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```Date: 05/27/2002 at 08:35:49
From: Natalie Bowlus

I'm doing a project in math on radians and degrees and I was
wondering where, exactly, did radians come from?  When did people
start to use them in calculations?  Thanks for the help.
```

```
Date: 05/27/2002 at 12:23:17
From: Doctor Sarah

Hi Natalie - thanks for writing to Dr. Math.

From Russ Rowlett's _How Many? A Dictionary of Units of Measurement_:

http://www.unc.edu/~rowlett/units/dictR.html

a unit of angle measure widely used in mathematics and science.
One radian is the angle at the center of a circle that cuts off
an arc of length equal to the radius. Since the circumference
equals 2 pi times the radius, one radian equals 1/(2 pi) of the
circle, or approximately 57.295 779 degrees. Using radians to
measure angles seems unnatural at first. However, when angles
are stated in radians the constant pi tends to disappear from
the equations, and this greatly simplifies calculation.

For example, the length of an arc is simply its radius
multiplied by its angular measure in radians, and the area
of a sector of a circle is simply its angular measure in

The radian was defined and named by James Thomson in 1873.
Thomson was a mathematics professor at Queens College, Belfast,
Northern Ireland, and the brother of the famous physicist
William Thomson, Lord Kelvin.

And see Pat Ballew's _Math Words, and Some Other Words of Interest_:

- Doctor Sarah, The Math Forum
http://mathforum.org/dr.math/
```
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High School Conic Sections/Circles
High School History/Biography
High School Trigonometry

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