Arcsecond: Angle unit equal to 1/60² of a degree
An arcsecond (second of arc), represented by the symbol ″ , is a unit of angle used in mathematics and astronomy.
An arcsecond is 1/60th of an arcminute (minute of arc), which in turn is 1/60th of a degree. Therefore, an arcsecond is equal to 1/3600th of a degree, 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, 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.
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