A second of arc is one sixtieth of a minute, which makes it one 3,600th of a degree. It is written with a double prime, as in 15", and is usually called an arcsecond.

This is the scale astronomers work at. Star positions, the apparent size of planets, and the tiny wobble used to measure distance to nearby stars are all quoted in arcseconds. Latitude and longitude are given in degrees, minutes, and seconds for the same reason: it pins a place down to a few tens of meters.

In writing, the second is usually shortened to "arcsec". One second is 0.0002777778 degrees, and that ratio is what every number on this page is built from.