Arcsecond
An arcsecond (second of arc), represented by the symbol ,[1] is a unit of angle used in mathematics and astronomy. An arcsecond is 1/60th of an arcminute (minute of arc),[2] which in turn is 1/60th of a degree.[3] Therefore, an arcsecond is equal to 1/3600th of a degree,[2] meaning that there are 360 × 60 × 60 = 1,296,000 arcseconds in a full circle.
One milliarcsecond (mas) is a thousandth (1/1000th) of an arcsecond. So there are 1,296,000,000 milliarcseconds in a full circle. The red supergiant stars Antares, Betelgeuse,[4] Mu Cephei, VV Cephei A, and VY Canis Majoris all have angular diameters (apparent sizes in the sky) of more than 10 milliarcseconds, when seen from Earth.