Minute of arc
A
minute of arc,
arcminute, or
minute arc, is a unit of angular measurement equal to 1⁄60 of one degree. Because one degree is 1⁄360 of a circle, one minute of arc is 1⁄21,600 of a circle, or π⁄10,800 radians. It is used in fields that involve very small angles, such as astronomy, optometry, ophthalmology, optics, navigation and marksmanship. The number of square arc minutes in a complete sphere is = approximately 148,510,660 square arc minutes. A
second of arc is 1⁄60 of an arc minute, 1⁄3,600 of a degree, 1⁄1,296,000 of a circle, and π⁄648,000 of a radian. This is approximately the angle subtended by a U.S. dime coin at a distance of 4 kilometres. To express even smaller angles, standard SI prefixes can be employed. In particular, the
milliarcsecond, abbreviated
mas, is commonly used in astronomy.