Ken Baker, Ph.D.
clock maker: “There will be 12 numbers on the clock face.”
Your average citizen: So the day will be divided into 12 segments?
“No, twenty-four.” Oh. And the day starts at one o’clock?
“No, twelve, and it’s at night.” Excuse me?
“And 6 on the clock means 30.” Uh …
It’s a marvel that any of us learn to tell time. One 24-hour “day” at noon is divided into two 12-hour parts – why 24 and 12? Each hour consists of 60 minutes by 60 seconds to a minute – why 60? Who thought that was a good idea?
Apparently, it was the ancient Egyptians and Babylonians.
The Egyptians and sundials were probably joints in the fingers responsible for the order of time
By 1500 BC, the Egyptians were using sundials to divide the daylight period into 12 segments. One explanation for their choice of 12 comes from the realization that there are about 12 lunar cycles (new moon to new moon) each year, which is also why most early cultures divided the year into 12 or 13 lunar months of 354 or 384 days.
A more entertaining possibility suggests that 12 stem from the number of knuckles of the four fingers (not the thumb) of one hand.
I imagine an Egyptian textile merchant around the time of Ramses II using the thumb of one hand to sort bundles of linen over the knuckles of his other hand as he brought them to his warehouses. Well, maybe not. But the counting system based on twelve is called the twelve system.
The Egyptians also divided the dark hours into 12 divisions based on the appearance in the night sky of 12 stars as the night progressed. So with 12 hours of daylight and 12 hours of night, 24 hours a day was created.
In this system, the daylight hour in summer is an hour longer than in winter. Although the use of fixed-length clocks is problematic, it was not until the invention of mechanical clocks in the 13th century that it gained widespread acceptance.
12-hour and 24-hour format: guess which one to use the most
As far as I can tell, here are 20 countries that rely primarily on the 12-hour clock (using AM and PM designations for pre- and post-noon): United States, Ireland, India, Pakistan, Mexico, El Salvador, Honduras, Nicaragua, Colombia, Canada (but not Quebec), Egypt and Saudi Arabia Jordan, Bangladesh, Australia, New Zealand, Singapore, Malta, Malaysia and the Philippines.
Although a 24-hour system is the norm in 170 other countries or so – what we call 3 PM, they call 15:00 – people in a good number of these places (UK and South Africa, for example) typically use AM and PM times in informal conversations. And even in 24-hour countries, almost all analog clocks and wristwatches read from 1 to 12.
For reasons that are unclear, about 3800 years ago, the Babylonians developed an aversion to a sexagesimal counting system based on the number 60. It probably had something to do with the fact that 60 is evenly divisible by 1, 2, 3, 4, 5, 6, 10, 12, 15 and 20 and 30.
The Greek astronomer, Eratosthenes (276-194 BC), adopted the Babylonian system not for time but for dividing the circle into 60 parts. A few decades later, Hipparchus increased the divisions to 360 degrees, and in AD 150, Ptolemy divided each degree into 60 smaller parts – win the first minute, where the word “minute” came from. All the moment It was further divided into 60 smaller parts – The second minute was wonthe source of the word “second”.
Watches got faces in the late fifteenth century AD
Although the angles and measurements of longitude and latitude have been based on the sexagesimal system since then, it was not until the late 15th century that clocks began displaying minutes on their faces.
Modern astronomers have limited 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 that the Earth’s rotation rate was slowly decreasing, the second was redefined to be 1/31,556,925.9747 of the time it takes the Earth to complete an orbit around the Sun.
Still not sufficiently accurate for modern technological needs, in 1967 the current definition of the time second was established as “the duration of 9,192,631,770.” periods of radiation corresponding to the transition between the two Ultra-fine levels from the state of the earth from Cesium 133 atom” (at a temperature 0 k And on average sea level).
One final development: the second now represents a fixed set of time. But between 1972 and 2022, there were 27 minutes that contained 61 seconds. While there are 86,400 seconds based on the atomic clock exactly 24 hours in a day, the actual time for Earth to complete one cycle is slightly longer than 86,400 seconds. So every now and then a “leap second” should be added to a day.
This is a bit of how we measure time. But how do we do that I know time? Are we alone in understanding notions of the past and the future, or can animals sense the passage of time as well?
Ken Baker is a retired professor of biology and environmental studies. If you have a topic about natural history that you would like Dr. Becker to consider in an upcoming column, please email your idea email@example.com.