Why are there 24 hours in a day?While each country has (in broad terms) historically had distinct measurements for distance, weights etc the method of splitting the day into 24 hours, one hour into 60 mins and one minute into 60 seconds seems to be the only one in use, and indeed to me the only one I know of. This non-metric measurement of time is far from ideal, but what other comparably accurate methods have been used historically?"The origin of our time system of 24 hours in a day with each hour subdivided into 60 minutes and then 60 seconds is complex and interesting" says Dr Nick Lomb, consultant curator of astronomy, from the Sydney Observatory.

Our 24-hour day comes from the ancient Egyptians who divided day-time into 10 hours they measured with devices such as shadow clocks, and added a twilight hour at the beginning and another one at the end of the day-time, says Lomb. "Night-time was divided in 12 hours, based on the observations of stars. The Egyptians had a system of 36 star groups called ‘decans’ - chosen so that on any night one decan rose 40 minutes after the previous one. Tables were produced to help people to determine time at night by observing the decans. Amazingly, such tables have been found inside the lids of coffins, presumably so that the dead could also tell the time”.

In the Egyptian system, the length of the day-time and night-time hours were unequal and varied with the seasons.

"In summer, day-time hours were longer than night-time hours while in winter the hour lengths were the other around," says Lomb.

Ancient Babylonians: hours and minutesThe subdivision of hours and minutes into 60 comes from the ancient Babylonians who had a predilection for using numbers to the base 60. For example, III II (using slightly different strokes) meant three times 60 plus two or 182.

"We have retained from the Babylonians not only hours and minutes divided into 60, but also their division of a circle into 360 parts or degrees" says Lomb. "What we have not retained is their division of a day into 360 parts called ‘ush’ that each equalled four of minutes in our time system".

Lomb says it’s likely that the Babylonians were interested in 360 because that was their estimate for the number of days in a year. Their adoption of a base 60 system was probably allowed them to make complex calculations using fractions.

Ancient ChineseThe ancient Chinese used a dual time system where they divided the day into 12 so-called, ‘double hours’, originally with the middle of the first double hour being at midnight. They also had a separate system in which a day was divided into 100 equal parts called ‘ke’, that are sometimes translated as ‘mark’ into English.

What complicated this arrangement was that the two systems did not mesh well since there were a non-integral number of ke in each double hour, specifically 8 1/3. Because of this inconvenience, much later on, in the year 1628 of our era, the number of ke in a day was reduced to 96.

Other culturesWhile many cultures had their own calendars, there doesn’t appear to be evidence for equivalent methods for keeping time.

"There is a lot of information available on the Mayan calendar, but I have not seen anything to indicate if, and how, they subdivided the day" says Lomb. "Similarly, though it is well known that the Australian Aboriginal people had seasonal calendars and used the sky to indicate the seasons, I have not seen anything on how they kept time".

Metric time?In 1998, the Swiss watch company Swatch introduced the concept of a decimal Internet Time in which the day is divided into 1000 ‘beats’ so that each beat is equal to 1 minute 26.4 seconds. The beats were denoted by the @ symbol, so that, for example, @250 denotes a time period equal to six hours.

"So far this system has not caught on" says Lomb.

"For each country the immense cost and difficulty in switching over to this or another metric time system would be enormous, possibly as great, if not greater, than it was for Australia to switch to decimal currency back in 1966" he says. "The insurmountable difficulty though would be the prior hurdle of getting each country in the world both to agree to change and to agree on a common system of decimal time. I think that I am safe in stating that there will be no change from the present system of time measurement in the foreseeable future".

Keeping timeWhile our units for measuring time seem to be here to stay, the way we measure time has changed significantly over the centuries. The Ancient Egypitians used sundials and waterclocks, as did several civilisations after them. Hourglasses were also an important time-keeping device before the invention of mechanical and pendulum clocks. The development of modern quartz watches and atomic clocks has enabled us to measure time with increasing accuracy. Today, the standard definition for time is no longer based on the rotation of the Earth around the Sun, but on atomic time. A second is defined as: “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.”

Read more about keeping time. [→ ABC]

(via understandingtheuniverse)