Circles and PiDegrees and Radians
So far in geometry, we've always measured angles in
The number 360 is very convenient because it is divisible by so many other numbers: 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, and so on. This means that many fractions of one circle are also whole numbers. But have you ever wondered where the number 360 comes from?
As it happens, 360 degrees are one of the oldest concepts in mathematics we still use today. They were developed in ancient Babylon, more than 5000 years ago!
At that time, one of the most important applications of mathematics was in astronomy. The sun determines the four seasons, which farmers have to know about when growing crops. Similarly, the moon determines the tides, which was important for fishers. People also studied the stars to predict the future, or to communicate with gods.
Astronomers noticed that the constellations visible at a specific time during the night shifted a tiny bit every day – until, after approximately 360 days, they had rotated back to their starting point. And this might have been the reason why they divided the circle into 360 degrees.
Of course, there are actually 365 days in one year (well, 365.242199 to be exact), but Babylonian mathematicians worked with simple sundials, and this approximation was perfectly adequate.
It also worked well with their existing base-60 number system (since
For many of us, measuring angles in degrees is second nature: there is 360° video, skateboarders can pull 540s, and someone changing their decision might make a 180° turn.
But from a mathematical point of view, the choice of 360 is completely arbitrary. If we were living on Mars, a circle might have 670°, and a year on Jupiter even has 10,475 days.
Rather than dividing a circle into some number of segments (like 360 degrees), mathematicians often prefer to measure angles using the
A has circumference
For a , the corresponding distance along the circumference is
For a , the distance along the circumference is
And so on: this way of measuring angles is called
Every angle in degrees has an equivalent size in radians. Converting between the two is very easy – just like you can convert between other units like meters and kilometers, or Celsius and Fahrenheit:
360° = 2π rad
1 rad =
You can write the radians value either as a multiple of π, or as just a single decimal number. Can you fill in this table of equivalent angle sizes in degrees and radians?
You can think of radians as the “distance traveled” along the circumference of a unit circle. This is particularly useful when working with objects that are moving on a circular path.
For example, the
The radius of the ISS orbit is 6800 km, which means that the actual speed of the ISS has to be
Can you see that, in this example, radians are a much more convenient unit than degrees? Once we know the speed of rotation, we simply have to multiply by the radius to get the actual speed.
Here is another example: your car has wheels with radius 0.25 m. If you’re driving at a speed of 20 m/s, the wheels of your car rotate at
For most simple geometry problems, degrees and radians are completely interchangeable – you can either pick which one you prefer, or a question might tell you which unit to give your answer in. However, once you study more advanced
Most calculators have a
Try using this calculator to calculate that
sin(30 rad) =
Using radians has one particularly interesting advantage when using the Sine function. If
This is called the small angle approximation, and it can greatly simplify certain equations containing trigonometric functions. You’ll learn much more about this in the future.