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Why a day is divided by 12/24 hours? Why the number 12? Why not using 10 or 6 or 14, 16? Who invented this? Any physical reasons behind this?

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Related: physics.stackexchange.com/q/57949/2451 –  Qmechanic Mar 27 '13 at 16:43
If you're concerned about the division like "Who did this?", then it totally goes to the Egyptians because they used the 12-based counting system ;-) –  Waffle's Crazy Peanut Mar 27 '13 at 16:50
No offense, we Chinese use 12 (Chinese zodiac) to mark time for a long time. It seems that number "12" is a common observation all over the world. –  Daniel Mar 27 '13 at 17:01
@Daniel: Maybe you're right. Zodiac uses 12 constellations generally (not only Chinese) for astrological purposes. Guessing that maybe the secret ;-) –  Waffle's Crazy Peanut Mar 28 '13 at 6:16
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4 Answers

up vote 7 down vote accepted

I'm guessing they started with inscribing a regular hexagon in a circle. Next draw the three diagonals, which are also diameters of the circle, and construct for each the perpendicular line through the center. Voila, you've divided the circle into 12 equal sectors. Alternatively, constructing an inscribed regular dodecagon in a circle isn't all that hard either:

enter image description here

By this rationale, 10 = 2*5 wouldn't have been a likely choice because the method of construction of a regular pentagon wasn't discovered until the time of Euclid, and 14 = 2*7 would be most inconvenient because the regular heptagon isn't constructable with compass and straightedge.

Image Source: http://upload.wikimedia.org/wikipedia/commons/2/2b/Regular_Dodecagon_Inscribed_in_a_Circle.gif

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How did you make that gif? It's cool. –  Mike Dunlavey Mar 27 '13 at 21:04
@DavidHammett you should cite sources –  raindrop Mar 28 '13 at 10:37
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12 h divides into many whole numbers and so it is easy to think about portions of a day:

  • A half = 6 h
  • A third = 4 h
  • A quarter = 3 h
  • A sixth = 2 h.

You can't do this with 10 h, 6 h, 14 h, or 16 h as easily.

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Then 36 is a good number too, so we can enjoy more church bells everyday. –  Daniel Mar 27 '13 at 17:07
It'd prefer 10, but there we go. –  user12345 Mar 27 '13 at 17:15
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how about you take a circle and split it in 4 quarters. then try to split each quarter further. Perhaps somebody thought that splitting the quarter to another 3 parts (hence ending up with 12 parts in total) made sense.

Then again, ~360 days per year (yes, I know that this is not accurate) i.e. split the circle by 360, which is divided nicely by 12.

Or just somebody with 6 fingers on each hand.

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Hey, I love your six fingers theory! –  Daniel Mar 27 '13 at 19:39
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It takes nearly 24h for the Earth to make one entire location with respect to a distant star. This makes a day (also knon as a stellar day). If you divide into night part and daily part you will get 12h.

See http://cseligman.com/text/sky/rotationvsday.htm for more info on the Earth rotation.

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This question is not about what sets the length, but why those specific numbers. –  Sparr Mar 28 '13 at 14:15
So the question should be more specific, as the length of the day is fixed. For example it could be "Why we favor a base twelve system in such a case?" –  luksmir Apr 2 '13 at 9:21
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