# Why do we have 360 degrees in a circle and why we need radians? [duplicate]

I have two related questions:

1- Why do we have 360 degrees in a circle?

2- I have seen in most of the mathematical concepts, angle is expressed in radians not in degrees. Why was radian introduced?

• A good place to start is Wikipedia: en.wikipedia.org/wiki/Degree_(angle), en.wikipedia.org/wiki/Radian. Jan 31 '13 at 6:48
• We don't need radians. But if we insist on using degrees, many important formulas, particularly in any field that uses calculus, get more complicated, In the same way, we don't need the usual metric system, we could use inches, feet, yards, furlongs, miles. But that is far messier. Jan 31 '13 at 6:50

Possibly the most important reason for using radians instead of degrees to measure angles is that it's the simplest way for computing the straight length subtended by a circular arc: if we have a circle of radius $r$, then we know that the length of the arc subtended by an angle $\theta$ measured in radians is just $r\times\theta$ - a relation that doesn't hold true if we measure $\theta$ in any other units. This in turn leads to a lot of observations that have important uses in 'higher' mathematics; for instance, the idea that a small piece of circular arc is approximately straight can be formalized as the statement that $\sin\theta\approx \theta$ for angles $\theta$ near $0$ - but only if we measure $\theta$ in radians! A ton of important formulas can then be derived from versions of this formula: for instance, it yields the formula $\dfrac{d}{d\theta}\sin\theta=\cos\theta$, which doesn't hold true if we measure $\theta$ in degrees (it's off by a constant factor), and the list goes on quite a bit. Radians are just, in an important sense, the most natural scaling factor for talking about angles.
A circle is defined to have $360$ degrees. That number is convenient because it has many factors and can be divided easily without getting fractions.