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Why is a day and night divided into 12 hours? Why not 10 hours? What is the significance of 12?

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There may be other reasons, but the most obvious one is that 12 can be evenly divided by 2, 3, 4, and 6. This makes it easier to work with portions of the day or night. (Similar reasons apply to the 360 degrees for angles, and the 1080 chalakim per hour for computing the molad.)

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  • For a system that uses 10 hours, see here: en.wikipedia.org/wiki/Decimal_time
    – Dave
    Dec 21, 2010 at 22:32
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    Take a look at a sundial. They are all round, but even if they were parallelograms one would get the point. It is easy to divide a circle into close to equal halves, and those halves into close to equal halves. And one could do so with a simple stick. Getting from 4 pieces to 12 could be done without too much skill, but getting from 4 to 10 takes some work. And it just one of those happy coincedences of life on Earth that the Earth is @24,000 miles around :-)
    – inSeattle
    Dec 23, 2010 at 6:34
  • In one of ibn Ezra's books on astrology, sefer ha-ta'amim, he makes this point when explaining why the ecliptic is divided into 360 degrees--that the number was chosen because it allows for easy division.
    – paquda
    May 12, 2014 at 14:55
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Wikipedia suggests that the original adoption of a 12-hour day by the ancient Egyptians, et al. may have been related to the 12 lunar cycles in a solar year.

There's also something about the Egyptians tracking the rising of 24 different stars over the course of the day, but it seems to me that the choice of 24 for the number of stars to track was due to the properties of the number, rather than the properties of the stars. Interestingly, the shift from using 36 stars for this purpose to 24 happened at some point during the "New Kingdom," which was also the period in which our Exile in Egypt happened.

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    definitely related to the lunar cycles: 12 months gives 12 constellations in the annular cycle, which also cross the sky every night. Except the 12 constellations take 24 hours to go around, so I'm off by a factor of 2 somewhere... eh, astrology and astrophysics are both order-of-magnitude estimates.
    – Jeremy
    Dec 22, 2010 at 13:32
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    That's a binary order of magnitude.
    – Isaac Moses ♦
    Dec 22, 2010 at 14:46
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The Midrash notes, that the twelve Shevatim correspond to the months of the year, hours of the day, and the different constellations of the sky.

http://www.cckollel.org/parsha_encounters/5767/vayigash.pdf

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