#130 Why do we have 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute?

I learned this today. We have 24 hours in a day because of Hipparchus. We have 60 seconds in a minute and 60 minutes in an hour because of Claudius Ptolemy, Al-Biruni, and the Babylonians.

Before we look at this topic, we have to remember that needing to know the time exactly is a fairly modern concept. The first clock that could accurately use seconds was invented in 1560. They weren’t widely used. Christiaan Huygens invented the pendulum clock in 1656 that could accurately tell seconds. John Harrison’s maritime chronometers could accurately tell seconds and they were used for finding longitude on boats. However, even then, most people didn’t need to know the time down to the minute. People working on farms would be ok with just knowing the hour, and often they wouldn’t even need to know that. They could see where the sun was. Once industrialization happened, knowing the minute as well became necessary for work, but it was the invention of the railway that really made it necessary. Train times had to use minutes in order to run on time. This is also the reason time zones started. Up until this point, towns generally used local time. From this point on, time was centralized.

Why do we have 24 hours in a day? Well, in the beginning, we didn’t. The earliest known clocks were sundials from 1500 BC, found in Ancient Egypt and Ancient Babylonia. They divided the daytime up into 12 sections. This comes from their use of the duodecimal system (base 12) as opposed to the decimal system (base 10) that we use. Nobody really knows why they used this system, but it could be something as simple as fingers. We use a decimal system because we have ten fingers. However, each finger is divided up into three joints so, using the thumb, it is possible to count to 12 on one hand. 12 is also a more convenient number than 10 because it has more divisors. 10 is divisible by 10, 5, 2, and 1. 12 is divisible by 12, 6, 4, 3, 2, and 1.

The Greeks inherited these sundials and developed them. One obvious problem with the sundial is that it only tells the time during the day. Another problem is that the length of the “hours” it tells will change with the seasons. You will have longer hours in the summer and shorter hours in the winter. However, again, these are both modern problems. If most people are involved with agriculture, then there is no need to know what time it is outside of morning or afternoon. And, with the only lighting source being expensive candles, there is no need to know the time through the night. People slept at sundown and woke up at sunup.

The Egyptians used stars to mark time through the night. There were 18 stars that appeared at different times throughout the night. 12 of these appeared during the period of total darkness. The invention of the water clock (about 1600 BC) could also be used through the night.

Hipparchus, a Greek astronomer and mathematician, proposed dividing the day into 24 hours. He lived from 190 to 120 BC. His ideas were accepted, but largely ignored by most people who still had no need of time.

In Hipparchus’ time, the Greeks were working with Babylonian math and astronomy, which is why the sexagesimal system was kept. Eratosthenes had used it to divide a circle into 60 parts, making rough lines of latitude. Hipparchus built on this to continue the system of longitude and latitude. He used 360 degrees. 300 years later, in 150 AD, Claudius Ptolemy divided each of the 360 degrees into 60 smaller parts that he called partes minutae primae and then he divided those 60 parts into partes minutae secundaei. He didn’t have timekeeping in mind when he made these, but that is where the words “minute” and “second” come from.

Iranian scholar Al-Biruni was the first person to divide an hour into minutes and seconds in about 1000 AD. He was tabulating the times of new moons and he wanted a more specific time than the hour. He divided the hour up into minutes, seconds, and then divided those up into thirds and fourths.

It was another 500 years before a clock could be made that could tell minutes instead of just the hour. And another 100 years after that before a clock could tell seconds. And yet, as I have said, most people still had no need of the time. It would take the invention of the railway before time telling became ubiquitous.

The definition of the second has changed over time. Today, it is defined as 9,192,631,770 energy transitions from a cesium atom. This is used in an atomic clock.

So, we have 24 hours in a day because of the Egyptians, Babylonians, and Greeks. We have 60 minutes in an hour and 60 seconds in a minute because of the Babylonians, the Greeks, and an Iranian scholar. And this is what I learned today.

Photo by Stas Knop from Pexels

Sources

https://en.wikipedia.org/wiki/Hour

https://en.wikipedia.org/wiki/Hipparchus

https://en.wikipedia.org/wiki/Minute

https://en.wikipedia.org/wiki/Second

https://www.theguardian.com/notesandqueries/query/0,5753,-1487,00.html

https://www.livescience.com/44964-why-60-minutes-in-an-hour.html