# National Aeronautics and Space Administration

## Angular Size Relationship

### The Relationship Between Angular Size and Actual Size

Angular size reflects how large an object actually appears to an observer. The equation describing angular size can be expressed in words as: the apparent size of an object decreases linearly as you get further away from the object. You already know this from real life: buildings look smaller as they are further away. So do people. If you know approximately how big something is (its physical size - for example, a person is usually around 5 or 6 feet tall) and you observe it to be a certain apparent size, you are able to automatically estimate how far away it is.

The equation, or mathematical expression, for this everyday phenomenon is useful because you can use it to calculate the third variable if you know the other two. Often that is the way the relationship is used in astronomy: to find the distances of objects like supernova remnants, for example, if their sizes and angular sizes are known.
 Practice finding and using angular size in an everyday situation.
 Practice finding and using angular size in an astronomical situation.