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).