radian

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radian (symbol rad) A unit of angle used in geometry. It is defined as the angle subtended at the centre of a circle by an arc of equal length to the radius of the circle. The whole circumference of the circle, of length 2π times the radius, subtends an angle of 2π radians; thus, 360° = 2π radians, and 1 radian = 57°.2958. A useful approximation for astronomers is that 1 radian is about 200 000 seconds of arc (actually 206 265). As a consequence of the definitions of the units, the number of seconds of arc in a radian equals the number of astronomical units in a parsec.

radian Angle formed by the intersection of two radii at the centre of a circle, when the length of the arc cut off by the radii is equal to one radius in length. Thus, the radian is a unit of angle equal to c.57.295°, and there are 2π radians in 360°.

ra·di·an / ˈrādēən/ • n. Geom. a unit of angle, equal to an angle at the center of a circle whose arc is equal in length to the radius.

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