Who Decided There Are 24 Hours In A Day?

There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. Who decided this?

Related on TestTube:
How was the Modern Calendar Invented?
How the Invention of the Clock Changed Everything

Why do our clocks measure out time the way they do? We probably take it for granted, but there are lots of different ways to measure the passing of time: the one we use just won out over all the other ones. Like the Gregorian calendar, (which we discussed in this episode of DNews), the clock we use has gradually evolved over time. Since the metric system uses base of 10, it may seem like the obvious choice to base our time keeping on, but ancient cultures used all sorts of different bases. The duodecimal system, which has a base of 12, was popular probably because it takes 12 lunar cycles to make one trip around the sun. Despite the fact that they're only actually equal on seasonal equinoxes, days and nights each got assigned 12 hours.

The Ancient Babylonians take credit for the hour being made up of 60 minutes. For reasons that remain unclear, they used a base 60 system of counting. They also divided the circle into 360 parts, which the Ancient Greeks built upon when they tried to divide the Earth into 360 lines of longitude and latitude. Ptolemy divided these into smaller parts which he called the "first minute", and those got split into 60 parts which he called the "second minute". Second minute became the second we know and use today. It wasn't until the 16th century that minutes and seconds were widely used, when more accurate mechanical clocks were able to keep up (the first-ever clock with a seconds hand dates back to Germany, ca. 1560).

By the time of the Atomic Age, scientists invented the most-accurate clock ever, the atomic clock, which measures one second by counting off 9,192,631,770 energy transitions from the cesium atom. But as accurate as this method of time keeping is purported to be, a few seconds have to be added to to Coordinated Universal Time (UTC) every decade since Earth's rotation around the sun slows ever so slightly over time. Scientists have tried to come up with more precise methods of measuring time---Metric time, Decimal time, even a New Earth Time (aka "Net Time")but it seems unlikely at this point the current system of time is going anywhere soon.

Learn More:
Keeping Time: Why 60 Minutes? (Live Science)
"How did we come to divide the hour into 60 minutes and the minute into 60 seconds?"

Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? (Scientific American)
"In today's world, the most widely used numeral system is decimal (base 10), a system that probably originated because it made it easy for humans to count using their fingers."

The Short, Strange History of Decimal Time (io9)
"For thousands of years, we've divided days into 24 hours, hours into 60 minutes, and minutes into 60 seconds. But why do we have to do that? Here's the story of the one gloriously failed attempt to decimalize time."