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 360×60×60/2/π ≈ 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 1″ (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.

Last updated: 08-17-2005 13:50:30