Sunday, May 18, 2014

http://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/

Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day?

Michael A. Lombardi, a metrologist in the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colo., takes the case.
In today's world, the most widely used numeral system is decimal (base 10), a system that probably originated because it made it easy for humans to count using their fingers. The civilizations that first divided the day into smaller parts, however, used different numeral systems, specifically duodecimal (base 12) and sexagesimal (base 60).
Thanks to documented evidence of the Egyptians' use of sundials, most historians credit them with being the first civilization to divide the day into smaller parts. The first sundials were simply stakes placed in the ground that indicated time by the length and direction of the resulting shadow. As early as 1500 B.C., the Egyptians had developed a more advanced sundial. A T-shaped bar placed in the ground, this instrument was calibrated to divide the interval between sunrise and sunset into 12 parts. This division reflected Egypt's use of the duodecimal system--the importance of the number 12 is typically attributed either to the fact that it equals the number of lunar cycles in a year or the number of finger joints on each hand (three in each of the four fingers, excluding the thumb), making it possible to count to 12 with the thumb. The next-generation sundial likely formed the first representation of what we now call the hour. Although the hours within a given day were approximately equal, their lengths varied during the year, with summer hours being much longer than winter hours.
Without artificial light, humans of this time period regarded sunlit and dark periods as two opposing realms rather than as part of the same day. Without the aid of sundials, dividing the dark interval between sunset and sunrise was more complex than dividing the sunlit period. During the era when sundials were first used, however, Egyptian astronomers also first observed a set of 36 stars that divided the circle of the heavens into equal parts. The passage of night could be marked by the appearance of 18 of these stars, three of which were assigned to each of the two twilight periods when the stars were difficult to view. The period of total darkness was marked by the remaining 12 stars, again resulting in 12 divisions of night (another nod to the duodecimal system). During the New Kingdom (1550 to 1070 B.C.), this measuring system was simplified to use a set of 24 stars, 12 of which marked the passage of the night. The clepsydra, or water clock, was also used to record time during the night, and was perhaps the most accurate timekeeping device of the ancient world. The timepiece--a specimen of which, found at the Temple of Ammon in Karnak, dated back to 1400 B.C.--was a vessel with slanted interior surfaces to allow for decreasing water pressure, inscribed with scales that marked the division of the night into 12 parts during various months.
Once both the light and dark hours were divided into 12 parts, the concept of a 24-hour day was in place. The concept of fixed-length hours, however, did not originate until the Hellenistic period, when Greek astronomers began using such a system for their theoretical calculations. Hipparchus, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days. Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries. (Hours of fixed length became commonplace only after mechanical clocks first appeared in Europe during the 14th century.)
Hipparchus and other Greek astronomers employed astronomical techniques that were previously developed by the Babylonians, who resided in Mesopotamia. The Babylonians made astronomical calculations in the sexagesimal (base 60) system they inherited from the Sumerians, who developed it around 2000 B.C. Although it is unknown why 60 was chosen, it is notably convenient for expressing fractions, since 60 is the smallest number divisible by the first six counting numbers as well as by 10, 12, 15, 20 and 30.
Although it is no longer used for general computation, the sexagesimal system is still used to measure angles, geographic coordinates and time. In fact, both the circular face of a clock and the sphere of a globe owe their divisions to a 4,000-year-old numeric system of the Babylonians.
The Greek astronomer Eratosthenes (who lived circa 276 to 194 B.C.) used a sexagesimal system to divide a circle into 60 parts in order to devise an early geographic system of latitude, with the horizontal lines running through well-known places on the earth at the time. A century later, Hipparchus normalized the lines of latitude, making them parallel and obedient to the earth's geometry. He also devised a system of longitude lines that encompassed 360 degrees and that ran north to south, from pole to pole. In his treatise Almagest (circa A.D. 150), Claudius Ptolemy explained and expanded on Hipparchus' work by subdividing each of the 360 degrees of latitude and longitude into smaller segments. Each degree was divided into 60 parts, each of which was again subdivided into 60 smaller parts. The first division,partes minutae primae, or first minute, became known simply as the "minute." The second segmentation, partes minutae secundae, or "second minute," became known as the second.
