# Radian

- The radian is the plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.

**SI Supplementary Unit**

The circumference of a circle is defined as 2π*radius. Considering a unit circle centred on a Cartesian axis. By tracing the x-coordinate (starting at (x=1,y=0) and travelling counter-clockwise) versus arc length we obtain a Cosine wave and similary the y-coordinate yields a Sine wave. From this derives the definition that 360° equals 2*π* radians.

**See also: **Angles, Plane Angle, Rise Run, SI Units.

**Subjects: ** Physics Trigonometry