### How long is a day?

It's the amount of time it takes for the Earth to rotate one time on its axis. But how long does it take the Earth to rotate? That is where things become completely arbitrary. The world has decided to standardize on the following increments:

- A
**day**consists of two 12-hour periods, for a total of 24 hours. - An
**hour**consists of 60 minutes. - A
**minute**consists of 60 seconds. **Seconds**are subdivided on a decimal system into things like "hundredths of a second" or "millionths of a second."

That's a pretty bizarre way to divide a day up. We divide it in half, then divide the halves by twelfths, then divide the twelfths into sixtieths, then divide by 60 again, and then convert to a decimal system for the smallest increments. It's no wonder children have trouble learning how to tell time.

### Why are there 24 hours in a day?

No one really knows. However, the tradition goes back a long way. Take, for example, this quote from Encyclopedia Britannica:

The Babylonians seem to be the ones who started the six fetish, but it is not clear why.

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

Again, it is unclear. It is known, however, that Egyptians once used a calendar that had 12 30-day months, giving them 360 days. This is believed to be the reason why we now divide circles into 360 degrees. Dividing 360 by 6 gives you 60, and 60 is also a base number in the Babylonian math system.

### What do a.m. and p.m. mean?

These abbreviations stand for **ante meridiem**, before midday, and **post meridiem**, after midday, and they are a Roman invention. According to Daniel Boorstin in his book The Discoverers, this simple division of the day into two parts was the Romans' first increment of time within a day:

Modern man bases time on the **second**. A day is defined as 86,400 seconds, and a second is officially defined as 9,192,631,770 oscillations of a cesium-133 atom in an atomic clock.

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