Minutes and seconds, however, were not used for everyday timekeeping until many centuries after the Almagest. Clock displays divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60. In fact, the hour was not commonly understood to be the duration of 60 minutes. It was not practical for the general public to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. Even today, many clocks and wristwatches have a resolution of only one minute and do not display seconds.
Thanks to the ancient civilizations that defined and preserved the divisions of time, modern society still conceives of a day of 24 hours, an hour of 60 minutes and a minute of 60 seconds. Advances in the science of timekeeping, however, have changed how these units are defined. Seconds were once derived by dividing astronomical events into smaller parts, with the International System of Units (SI) at one time defining the second as a fraction of the mean solar day and later relating it to the tropical year. This changed in 1967, when the second was redefined as the duration of 9,192,631,770 energy transitions of the cesium atom. This recharacterization ushered in the era of atomic timekeeping and Coordinated Universal Time (UTC).
Interestingly, in order to keep atomic time in agreement with astronomical time, leap seconds occasionally must be added to UTC. Thus, not all minutes contain 60 seconds. A few rare minutes, occurring at a rate of about eight per decade, actually contain 61.
References
  • Time's Pendulum. Jo Ellen Barnett. Plenum Press, 1998.
  • A History of Mathematics. Florian Cajori. MacMillan and Co., 1894.
  • History of the Hour. Gerhard Dohrn-van Rossum. University of Chicago Press, 1996.
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IndigoDingoOctober 24, 2009, 3:08 PM
so if the Egyptians observed a group of 36 stars that divided the circle of heavens into equal parts, could this also be related to 360 degrees in a circle? Just a thought...
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Mano57January 22, 2010, 6:22 AM
The concept of fixed-length hours, however, did not originate until the Hellenistic period, when Greek astronomers began using such a system for their theoretical calculations. Hipparchus, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days. Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries. (Hours of fixed length became commonplace only after mechanical clocks first appeared in Europe during the 14th century.)
I am sorry it is totally a baseless darkage european invention.
check Persian nowrouz from 600BC
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Mano57January 22, 2010, 6:26 AM
The concept of fixed-length hours, however, did not originate until the Hellenistic period, when Greek astronomers began using such a system for their theoretical calculations. Hipparchus, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days. Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries. (Hours of fixed length became commonplace only after mechanical clocks first appeared in Europe during the 14th century.)
I am sorry to say that these statements and alike are made up mostly during darkages of europe
Chinese Indians and persians 800 to 550 BC had already a very developed time division based calander specially the Koshanian and Hakhmaneshiyans.
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Jose Garcia LTJanuary 14, 2011, 6:37 PM
Time has to be logic, simple and easy to learn. The 24 hour system is not logic, simple and easy to learn. So I developed a new time system that I called "Logic Time" (LT). It corrects most of the "build in errors" of our conventional 24 hours system. The space that is available on this page isn't enough to explain in detail how LT works so if you are interested in knowing more about it take a look at an online example of a LT clock (with detailed explanation).http://dhost.info/jose/Logic_Time_v3.00_DTD_html_4.01.html
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infosFebruary 2, 2011, 2:19 AM
We cannot completely explain anything considering time measuring. That's could be clear on paper but in real life its not. How many <a style="color: black; text-decoration: none;" rel="dofollow" href="http://www.uk-dissertation.com">dissertations</a> and projects have been written on this theme, and have they really revealed relative nature of time? No. Because the matter of "time" is purely subjective for every person, regardless of its measuring.
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MuddlerJuly 14, 2011, 8:48 PM
This is pure speculation but just suppose, way back when, the average heart beat was around once per second. No one would have known this but somebody got a wild hair about the heartbeat, probably a profound personal and spiritual part of ife, and a tribal member was chosen.
