# National Aeronautics and Space Administration

## Goddard Space Flight Center

### Angular Size

An object's angular size is the angle between the lines of sight to its two opposite sides. For example, the angular size of the horizon is about 180 degrees. An object's angular size is a measure of how large the object actually appears to be (which is a function of both actual size and distance away). In astronomy, the angular sizes of most objects are much smaller than even a single degree. To measure these tiny angular sizes, astronomers use units of arcminutes (') and arcseconds ("). There are 60 arcminutes in a degree and 60 arcseconds in an arcminute, so there are 648,000 arcseconds in the horizon.

For small angles (less than ten degrees), the angular size, physical size, and distance of an object are related in a useful way, namely:

You are already familiar with this relationship; the closer you get to something (the smaller the distance), the bigger it appears (its angular size is larger).

 Tell me more about real life examples of using angular size.
 Practice finding and using angular size in an everyday situation.
 Practice finding and using angular size in an astronomical situation.