arcmin

An arcminute, abbreviated arcmin and sometimes written with the prime symbol (′), is a unit of angular measure equal to one-sixtieth of a degree or sixty arcseconds. It is commonly used in astronomy, navigation, and surveying to express small angles on the sky or on the ground.

In SI terms, the arcminute equals pi/10800 radians, about 0.000290888 radians. In degrees, it is exactly 1/60 of a degree, or about 0.0166667 degrees. Because 60 arcminutes make up a degree, 60 arcseconds make up an arcminute.

The arcminute is used to describe angular separations between celestial objects, field of view in telescopes, and apparent sizes on the sky. For reference, the Sun and Moon each subtend about 31 arcminutes on average.

In practice, many astronomical measurements are quoted in arcminutes or arcseconds; the radian remains the SI unit for angular measure. The prime symbol (′) is often used to denote arcminutes, while arcseconds use a double prime (″).

Because arcminutes measure angles, the actual linear size they subtend depends on distance, via the small-angle approximation s ≈ r·θ (with θ in radians). For example, at the Moon’s average distance (~384,000 km), 1 arcminute corresponds to about 110 km, and the Moon’s diameter (about 3474 km) spans roughly 31 arcminutes.