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minute of arcarcminute, or minute arc (MOA), is a unit of angular measurement equal to one sixtieth (160) of one degree (circle21,600), or (π10,800radians. In turn, a second of arc or arcsecond is one sixtieth (160) of one arcminute. Since one degree is defined as one three hundred and sixtieth (1360) of a rotation, one minute of arc is 121,600 of a rotation. It is used in those fields which require a unit for the expression of small angles, such as astronomyoptometryophthalmologyopticsnavigation and marksmanship.

The number of square arcminutes in a complete sphere is

or approximately 148,510,660.498 square arcminutes.

The arcsecond is 13,600 of a degree, or 11,296,000 of a circle, or (π648,000radians, which is approximately 1206,265 radian.

To express even smaller angles, standard SI prefixes can be employed; in particular, the milliarcsecond, abbreviated mas, is used in astronomy.

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