This page compares and contrasts the two systems of measuring angles in math: radians and degrees, and explains why radians are the preferred unit for angles. This page assumes a reasonable level of familiarity with the degrees system and an introductory level of experience with radians.

The short answer for why radians are preferred is that the radian system leads to more succinct and elegant formulas throughout mathematics[1]. For example, the derivative of the sine function is only true when the angle is expressed in radians.

There are two systems for measuring degrees in math: degrees and radians. The degree system divides a full rotation into and often introduces the concept of angles to students. The degree system is rooted in the origins of trigonometry and is a useful system for measuring angles. However, as time has gone by and math has evolved, the radian system has replaced degrees as the preferred system because of its natural connection to the geometry of the circle.

The concept of angles and circles are intimately related. Consider the angle (theta) formed by two rays as illustrated in figure 3 whose vertex is at the center of two cocentric circles of radii , and arc-lengths , . Observe that the angle can be defined both in terms of the first circle and the second circle.

This observation suggests a natural definition for radians. In both cases, the arc-length is proportional to the radius which implies that ratio of arc-length to radius is the equal. This is stated more formally in math notation as:

This relationship gives us the definition of radians: A radian angle is equivalent to the ratio of arc-length to radius, as shown in the equation below. Since radians measure ratio this is why they are referred to as a “dimensionless unit” or having a “radius-invariant” property.

Note, the equivelant symbol () represents that all angles on the domain of real numbers are equivalent to a corresponding angle in the domain of to a full rotation. For example, the angles and , measured using the circle constant , are equivalent.

### The Circle Constant

As a result of the definition of radians discussed above, a full rotation or “one turn” in radians is approximately equal to as shown in figure 4. This special number is called the circle constant represented by the symbol (tau) and appears in many areas of math, statistics and physics. One way to visualize this number is to divide any circle by its radius as shown in figure 5.

The circle constant makes talking about and using radians much easier. Take, for example, the interactive below which visualizes angles measured in radians and annoted using the circle constant. Everything becomes a fraction of one “turn”.

### Succinct and Elegant Formulas

The advantages of the radian system become apparent when definining complex formulas such as the area of the circle, definition of the trigonometric function sine and cosine, normal distribution and Euler’s formula.

This section is still in development, the derivations are hopefully coming soon. Thanks for your patience!

TODO: derive area of circle formula

TODO: derive definition of sine and cosine

TODO: derive normal distribution formula

TODO: euler’s formula

## references

1. No, really, pi is wrong: The Tau Manifesto
Michael Hartl