As we all know – a day is the amount of time it takes the earth to spin around its axis. But why is that period of time divided into 24 periods? Seems weird. If you were a space explorer and discovered a new planet that spun on its axis would you decide that a day should be divided into 24ths?

To understand why 24 hours was chosen you must first understand that different civilizations used varying base systems of numbers. From the very interesting IFOD History of Numbers:

Some early numbering systems were binary base systems with numbers progressing as follows: one and two were their own discrete numbers, then three was “two and one” and four was “two and two” and five was “two and two and one” and so on. Others were base-five systems, supposedly because we have five fingers on each hand (and five toes on each foot). Base-10, 12, 20 and 60 systems all have been used as well.

Note that a “base” means that when a number is reached, whether 2, 5, 10, 12, or 60, a different mark is made representing all the previous numbers. At that point, units continue to be added to the base until the next base is reached, and so on. Our numbering system is a base-10 system.

Given our base-10 numbering system, it would make sense if a day were sliced into 10ths or 20ths. The reason days were sliced into 24ths is because ancient Egyptians used a base-12 system of numbers and they divided the daytime and nighttime each into 12ths.

Interestingly, having hours as that are fixed in length is a relatively recent practice. The Egyptians used sundials to keep time during the day and of course the amount of daylight varies as the seasons change. So, hours varied in length as the amount of daylight cycled through seasons. The concept of using hours of a fixed length wasn’t proposed until 1000 years later by the Greeks and fixed hours weren’t widely adopted until mechanical clocks were introduced in Europe in the 14th century.

Why there are 60 minutes in an hour and 60 seconds in a minute?

Again it’s due to a different base numbering system. The ancient Babylonians used a base-60 system of numbering and made astronomical calculations with that system. They gave us the 360 degrees in a circle. The Greeks, relying on Babylonian astronomy divided the globe into latitudes and using the Babylonian numbering system subdivided latitudes into 60 sub-parts which were in turn divided by a further 60. “The first division, partes minutae primae, or first minute, became known simply as the ‘minute.’ The second segmentation, partes minutae secundae, or ‘second minute,’ became known as ‘the second.'” Source.

Primary source for this IFOD is from Scientific American.