*Hanging Garden of Babylon*

**Editor’s Note: While we were researching about watches and clocks some years ago, we cannot help but to question who designed the notion that a day consists of twenty four hours, an hour of sixty minutes and a minute of sixty seconds. Why, after all, a time measurement has to be divided by twelve and not ten? As such, we commissioned a researcher to create an article for us in hope to bring closure to our question.**

**Written By: Anantari**

The hour is a legal unit of time and describes the twenty-fourth part of a day. But why is an hour divided into 60 minutes? Wouldn't it be much easier if an hour had 10 "deci-hours" and 100 "centi-hours" (instead of 60 minutes and 3,600 seconds)? How do we come up with the number 60?

**How did the timing as we know it today come about?**

Physics teaches that there is no absolute time. So there is no super clock somewhere in the cosmos to which everything is aligned. Time is experienced subjectively. However, the human community and its needs require a formal framework to work. Its development runs through thousands of years. In the course of history people have always divided the day differently, measured hours differently depending on the season and time system.

What is documented is that the ancient Babylonians began to divide time as we know it today. They divided the year according to the moon cycles into twelve months, day and night into twelve parts of equal length = 24 hours.

**Why 12 and 60 as base numbers around the time?**

The Babylonian period was a golden age for astronomy and mathematics. The number 12 also had religious significance for the Babylonians and they considered it a perfect number. In everyday life, they counted to twelve with the three knuckles of their four fingers, using the thumb as a pointer. Meanwhile 60 is a multiple of 12, a highly composite number with a total of 12 divisors, making it much easier to calculate with. This so-called sexagesimal system was already used by the Sumerians in the third millennium B.C. and passed on to the Babylonians. The ancient Greeks used it to divide a circle into 60 parts and devised an early system of longitude and latitude, the basis for minutes and seconds as we define them today.

**Count with your fingers**

It's easy, just count on your fingers! You only count to 10.

However, the Babylonians, who lived from 3,000 to 500 B.C. living in Mesopotamia could count to 12 with one hand and 60 with their two hands. How did you do that? Very simple, with one hand they counted the segments from the other 4 fingers with their thumbs (4 fingers x 3 segments = 12). Using the fingers of the second hand, they counted the number of times they had in hand one (12 segments) times 5 fingers in the hand two

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**Calculate the number 60**

To represent a number in writing, we still use the 10 different "Arabic numerals" (1, 2, 3, 4, 5, 6, 7, 8, 9, 0), and only after the number 9 is a digit added (the number "9" has 1 digit and the number 10 has two digits).

In simplified form, one can imagine the Babylonians as having 59 digits to write their numbers and only starting with the number 59 were they extended by one digit. According to this, the number 124 for the Babylonians was made up of two times 60 plus 4 (and not, as for us, of (1x10^{2}) + (2 x10^{1}) + 4). So the Babylonians had a sexagesimal system (base 60 number system) compared to our decimal system (base 10 number system).

For many centuries, minutes and seconds had no meaning in everyday life, they were mainly mathematical quantities and were used for astronomical calculations. They only became interesting for timekeeping with the invention of correspondingly more precise clocks.

*Trivia*

With the introduction of the French Revolutionary Calendar in 1792, an attempt was made to switch the division of a day to the decimal system. A day was divided into ten hours, one hour into one hundred minutes, and one minute into one hundred seconds. According to this, a day had 100,000 seconds compared to 86,400 seconds of conventional definition. For the smallest unit of time, this decimal time had an imperceptible effect, being only about 15% shorter. The effects on the next two higher time units were significantly stronger. The decimal minute was almost one and a half times as long, the decimal hour almost two and a half times as long.However,this "heurerépublicaine" (republican hour) could never be implemented in practice and because of this 1 hour still has 60 minutes today.

**One Second’s Definition**

Since 1967, one second has been 9,192,631,770 times the period of radiation corresponding to the transition between the two hyperfine levels of the ground state of atoms of the 133 Cs nuclide. With the revision of the SI in 2019 by the 26th General Conference on Weights and Measures, the exact wording of the definition was changed, but the content of the definition of the second remained.

