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.