This page compares and contrasts the two systems of measuring angles in math: radians and degrees, and explains why radians are preferred. This page assumes a reasonable level of familiarity with the degree 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 whose vertex is at the center of two cocentric circles of radii , and arclengths , . Observe that the angle can be defined both in terms of the first circle and the second circle. This is illustrated below.
This observation suggests a natural definition for radians. In both cases, the arc length is proportional to the radius which implies that the ratio of arc length to radius is 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 the 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 “radiusinvariant” 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.
As a result of the definition of radians discussed above, a full rotation or “one turn” in radians is approximately equal to . 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 below.
The circle constant makes talking about and using radians much easier. Take, for example, the interactive below which visualizes angles measured in radians and annotated using the circle constant. Everything becomes a fraction of one “turn”.
The claim here is that the radians system leads to succinct and elegant formulas. Here we see a relationship between the functions sine and cosine that wouldn’t be true if we had instead used the degree angle system. If we take the derivative of sine expressed below as a power series, we are left with the power series definition of cosine.
Applying the same process we can show that the derivative of cosine is equal to negative sine.
The advantages of the radian system become apparent when defining complex formulas such as the area of the circle, the 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

No, really, pi is wrong: The Tau ManifestoMichael Hartl