How to Read a Star Chart and Find the Pole Star

A star chart can look like a page of scattered dots until someone shows you how to hold it. Once you know the trick, a clear night turns into a map you can read: you look up, find a pattern you know, and hop from it to the next. This short guide uses the northern sky as seen from Britain, northern Europe and North America.

Before you start: give your eyes time

Your eyes take around 20 to 30 minutes to adapt fully to the dark, and one glance at a bright phone screen resets the clock. Find a spot away from street lights, let your eyes settle, and use a red torch to read your chart. Red light keeps your night vision far better than white light.

Step 1: find the Plough

Most people in the UK already know one star pattern, even if they don't know its name: the Plough, seven bright stars shaped like a saucepan. In North America it's called the Big Dipper. It's part of the larger constellation Ursa Major, the Great Bear, and from the UK it never sets, so you can find it on any clear night of the year. In autumn evenings it sits low in the north; in spring it's high overhead.

Step 2: follow the pointers to the Pole Star

The two stars at the end of the saucepan's bowl, Merak and Dubhe, are known as the Pointers. Draw a line from Merak through Dubhe and carry on for about five times the gap between them. You'll reach a fairly modest star sitting on its own: Polaris, the Pole Star.

Polaris isn't the brightest star in the sky, despite what many people think. What makes it special is its position. It sits almost exactly above Earth's north pole, so while every other star wheels slowly around the sky, Polaris hardly moves. Face Polaris and you're facing north. Its height above the horizon is roughly equal to your latitude, so from London it stands just over halfway up the sky, at about 51 degrees.

Step 3: turn the chart to match the sky

This is the step most beginners miss. A chart of the whole northern sky is drawn around Polaris, but the sky is always turning, so the chart only matches it when you rotate it the right way.

On our Northern Night Sky star chart, the months run around the outside edge. Face north, hold the chart up in front of you and turn it until the current month is at the top. Polaris at the centre now matches the Pole Star, and the stars around it match the sky. The line from the centre to the month shows the stars that are overhead and due south at about 10 pm (11 pm in summer time). Stars near the edge of the chart are low on the horizon behind you, or below it.

Later in the evening, the sky will have turned. Each star rises about four minutes earlier every night, which adds up to about two hours a month, so two hours after 10 pm the sky looks like the chart for next month.

Step 4: star-hop

From the Plough and Polaris you can find almost anything. A few easy hops:

  • Cassiopeia: on the opposite side of Polaris from the Plough, a bright W or M shape. When the Plough is low, Cassiopeia is high, and the other way round.
  • Arcturus: follow the curve of the Plough's handle away from the bowl and you'll "arc to Arcturus", a bright orange star that dominates spring and early summer evenings.
  • Vega, Deneb and Altair: three bright stars that form the Summer Triangle, high on summer and early autumn evenings.
  • Orion: the winter hunter, with three stars in a row for his belt. Follow the belt down and to the left to find Sirius, the brightest star in the night sky.

What "magnitude" means

Star charts show brighter stars as bigger dots, and they measure brightness in magnitudes. The scale runs backwards: the smaller the number, the brighter the star. Each step of one magnitude is about two and a half times fainter than the one before. Sirius is about magnitude −1.5. Polaris is about magnitude 2. The faintest stars on our chart are magnitude 5. From a town centre you might see stars down to magnitude 3 or 4. From a dark country site, on a moonless night, you can see fainter than our chart goes.

Planets are the visitors

If you see a bright "star" that isn't on your chart, it's probably a planet. Planets move against the background stars over weeks and months, so they can't be printed on a star chart. They always stay close to the ecliptic, the path of the Sun and Moon, which is drawn as a dashed line on our chart. Venus and Jupiter are brighter than any star, and Mars has a distinct orange-red colour.

Take it outside

The best way to learn the sky is a few minutes at a time, a few nights a month. Start with the Plough and Polaris, add one new pattern each time, and by the end of the year you'll have met most of the northern constellations. If you'd like a map for your wall, our Northern Night Sky print shows all 1,266 stars to magnitude 5, or you can print it yourself at home. Clear skies!

Sources

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