An arcsecond, also called a second of an arc, is a unit of measurement that amounts to one sixtieth of an arcminute. Simply put, it is equal to 1/3600 degrees of an arc. The symbol used to mark an arcsecond is the double quote. As an example, 1 arcsecond is written as 1”. The term arcsecond is also abbreviated to arcsec, but this term is often confused with the arc secant, which is a trigonometric function that bears the same abbreviation.

This unit of measurement is extremely useful in astronomy where the apparent size of a celestial object is usually gauged by means of angular measurements. These angles are most of the time so small that they can only be denoted using arcminutes or arcseconds.

Arcsec is also often taken to be related to parsec, another term in astronomy. To clear things up, arcsec is a measure of an angle, while parsec is a measure of distance. For example, suppose that a celestial body moved a distance of about 1 arcsec in contrast to other objects in its background, the body then is said to have moved 1 parsec away.

Aside from astronomy, there are also other fields where arcsecond is used and one of them is the firearms industry where the accuracy of high-caliber guns is measured in terms of arcseconds and arcminutes.