What you need
A clear night sky
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IMAGINE there is a shell encasing Earth, and every star is painted on it. The shell constantly rotates, so the stars rise and set like the sun. But one bright star doesn鈥檛 move: Polaris, the North Star.
That is because its position is directly above the North Pole and therefore Earth鈥檚 axis of rotation. This means that if you are north of the equator it can be used for navigation, as it can always be seen at due north at an angle in the sky equal to the latitude you are seeing it from.
The shell of stars doesn鈥檛 make a full rotation each day, though. If you looked up at 7 pm one day, and again the next, the stars would be shifted by four degrees. This is because we measure time in solar days 鈥� the time it takes Earth to rotate with respect to the sun鈥檚 position 鈥� rather than sidereal days, the time it takes Earth to rotate with respect to the stars. The two are different because Earth is also orbiting the sun as it rotates, meaning a solar day is slightly longer and the stars rise 4 minutes earlier each day.
Polaris is so bright despite being 433 light years away because it is a triple star system. A yellow supergiant called Polaris Aa orbits smaller Polaris Ab, and the pair orbits another star, Polaris B. It is part of the constellation Ursa Minor, or the Little Bear.
You can find Polaris using the constellation Ursa Major, part of which is the Plough, or Big Dipper. This moves around the North Star, but two of its stars always point to Polaris: Dubhe and Merak. Dubhe is a multiple star system 123 light years away, and Merak is a subgiant, meaning it has used up all its hydrogen and is now cooling.
To find Polaris, draw a line from Merak to Dubhe, and then extend it five times the distance between them. The line ends at the North Star.
There is no bright star over the South Pole, but you can find due south using the Southern Cross, part of Crux, the smallest of the 88 official constellations.
The Southern Cross has four main stars: Alpha Crucis, Beta Crucis, Gamma Crucis and Delta Crucis. These are easy to spot as they have the same brightness. Plus there are two bright stars in the constellation Centauraus that point to it: Alpha Centauri and Beta Centauri. Alpha Centauri is notable for being the closest star system to Earth, at just 4.37 light years away.
Once you have found the Southern Cross, draw a line from the top star in the cross to the one at the bottom. Extend it 4.5 times the distance between the stars and you are at the south celestial pole and are looking due south.
Next week, I will show you how to work out the light pollution level in your area and so gauge how many planets and stars it will be possible to see.
To download a printable northern hemisphere聽聽version of this page click here
To download a printable southern聽hemisphere version of this page click here
For next week
A star chart or phone with a stargazing app will be helpful but not essential
Next in the series
2 Find the North Star and Southern Cross
3 Test your area鈥檚 light pollution. It鈥檒l help you plan stargazing trips
4 Identify the craters of the moon
5 Orion and Sirius: how to star-hop
6 Planet spotting: Mars, Mercury and Uranus
7 Taurus and the zodiacal constellations
Stargazing at home online
Projects will be posted online each week at Email: maker@newscientist.com
