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When the length of an arc of a circle is equal to the radius of the circle, the angle subtended by that arc equals one radian.

## ra·di·an

(rā′dē-ən)
A unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°17′44.6".

(ˈreɪdɪən)
n
(Units) an SI unit of plane angle; the angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius. 1 radian is equivalent to 57.296 degrees and π/2 radians equals a right angle. Symbol: rad

## ra•di•an

(ˈreɪ di ən)

n.
the measure of a central angle subtending an arc equal in length to the radius: equal to 57.2958°. Abbr.: rad
[1875–80]

## ra·di·an

(rā′dē-ən)
A unit of angular measure equal to a little more than 57°. This is the measure of an angle whose vertex is the center of a circle and whose two rays intersect the circle so as to form an arc with the same length as the circle's radius.
ThesaurusAntonymsRelated WordsSynonymsLegend:
 Noun 1 radian - the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees)radangular unit - a unit of measurement for anglesmilliradian - a unit of angular distance equal to one thousandth of a radian
Translations
радиан

[ˈreɪdɪən] n
References in periodicals archive ?
The ability to compute measurements in the single-digit nanoradian realm.
Astronomers with the Optical Hawaiian Array for Nanoradian Astronomy (OHANA) project are using optical fibers to combine the light from some of world's largest telescopes atop Mauna Kea for a maximum baseline of 800 meters.
5 nHz at 1 kHz and a phase resolution of 23 nanoradians.

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