2.04 SI units for angular measurement
There are two other derived SI units. These are both used for angular measurement they are the radian and the steradian.
In the previous section we have seen that angular measurement is a dimensionless unit. Angular units are derived from the unit of length (the metre) however the units of length cancel out to result in angular measurement having no corresponding base units.
Angular measurement radian (r)
A radian is equal to the angle between two radii (radii is just the plural of radius) in a circle when the length of the arc between them at the circumference is equal to the length of the radius
Solid angular measurement steradian (sr)
A steradian is the angle of a conic section of a sphere that has and angle of 1 radian at the apex and encloses an area on the surface of the sphere equal to the radius squared