## March 28, 2008

### 24 Hours

Stupid Question ™
Aug. 30, 2001
By John Ruch

Q: Why is a day divided into 24 hours rather than, say, 9 zorlons? Does it have anything to do with the 12-month year?
—H.C.

A: We get our 24-hour day from ancient Rome, which got it from Greece, which got it from Egypt. Likewise, the word “hour” comes via the Romance languages from the Latin hora, which came from a Greek word for “season,” “hour” or “auspicious time.”

The practice of chopping the day up into relatively discreet units based on the general position of the sun in the sky is commonplace. We still use the primitive divisions of “morning,” “afternoon” and “evening.”

A later development was the “mathematical hour,” in which the day is divided more precisely into an even number of abstract units, often with each unit assigned a number. This made time-keeping and calendar calculation possible, and is known as far back as ancient Babylon.

Our 24 hours apparently come directly from ancient Egypt—by most accounts, the only ancient society to use a 24-unit division. By some guesses, they divided the day into 10 hours of daylight, two hours of twilight and 12 hours of night.

Why 24? The number is a multiple of 12, which is mathematically convenient as a basis for measuring systems (even more convenient than 10, because it has four divisors besides itself and 1). That’s why the Romans used fractions of 12 in their monetary system and some measurements, and possibly why 24-hour time felt natural to them.

However, it’s also possible that 24 hours indeed relate mathematically or numerologically to the 12-month year.

Nearly every major calendar—including the Roman and Egyptian ones—has had 12 months. That’s because most calendars originally tried to measure both seasonal (solar) and lunar changes, and on average, there are 12 complete cycles of the Moon (a “month”) for every one cycle of the seasons (a solar year, or about 365 days). Likewise, most calendars have used months of about 30 days, which is the average number of days in a lunar cycle.

Cross-cultural comparisons show that days were often divided into a number of units that was a multiple of 12 and/or 30. The Babylonians and ancient Chinese had 12-“hour” days, some parts of India had 30 and Jewish reckoning sometimes uses 1,080. (Some claim that Babylon later used 24 “hours,” too.)

It’s also possible that the Babylonians used a base-60 number system because of astronomical symbolism. (Among other things, 60 is the lowest common multiple of 12 and 30.) Incidentally, their base-60 system remained a mainstay of astronomy and is where we get our 60 minutes per hour and 60 seconds per minute.

However, there are counter-examples: the 20-day “month” of the Mayan calendar is pretty clearly a result of their base-20 number system, not the cause of it.

In any case, astronomical matters were always treated in ancient societies with reverence and mysticism. So the number of “hours” in day undoubtedly meant something—be it easy math or celestial metaphor.