On degrees

I gave a talk (some months ago now) on the history of $\pi$ (which is well discussed in my unreliable history of maths, Cracking Mathematics, available wherever good books are sold.)

At one point, I put up a slide generally excoriating degrees as a measurement of angle, and stating that for small $x$, you have $\sin\br{x^\circ}\approx \frac{\pi}{180}x$.

And one member of the audience was Not Having It. He was certain that the $x$ on the right also needed a $^\circ$ after it.

Here’s why (I think) he is mistaken

Let’s start with the idea of units. If I write something like 15m, the m means “this number represents a distance, in the proportion such that 40,000,000 of them make a circumference of the earth” (or whatever the precise definition is these days).

My understanding of the $^\circ$ symbol is that – even though angles are dimensionless – it functions as a unit: it states “this number represents an angle, in the proportion such that $360^\circ$ is a complete revolution.”

The sine function is a mapping from angles to real numbers (in particular, real numbers between -1 and 1). It’s usually convenient to deal with angles in radians, which are angles in the proportion such that $2\pi$ represents a complete revolution – and it’s so convenient to deal with it this way that we don’t normally mention the unit – it’s implied. Angles (in radians) generally behave just like numbers, so there’s no difficulty.

Putting a $^\circ$ on the right-hand side of the equation would state “the right hand side is an angle” – which it isn’t – and that 360 of those angle-units made up a circle – which they don’t.

It would perhaps have been better to write it as $\frac{\pi}{180^\circ}x^\circ$ – but I stand by what I stated.

And I can only apologise to the audience for what must have seemed like an interminable, technical conversation.


Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.


2 comments on “On degrees

  • Andrew Old

    I think it’s pretty obvious that you were right. Difficult to see any grounds for objecting that aren’t just a massive misunderstanding of basic notation.

    • Colin

      Thanks, Andrew.

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