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I know jewish culture tend to like number 7. What about other cultures? Do we have 7 days in a week from bible?

Why do we derive 7 days a week from jews? Why not from more prominent culture, like roman, greek, etc.

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closed as off-topic by Louis Rhys, Kobunite, Tea Drinker, DVK, Fitri Nov 23 '13 at 7:46

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Does this article help? en.wikipedia.org/wiki/Seven-day_week –  Louis Rhys Nov 22 '13 at 4:48
    
A better question might be where we got the concept of a week from at all. The year, the month and the day are all derived from natural cycles of the sun (day and year) and moon (month). The week appears to have no corresponding natural cycle, be it 7,8,10 etc. Why divide time into weeks? –  Vector Dec 4 '13 at 23:32

2 Answers 2

up vote 6 down vote accepted

It's because of the moon. Mostly.

The new moon was the start of a new lunar month (the word itself being derived from words for moon). A lunar month is 29.53 days and is a very prominent cycle in all early cultures.

7 days is, very roughly 1/4 of a lunar cycle or "lunation" (more like .23 than .25 as you can see from the length of the lunar month). And yes, 7 days does work out nicely with some religious beliefs. But, unfortunately, the Moon's orbit is complicated and doesn't neatly divide as it's independent of the rising and setting sun. So then you fudge the number of days at the end of a month when the next new moon arrives too early or you add a day here and there to make it work out overall (which is why our months are not all the same).

Why 12 months? Well, the four seasons start over after 12 months.

Or, a gēar is twelfe mōnath, and seofon daeg make a mōnath, m ra and m re. :)

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I would certainly agree that the cycle of the moon is a good starting point. The phases makes for a straightforward reckoning of time passing without special instruments. –  Mario Elocio Nov 22 '13 at 7:58
    
Three weeks of ten days work better than four weeks of seven (half a day of error, vs one and a half days of error), and this was used in for example the Egyptian calendar. So I disagree it's mostly because of the moon. It's rather a combination of 1. Seven being seen as magical, as it is a prime number. 2. 7*4 being roughly equal to a month. –  Lennart Regebro Nov 22 '13 at 10:25
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@LennartRegebro That's all very reasonable, but it strikes me as a bit anachronistic. They weren't trying to evenly partition the month as you suggest. Rather, they were more interested in tracking the days at which the new moon, first quarter, full moon, and last quarter occur. The week and the month were not originally defined as any set number of days, but instead the time between distinct regular lunar events. –  David H Nov 22 '13 at 12:07
    
@DavidH That seems like a reasonable explanation. –  Lennart Regebro Nov 22 '13 at 12:21

I'm afraid the answer is that we don't know.

The Romans gradually replaced their 8 day week (the imperial nundinal cycle) with a 7 day week over a course of a century, after Julius Caesar's calendar reform in 46 BC. Their reasons for doing so are unclear, however we do know that the two cycles co-existed for quite some time. Ultimately, the nundinal cycle fell in disuse and the 7 day cycle prevailed.

One interesting hypothesis, supported by the names of the days in the Greco-Roman world, is that the 7 day cycle prevailed over the nundinal cycle because of its astrological symbolism: Each day represents one of the seven classical planets. While we don't know for sure what prompted the switch, Hellenistic astrology seems like a far more likely influence for the Romans than Jewish culture.

In 321 AD, the 7 day week - by then the norm - was officially adopted by Emperor Constantine. Constantine's primary reason for normalizing the length of the week seems to have been for all his subjects, regardless of religion, to observe the day of the sun.

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I find the switch from an 8 day week to a 7 day week interesting. After all, it took a good bit of astronomical study to realize there only 7 classical planets. Originally, there were 8 planets because people were mistakenly counting Venus twice, as the morning star and the evening star. –  David H Nov 22 '13 at 3:28

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