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Asaf Pe'er

Physics for all

Why is there 24hrs in a day?

Interestingly, while different countries use different measures of distances (e.g., meters, foot, etc.) and weight (kilogram, pound, etc.) it seem that the division of a day into 24 hours, each hour into 60 minutes and each minute into 60 seconds is a convention accepted worldwide.

The historical origin of the division of a day into 12 hours originate from ancient Egypt. From what I read, Egyptian hieroglyphs from as early as 3000 BC show the use of a base-10 decimal system. This was measured by shadow clock. Two twilight hours were added, one at the beginning and one at the end of each day, reaching total of 12 hours per day. Many believe that the base-12 system arose from a counting system the Egyptians inherited from the earlier Sumerian culture, counting not by the whole finger but by each individual knuckle. That is, if you open your left hand and use the tip of your thumb to touch each of the three knuckles in your four fingers, you'll total 12. To measure time using this method, the Egyptians divided the day into 12-hour halves - or, more accurately, a ten hour day, two hours of morning and evening twilight, and 12 hours of darkness. The night time division was done using stars. Clearly, this system is season-sensitive: during the summer time, one day-hour is longer than a winter day-hour.

Web resources I found on the issue are: http://gizmodo.com/5926491/why-there-are-24-hours-in-a-day http://www.abc.net.au/science/articles/2011/11/15/3364432.htm

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