arc second (arcsec; second of arc) Symbol: ″. A unit of angular measure equal to 1/3600 of a degree. It is used, for example, to give the apparent diameter of objects, the angular separation of binary stars, and the proper motion and annual parallax of stars. An arc minute, symbol: ′, is 60 arc seconds, i.e. 1/60 of a degree. The full Moon is about 30′ in diameter.