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THIS week, instead of looking for a particular target, we will try a technique for measuring distances in the sky. Once you know how, you will be able to find your way from the constellations or stars you know to anything you wish to find. The great thing is, it is a simple method that works well for kids and adults alike.

Astronomers measure the sky in degrees. There are 90 degrees between the point directly above your head, called the zenith, to the horizon. Each degree is divided into 60 arcminutes, which are in turn made up of 60 arcseconds. For our purposes, only the degrees are important. We can determine the distance from one star to another in degrees.

All you need to be able to measure the sky in degrees is your hand. With your arm outstretched and one eye closed, make a fist. The width of your fist is about 10 degrees. If you hold up just your three middle fingers, that is about 5 degrees. Your little finger on its own is 1 degree. If you want to measure distances bigger than 10 degrees, hold up just your index and little fingers, stretched apart. This width is about 15 degrees. Stretching out your thumb and little finger as far as they will go, with your other fingers held down, is about 25 degrees.

Of course, everyone has different-sized hands, but this technique usually works because most people’s hands are in proportion to the length of their arms. That is why holding your arm outstretched is important.

In any case, you can check that your hand does indeed match up to these distances. In the northern hemisphere, use the constellation Ursa Major, which contains the asterism known as the Plough or Big Dipper. The width of the entire asterism is 25 degrees, so should be the width between your outstretched thumb and little finger. The width of the bowl of the Plough is 10 degrees, so the size of your fist, and there is a 5-degree distance between the two right-hand stars of the Plough, Merak and Dubhe, which point towards the North Star.

In the southern hemisphere, try it instead with the seven stars in Orion often referred to as the saucepan. The three bright stars, Orion’s belt, make up the bottom of the saucepan, while its handle stretches out to a star called Saiph. The pan is 25 degrees across.

In both hemispheres, you can also check your little finger is about 1 degree by holding it up and covering the moon. The full moon should be about half the width of your little finger – or roughly half a degree across. It often seems bigger than this because, when the moon is near the horizon, our brain tricks us into comparing it with objects that are much closer, like trees or mountains. This is called the moon illusion. The next time you see a full moon that looks huge, you can check its size with your little finger.

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