Since the late 19^{th} century, a unit used
to measure plane angles in SI and elsewhere, equal to the
angle between two radii of a circle that cut off a piece of the circumference
whose length is equal to the length of the radius. Symbol, rad.

Since the circumference of a circle is pi times the diameter, or two radii, one radian is 360 divided by 2 pi degrees, or approximately 57° 17′ 45″ (about 57.295 779 513°).

According to Florian Cajori,^{1} the term “radian” was first used in
print by James Thomson in 1873. (Thomson was a
brother of Lord Kelvin.)

A variety of symbols have been used for the radian, including, in a raised
position like an exponent, R, r, (r) and c. In 1929
Cajori wrote “Evidently there is, as yet, no approach to uniformity in the
designation of radians.”^{2}

1. Florian Cajori. *History of Mathematics* (2nd ed.,
1919), page 484.

2. Florian Cajori. *A History of Mathematical Notations.*
La Salle, Illinois: Open Court, 1929. Volume 2, page
148.

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