Here’s my alternative:
Consider one pair of hands – use them to count seconds. Two hands count 10 beats (the second hands). A second pair of hands is added. The third hand counts multiples of 10 heartbeats up to 50. On the 60th count, the first three hands revert to no counts and the fourth becomes the ‘minute hand’. A third set of hands is added. Each time the previous four hands accumulate a maximum number of counts, the fifth and sixth hand accumulate a six minute block. A fourth set of hands is added. The seventh hand becomes the ‘hour hand’. The eighth hand accumulates blocks of 6 hours. This starts at sunrise or sunset and concludes at the same point of the next day. It is repeated over the seasons from solstice to solstice for years/decades until an accepted total count per day is established and the method refined for repeatability. Over generations, the concepts are proven and easily passed on by the use of finger counts without the difficulty of mechanically manipulating finger joints and the difficulty of perceiving the movement of those joints.
As with most of my ideas, I am probably full of crap but I hope my idea can generate a discussion or two. If nothing else, maybe someone can, gently, show me the error of my ways.
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nuiunNovember 18, 2011, 2:56 AM
I'm unpleasantly surprised to note that most of the comments here contain a vast amount of spelling errors. Here I was thinking that SciAm readers were literate. Anyway, very interesting article. However there is a mistake. It is mentioned above that Egyptians were the first civilization to break the day into smaller parts. What about the Sumerians? They were around long before Egypt, and from what I read they were very advanced for their time. Surely they would have used complex measurements of time and split their days into segments, but I am guessing now. The other correction I am not guessing the answer: Egyptians liked to put their deeds on the walls of their buildings in great artistic detail. In the Great Pyramid at Giza there is a wall painting that depicts either Anubis or Horus holding up what looks like an elongated light bulb. It even shows the little copper coils inside and a wire coming out of the end of the bulb. Inside this room there are no carbon marks on the walls or ceiling from torches, and the room itself is so isolated in the pyramid that even with their best mirrors, reflected sunlight would not be sufficient. Related to this, has anyone heard of the Baghdad Battery? Archaeologists a few years back discovered a clay jar that contained layers of alkaline metals and acids, penetrated by two copper wires which protruded from the lid of the jar. When applied to a voltmeter, this object gave off 4.something volts of electricity. They dated the jar to be about 4,000 (or was it 14,000?) years old. Google this, I'm not bullshitting you. If people 4,000 years ago could make a battery, I'm sure the 'almighty' Egyptians could figure out how to make artificial light.
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barryb83110March 1, 2012, 9:05 PM
Perhaps the measurement of time, and the degrees of a circle, both of which lend themselves to navigation were taught to us by a civilization that had 12 fingers, hence a base 12 compared to our base 10 which I assume is because of our 10 digits. I am on this site because I was wondering why a circle is measured by 360 degrees and not 100. I've read the remarks about planets and they could have easily chosen 10 planets to denote the passage of the night rather than 12. Planets lining up every 60 years makes some sense but it seems a stretch as a reason for 360 degrees in a circle.It's not like I'm saying 12 fingers isn't a stretch, but there you go.
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mcsean2163November 29, 2012, 8:45 AM
Actually, as early as 1500 B.C., the Egyptians had developed a more advanced sundial. A T-shaped bar placed in the ground, this instrument was calibrated to divide the interval between sunrise and sunset into 14 parts.
However, Theodontus the Weird, proposed that rather than 14 parts it should be 12. He was givent the appellation, "the Weird", due his a strange genetic abnormality which he had inherited from his mother, namely he had an extra digit on each hand.
He was also a peerless warrior, this ability also being attributed to his extra digits. He swore vengeance of the 14 parters and left a terrible legacy of slaughter in his wake. In twelve days all would divide the sunset into 12 parts.
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brettc286

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