60 Seconds in a Minute, 60 Minutes in an Hour

In today's computerised, digital world we're used to everything being calculated using the decimal system (base ten) and usually being divisible by 100. So why are there 60 seconds in a minute? Why are there 60 minutes in an hour? Why not have a more "sensible" number like 100?

Why Sixty?

Many people believe that the origin of the sixty minute hour goes back to the ancient Sumerians. They used a sexagesimal system - one based around the number 60. But why?

The answer would appear to be arithmetic. The number 60 may not be popular with computers but it's very useful for people. It can be divided easily by 2, 3, 4, 5 and 6 - as well as the ubiquitous 10. Although 100 is divisible by 2, 4 and 5 it is not cleanly divisible by 3. A third of an hour would thirty three and a third minutes, which is not pretty.

It's interesting to note that the number six has a special place in arithmetic as the smallest "perfect number". That is:

1 + 2 + 3 = 6 = 1 * 2 * 3

Using the number 60 is also in alignment with geometry. There are 360 degrees in a circle, and 360 equals 6 times 60. So dividing up a clockface is a straightforward matter: each minute of time equates to exactly 6 degrees of arc.

There is also a connection with astrology: the Western Zodiac is divided into 12 signs and 5 times 12 is 60.

Despite the clear advantages of the number 60, there are still people who advocate switching to a system of decimal time with 100 minutes per hour and 100 seconds per minute.

Incidentally, not all minutes are exactly 60 seconds long. Every few years a leap second is added to or subtracted to one minute at the end of June or December.