Arcsecond

A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7.7×10^-7 of a circle. It is the angular diameter on object of 1 unit diameter at a distance of 360×60×60/(2π) ≈ 206,265 units, such as (approximately) 1 cm at 2.1 km, or, directly from the definition, 1 astronomical units at 1 parsec .

Correspondingly, 1 radian ≈ 206,265 arcseconds.

The symbol for marking the arcsecond is the double prime (″) (U+2033, ″). One arcsecond would be " (or 1").

It can be abbreviated as arcsec, but should then not be confused with the inverse trigonometric function arc secant, which has the same abbreviation.