So, by definition, the second is the multiple of the period of a microwave that resonates with a selected level transition in the cesium atom. Therefore it is called the atomic second. Atomic clocks are based on measuring this transition. Their precision has been increased by more than four powers of ten to 10^{-16} since the definition is given.

Until 1967, the second was based on astronomical measurements:

- Solar second (until the 1950s)

The 1/86,400 fraction of the mean solar day. This definition was introduced so that an average solar day is 24x60x60 seconds. This corresponds to the time after which a fictitious mean sun is back in the same place. (The solar day is about 4 minutes longer than the Earth's rotation period because the Earth moves around the Sun during the day and therefore it takes a little longer for a point on Earth to face the Sun again.)

- Ephemeris second

The fraction 1/31,556,925.9747 of the tropical year on January 0, 1900 (= December 31, 1899) at 12 p.m. UT. It refers to the relationship between the length of the year and the rotation of the earth at that time. The ephimeris second was defined as a second by the International Committee of Weights and Measures (CIPM) in 1956.

**Why do most clocks show 12 instead of 24 hours?**

The dial with a 12-hour display has prevailed for showing the time. The reasons for this have grown historically and are probably more of a practical than a logical nature. Tower clocks used to announce the time every hour with the chimes of the clock, and counting the late hours precisely would have required the utmost concentration. In addition, a dial with a few subdivisions also made it easier to read from a distance. Since humans are known to be creatures of habit, the dial with a 12-hour display was ultimately able to assert itself. For a uniform understanding on a global level, the day is nevertheless divided into 24 clear hours by the coordinated universal time (UTC), which clearly separates day and night times.

**Future Developments**

The precision with which the second can be realized (as of 2018: approx.), limits the achievable accuracy of time measurements in the unit “second”. Clocks have now been developed that are not based on cesium atoms and are up to two orders of magnitude more precise. With such clocks as the primary reference, even more accurate time measurements could be achieved. A prerequisite for this would be a redefinition of the unit of measurement. This could happen in 2030.

Minute, hour and day are common as larger units of time and are legal units of measurement in the European Union and Switzerland. Magnifying SI prefixes such as "megasecond" (= 1,000,000 s = 11 days 13 h 46 min 40 s) are permissible, but very unusual. Derived from the second are the units "Hertz" (1 Hz = 1 s^{-1}) for periodic events and "Becquerel" (1 Bq = 1 s^{-1}) for radio activity.

**REFERENCES**

Consulting Committee for Units (CCU). (2016). Report of the 22nd meeting. Chapter 11.1: Conditions and timeline for the redefinition of the second. Bureau International des PoidsetMesures.

Draft Resolutions for the 27th General Conference on Weights and Measures in November 2022. (2022).BIPM.

Humphrey, I. (2020). Why 1 minute has 60 seconds, 1 hour has 60 minutes, and 1 day has 24 hours? Retrieved from https://tipsmake.com/why-1-minute-has-60-seconds-1-hour-has-60-minutes-and-1-day-has-24-hours.

Kinnebrock, W. (2012). "What does time do when it passes? How science explains time". Verlag CH Beck, 2nd edition.

Resolution 1 of the 13th CGPM. (1967). SI units of time (seconds). Bureau International des PoidsetMesures.

Resolution 1 of the 26th CGPM. On the revision of the International System of Units (SI). Bureau International des PoidsetMesures.

Resolution 9 of the 11th CGPM. (1960). Definition of the unit of time (second). Bureau International des PoidsetMesures.

SI Brochure: The International System of Units (SI), 9^{th}ed. In: bipm.org. Bureau International des PoidsetMesures.

SI Brochure 9th Edition. (2019). Appendix 4, Part 1. Bureau International des PoidsetMesures.

Wikiwand. 24-hour analog dial. Retrieved from https://www.wikiwand.com/en/24-hour_analog_dial.