How Come There Are 60 Minutes In An Hour

An interesting story I learned recently while reading the excellent book Time’s Pendulum by Jo Ellen Barnett is the history of how we came to have 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. Apparently, the 24-hour convention began in ancient Egypt, and the sixty divisions of minutes and seconds derive from the sexagesimal (base 60) number system of the Mesopotamians. (Fractional numbers had not been invented by the time of the Mesopotamians and they favoured whole numbers that can be divided in several different ways. The number 60, being divisible by 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30, turned out to be a convenient choice.)

It’s remarkable to me how everyday items we take completely for granted often have a historical context that goes back thousands of years. As Barnett thoughtfully pointed out: “Had history gone differently, our time system might have been the decimal (based on ten) which we use for most of our other measurements, or the digital (based on two) of our computers, or even the vigesimal system (based on twenty) of the Maya, but what lives on in our clock time is the sexagesimal system of the ancient Fertile Crescent.”

The other mental model I took away from the story is the staying power of cultural habit. For a short period after the French Revolution, from 1793 to 1795, the leaders of the Revolution changed the time division to a decimal system: 10 hours in each day, 100 minutes in each hour, and 100 seconds in each minute. This proved difficult for people and clocks were made with both decimal time and sexagesimal time for a while. After less than 3 years of experimentation and no doubt a lot of attendant confusion, the Revolutionaries gave up and France reverted to the old system.


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