Radian

The radian is the SI supplementary unit of angle. It is defined as the angle subtended at the center of a circle by an arc of circumference equal in length to the radius of the circle.

There are 2π (about 6.283185) radians in a complete circle. One radian is equal to 180^o/π or approximately 57.29578^o.

In calculus, the use of radians leads to the simple identity

<math>\lim_{h\rightarrow 0}\frac{\sin h}{h}=1</math>

For measuring solid angles, see steradian.