# Invention of the SI unit of plane angle:Radian

As I was reading on the topic plane angle then I came across a term radian.Hence I want to know how radians were discovered as well ?

• Regarding the measure itself, Roger Cotes is credited with first using it in its current sense (1714) although other measures of angle by arc length were used previously. Regarding the name "radian", Jeff Miller's site states that it was Lord Kelvin's brother, James Thomson, and James Muir who first introduced the term which is a shortened form of radial angle in the late 19th century. – Nick Jul 16 '19 at 15:55
• It's not really a discovery.... other than that $circumference = 2*\pi*radius$ – Carl Witthoft Jul 17 '19 at 13:07