Minute of arc
minute of arc,
arcminute, or
minute arc (
MOA), is a unit of angular measurement equal to one sixtieth (1⁄60) of one degree (circle⁄21,600), or (π⁄10,800) radians. In turn, a
second of arc or
arcsecond is one sixtieth (1⁄60) of one minute of arc. Since one degree is defined as one three hundred and sixtieth (1⁄360) of a rotation, one minute of arc is 1⁄21,600 of a rotation. It is used in those fields which require a unit for the expression of small angles, such as astronomy, optometry, ophthalmology, optics, navigation and marksmanship.
The number of square arcminutes in a complete sphere is
or approximately 148,510,660.498 square arcminutes.
The arcsecond is 1⁄3,600 of a degree, or 1⁄1,296,000 of a circle, or (π⁄648,000) radians, which is approximately 1⁄206,265 radian.
To express even smaller angles, standard SI prefixes can be employed; in particular, the
milliarcsecond, abbreviated
mas, is used in astronomy
Symbols and abbreviations
The standard symbol for marking the arcminute is the prime (′) (U+2032), though a single quote (') (U+0027) is commonly used where only ASCII characters are permitted. One arcminute is thus written 1′. It is also abbreviated as
arcmin or
amin or, less commonly, the prime with a circumflex over it (
).
The standard symbol for the arcsecond is the double prime (″) (U+2033), though a double quote (") (U+0022) is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″. It is also abbreviated as
arcsec or
asec.
In celestial navigation, seconds of arc are rarely used in calculations, the preference usually being for degrees, minutes and decimals of a minute, written for example as 42° 25′.32 or 42° 25′.322. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the latter format by default.
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نوشته اصلی توسط Ali Ahmadi
