Ken Baker: Why 24 hours to the day and 60 minutes to an hour?
Inventor of the clock: "There will be 12 numbers on the face of the clock."
Your average citizen: So the day will be divided into 12 segments?
“No, twenty-four.” Oh. And the day starts at one?
“No, twelve, which is at night.” Excuse me?
“And 6 on the clock means 30.” Uhh…
It’s a marvel any of us ever learns to tell time. One “day” of 24 hours divided at noon into two 12-hour chunks — why 24 and 12? And each hour consists of 60 minutes with 60 seconds to a minute — why 60? Who thought that was a good idea?
Apparently, it was the ancient Egyptians and Babylonians.
Egyptians and sundials, maybe joints in fingers, responsible for time system
By 1,500 BCE, Egyptians were using sundials to divide the period of daylight into 12 segments. One explanation for their choice of 12 comes from their recognition there are about 12 lunar cycles (new moon to new moon) per year, which is also the reason most early cultures divided the year into 12 or 13 lunar months of 354 or 384 days.
A more entertaining possibility suggests 12 stemmed from the number of joints on the four (non-thumb) fingers of one hand.
I imagine an Egyptian textile merchant around the time of Ramesses II using the thumb of one hand to tally sheaves of flax on the knuckles of his other hand as they’re brought into his warehouse. Well, maybe not. But a counting system based on twelve is called a duodecimal system.
The Egyptians also divided the dark hours into 12 sections based on the appearance in the night sky of 12 stars as the night advanced. So with 12 hours of daylight and 12 of night, the 24 hour day was established.
In this system, an hour of daylight in the summer would be longer than one in the winter. Though a problem, the use of hours with a fixed length didn’t gain widespread acceptance until the invention of mechanical clocks in the 1300s.
12-hour clock, 24-hour clock: Guess which is used more
As nearly as I can determine, here are the 20 countries that primarily rely on the 12-hour clock (using AM and PM designations for before and after noon): the U.S, Ireland, India, Pakistan, Mexico, El Salvador, Honduras, Nicaragua, Colombia, Canada (but not Quebec), Egypt, Saudi Arabia, Jordan, Bangladesh, Australia, New Zealand, Singapore, Malta, Malaysia, and the Philippines.
Although the 24-hour clock is the norm in the other 170 or so countries — what we call 3 p.m., they call 15:00 — folks in a fair number of these places (the U.K. and South Africa, for example) commonly use AM and PM times in informal conversations. And even in solidly 24-hour clock countries, almost all analog clocks and wristwatches read 1 to 12.
For unclear reasons, some 3,800 years ago the Babylonians developed an affection for the sexagesimal system of counting based on the number 60. Perhaps it had something to do with the fact that 60 is evenly divisible by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30.
The Greek astronomer, Eratosthenes (276-194 BCE), adopted the Babylonians' system not for time but for dividing a circle into 60 parts. A few decades later, Hipparchus increased the divisions to 360 degrees, and in 150 AD Ptolemy divided each degree into 60 smaller parts — partae minutae primae, which is where the word “minute” came from. Each minuta was further divided into 60 even smaller parts — partae minutae secundae, source of the word “second.”
Clocks got faces in late 1500s
Although angles and measures of latitude and longitude have relied on the sexagesimal system ever since, it wasn’t until the late 1500s that clocks began displaying minutes on their faces.
Modern astronomers standardized the length of a second to 1/86,400 of the time it took the earth to complete one revolution on its axis. In the 1940s, after it was determined the Earth’s rate of rotation is slowly decreasing, the second was redefined to equal 1/31,556,925.9747 of the time it takes Earth to complete an orbit around the sun.
Still not accurate enough for modern technological needs, in 1967 the current definition of a second in time was established as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K and at mean sea level).
Clear enough?
One final twist: So a second now represents a set, invariable length of time. But between 1972 and 2022, there have been 27 minutes that have contained 61 seconds. While there are exactly 86,400 atomic-clock based seconds in a 24-hour day, the actual time for the earth to complete one rotation is slightly longer than 86,400 seconds. So every now and then, a “leap second” has to be added to a day.
So that’s a bit on how we measure time. But how do we know time? Are we alone in understanding the concepts of past and future, or can animals sense the passage of time too?
Next time.
Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.
This article originally appeared on Fremont News-Messenger: Timing is everything: Why 24 hours to day and 60 minutes to hour?
