Starwatch: Count these stars to find out how dark the sky is where you are

This week, we are going to locate the Great Square of Pegasus and count stars to see how dark your sky is.The chart shows the view looking south from London at 2000 GMT on 9 November. High in the sky is the Great Square of Pegasus, a distinctive pattern of four stars that marks the body of the mythical winged horse.The four corner stars – Alpheratz, Scheat, Markab and Algenib – are bright enough to be visible even from suburban locations. The question is, how many stars can you see inside the square?To do the experiment, find somewhere away from direct lighting and wait for at least 20 minutes to allow your eyes time to adjust to the darkness. No checking your phone to pass the time, as the bright screen will ruin your dark adaptation. Then look carefully inside the square and count every star that you can see with your naked eyes. Do not include the four corner stars.If you can see just one star, your sky is relatively poor. Four stars indicate an average sky, while seven suggests good conditions. Count 13 and your sky is very good indeed. If you can see about 35, you are observing under an exceptionally dark sky.