and a half times the cross's length. There, in a patch of almost empty sky, lies the south celestial pole. Drop a line straight down to the horizon from that point and you are facing due south.
Your Latitude Written in the Stars
Here is the payoff, and it has guided sailors for centuries. The height of the celestial pole above your horizon equals your latitude. Stand at the North Pole and Polaris shines directly overhead, 90 degrees up. Stand on the equator and it rests on the northern horizon, at zero degrees. Anywhere in between, it sits at an angle that matches how far north you are.
You can measure that angle with your own hand. Hold your arm straight out and make a fist. It covers roughly 10 degrees of sky. From London, at about 51 degrees north, Polaris sits a little over five fists above the horizon. From Edinburgh it rides slightly higher. In Sydney, about 34 degrees south, the starless southern pole hangs a little over three fists up.
Tonight's Challenge
On the next clear night, find the Plough and follow its Pointers to Polaris, or in the south, sweep from the Pointers to the Cross and on to the pole. Stack your fists from the horizon and estimate your latitude. Then check it against a map. Come back an hour later and notice how far the wheel has turned.
Key takeaways
- The stars rise in the east and set in the west because the Earth spins from west to east beneath them.
- The sky turns around two fixed hubs, the celestial poles, where the Earth's axis points into space.
- Follow the Plough's Pointers, from Merak through Dubhe, about five times their gap to reach Polaris.
- In the south, extend the Southern Cross's long axis about four and a half times to find the starless south celestial pole.
- The pole's height above your horizon equals your latitude, and a fist at arm's length covers about 10 degrees.