What is a second of arc?
A full circle consists of 360 degrees. One degree can be divided into
60 minutes of arc. These minutes of arc should not be confused with
minutes of time. Each minute of arc contains 60 seconds of arc, so a second
of arc is an angle that is 1/3,600 of a degree.
When we stand on Earth and look at the sky, the full moon covers about
one-half degree of arc, which is equal to 30 minutes of arc, or 1,800 seconds
A second of arc is a tiny angle. If you look at the head of a pin from
the distance of the length of two football fields, the diameter of the
pinhead would be 1 second of arc wide.