Radians (rad)
A radian is the SI unit of plane angle defined as the angle subtended at the center of a circle by an arc equal in length to the radius. One radian equals approximately 57.2958 degrees or exactly 1 rad = radius/arc length ratio. It is a dimensionless unit used extensively in mathematics, physics, and engineering [bipm-si-brochure].
Origin
The radian concept was formalized in the 19th century, with mathematicians like Roger Cotes contributing to its development. The radian was adopted as the standard angular unit in the SI system to provide a natural measure of angles based on circle geometry [bipm-si-brochure].
Worldwide Use
Radians are the standard unit of angular measurement in science, engineering, and technology worldwide. They are used in trigonometry, rotational dynamics, and signal processing, with formal adoption by the BIPM and ISO 80000 standards [bipm-si-brochure, iso-80000].
Common References
- A full circle measures 2π radians
- Angles in trigonometric functions are expressed in radians
- A right angle is π/2 radians
- Radians simplify calculations in calculus
- Angular velocity is commonly measured in radians per second