Angular Measurements

In astronomy, we often use angular measurements to describe the apparent size of an object in space and the apparent distances between objects. Often these angles are very small. Angles are also used to describe an object's location in space.

The angular measure of an object is usually expressed in degrees, arcminutes or arcseconds. Just as an hour is divided into 60 minutes and a minute into 60 seconds, a degree is divided into 60 arcminutes and an arcminute is divided into 60 arcseconds. To give you an idea of how small an arcsecond is, imagine the width of a dime as seen from 2 kilometers or 1 1/4 miles away.

1 degree = 1° = 1/360 of a circle

1 arcminute = 1' = 1/60 of a degree

1 arcsecond = 1" = 1/60 of an arcminute = 1/3600 of a degree

To get a rough estimate of the angular size of objects in space, you can go out on clear night when the moon is up. Extend your arm towards the sky. Your fist, at arms length, covers about 10 degrees of the sky, your thumb covers about 2 degrees, and your little finger covers about 1 degree. If you look at the Moon, it should take up about 1/2 a degree in the sky. The Big Dipper should be about 20 degrees (two fists at arms length) from one end to the other.