FIGURE: Units of angular measure. The radian is a unit of
measure which gives an arc on a circle equal to the radius of the
circle. It is about 57.3 degrees in size.
A unit of angle equal to about 57 degrees. The length along the arc of a circle covering by one radian is equal to the radius of the circle. The complete angle around the circle (360 degrees) is equal to 2 pi radians. The radian is particularly useful because if you know the distance to some object and you measure its apparent size as the angle it subtends in your field of view in radians, then the actual size is just that number of radians times the distance to the object. For example, a meter stick held up at a distance of 100 meters makes an apparent angle in your field of view of 0.01 radians.
FIGURE: Units of angular measure. The radian is a unit of
measure which gives an arc on a circle equal to the radius of the
circle. It is about 57.3 degrees in size.