Angular Sizes

To discuss what we see in the night sky, we need to be able to describe how big objects appear (e.g. the Moon, or M31, the Andromeda galaxy). Astronomers use angles to describe these apparent sizes. To see how this works, hold up the tip of your pointer finger to your eye to see what objects around your room you can cover up with it.

Notice that the closer an object is to your eye, the larger it appears. Your fingertip can have the same angular size as a physically much larger object… if the smaller object is much closer to you than the large one!

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The angular size (also called apparent size) of an object can be measured in degrees. Your hand is a very “handy” tool with which to measure the angular sizes of various things in the sky (e.g. the Moon)! Below are the directions on how to use your hands to determine the apparent sizes of objects in the sky:

• Use the image below to make the first hand gesture with your pinky

• Hold your arm stretched out straight out in front of you (don’t let your elbow bend!).

• Close one eye (or cover it with the other hand).

Figure 1: Image Source: http://www.fortworthastro.com/beginner1.html

 

1) Tape a standard 8.5×11 inch piece of paper to the wall (if you don’t have tape, find a way to make it stand up). How far away do you have to stand for you to cover the paper with three fingers (the 5° hand gesture)? This is your best guess, don’t worry about having perfect measurements. Don’t forget to completely stretch out your arm!

Note: Not all hands are the same size, so they will not all measure the same angular sizes. The guide above is a rough estimate. To check your own hand measurements, you can compare it to the Big Dipper. At some point during the semester, try to find the Big Dipper (we will discuss how to do this later in the semester) and hold up these hand gestures to compare angular sizes.