Why does a day have 24 hours?

We take for granted that a day has 24 hours, an hour has 60 minutes, and a minute has 60 seconds. But why those specific numbers and not others? Earth doesn’t “know” how to count, so the answer isn’t in astronomy but in history: in decisions made by civilizations thousands of years ago. It’s a surprising story.

It’s not a natural number

The first thing to understand is that dividing the day into 24 parts is a human decision, not a law of nature. Earth takes roughly a day to spin on its axis, yes, but how many parts we assign to that spin is a cultural choice. We could have chosen 10, 20, or 100. We chose 24. Why?

The Egyptian origin

The main clue takes us to Ancient Egypt, more than 3,000 years ago. The Egyptians divided the day-and-night cycle in a peculiar way:

• They divided the night into 12 parts, guided by the rising of certain stars.
• They divided the day (the hours of light) into another 12 parts, using sundials.

Adding the 12 of night and the 12 of day, they got 24. As the Royal Observatory Greenwich notes, that’s essentially where our 24-hour day comes from.

The obsession with the number 12

And why 12 and not 10, which would seem more logical with our ten fingers? Here comes a fascinating curiosity. Many ancient cultures valued the number 12 because it’s very divisible: it can be split into halves, thirds, quarters, and sixths without decimals (12 divides by 2, 3, 4, and 6). This made it very practical for trade and calculations.

There’s also a curious theory: you could count to 12 on a single hand, using the thumb to point to the three segments of the other four fingers (4 fingers × 3 segments = 12).

Minutes and seconds: blame Babylon

And why does an hour have 60 minutes and not 100? The answer lies with another great civilization: the Babylonians. They used a sexagesimal system, that is, base 60 (instead of our decimal base 10). Like 12, 60 is a very divisible number, ideal for fractions.

From that Babylonian legacy we inherited the hour having 60 minutes and the minute 60 seconds. The same reason a circle has 360 degrees (60 × 6).

A failed attempt to change it

Curiously, there have been attempts to change this system. During the French Revolution, decimal time was proposed: days of 10 hours, hours of 100 minutes. It was more “logical,” but it failed spectacularly: the habit of 24 hours was too ingrained. And so it remains.

Heirs of Egyptians and Babylonians

Every time you look at the clock you’re using a system thousands of years old, inherited from the Egyptians (the 12 and the 24) and the Babylonians (the 60). There’s nothing “natural” about 24 hours: it’s pure history and convention. A reminder that sometimes the way we organize the modern world comes directly from the ideas of very ancient civilizations.