When describing how to find an object in the night sky, the location is often given as an angle from a bright, easy to find object. This is particularly true for “naked eye” and binocular objects where you don’t have a fancy GoTo computer controlled mount to point the telescope for you. Statements like “Mercury will be 2° below the moon on Oct 6th, 2013” may sound great, but what does 2° look like?
You can get a pretty good approximation of an angle by using your own hand as a scale. Hold your hand up in front of the object, straiten your arm and use various parts of your hand to gauge the angles. This works for everyone since generally those with longer arms have larger hands – so the angles will be roughly the same.
With your fingers stretched open, the distance from the tip of your thumb to your pinky is about 20°. The distance across your palm is about 10° and a finger is about 2°.
You can also calibrate your hand scale by checking it against a known object. The Big Dipper asterism is very convenient for this since it’s a familiar object and visible mostly all times of the year. The chart below shows the distances between some stars: 4° – two fingers, 10° – palm and 20° – tip to tip fingers.
While you’re looking at the Big Dipper, just for fun try to see the separation between Mizar and Alcor. They are about 11’ (1/6 of a degree) apart which is about the limit of normal vision. These two stars are part of a multiple star system that revolve around each other (in a few hundred thousand years).
So if you wanted to find Mercury when it’s 2° from the moon, line up the side of your finger with the moon and Mercury will be just on the other side.