[yor_on (OP)]

As I say

How come we choose seven instead of ten for that one?
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You might want to move it? To me it's a question about the mathematics we use.

[bizerl]

This question has been on the tip of my fingers for ages!

I can understand years (obvious, Earth going around the sun) and months (moon going around the Earth - sort of-ish) and days (duh! Sun going around the Earth []), but why weeks?

I thought it may just be a convenient division of a month into four. But there's nothing convenient about a month.

The obvious answer is because the Bible says so. I would be interested to know if any non-Moses based cultures had any concept of a week and if so, how long was it?

[CliffordK]

Perhaps it is a union thing.
Imagine the strikes if one transitioned to a 10 day work week.

Christianity, and Judaism (Old Testament) state:
In 6 days God made the Earth
And, then he rested.

It appears as if the 7 day week was adopted with the Julian Calendar (basis for Gregorian Calendar), and thankfully replaced an 8 day week that was prevalent at the time.

Some theories indicate that the days were based on the classic known astronomical bodies:

Sun, Moon, Mars, Mercury, Jupiter, Venus, & Saturn. 

If they had known about Uranus, Neptune, and Pluto, we would have had a 10 day week.  Well, that is until Pluto was demoted from a planet to a mere Dwarf.  And, then what about all the other moons of the solar system, some of which are larger than our own moon.

More history about the week.

[yor_on (OP)]

You know Bizerl, maybe you have something there? You can split 7 in a year and it comes out almost perfect, but doing the same with ten makes a weird 'half week'? Aborigine's don't believe in time I read somewhere, so they probably never used weeks? The bible Clifford? But what about before that?
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On the other tentacle, assuming we would have used ten instead, what would have stopped us from giving a year 370 days? Is it astronomically impossible to do so? And how did they know?

[Airthumbs]

I have a sneaky suspicion they knew about the number of days in a year well before the Bible was written.  Places like Stone Henge were used to calculate the passing of time in a year.  They knew when both the Summer and Winter Solstice took place so probably worked it out from that.....  that is my guess anyway. 

[CliffordK]

If we only had a little sunshine, it would be easy enough to determine the winter solstice with a sundial (longest noon time shadow cast by the sun dial).
And, the summer solstice would be the shortest noon shadow.  The device to do the measurements could even be quite crude.

Then one merely has to count the days between the two.

Once one knows the northern and southern extent of the sun in the sky, one can also design an apparatus that would be sensitive specifically to those angles.

[caKus]

A basic unit of time was (always is) the lunar month. It's duration is 28 days. It was a natural unit of time for many civilizations. But our ancestors thought it was a rather long period of time and looked for a smaller interval. 28 is divisible by 7, 4 and 2. And they choose an interval of 7 days.

I must say it was an unfortunate choice, imagine they choose 4 days per week : monday, tuesday, saturday and sunday. Life would have been easier.

[Pmb]

It's a nice coincdence that you ask this now. I'm reading a book which touches on this topic. It's a mechanics text which is describing the system of units we use in physics and elsewhere.

This text Analytical Mechanics by Fowles and Cassiday, states on pages 4-5

Astronomical motions provide us with three great "natural" time units: the day, month and year. The day is based on the Earth's spin, the month on the moon's orbital motion about the earth, the year on the earth's orbital motion about the sun. Why do we have ratios of 60:1 and 24:1 connecting the day, hour, minute and second? These relationships were born abouty 6,000 years ago in Mesopotamia (now Iraq) where civilization and city-states first appeared on earth. The Mesopotamiam number system was based on 60, not 10 like ours. It seems likely that the early Mesopotamians were more influenced by the 360 days in a year, the 30 days i a month, and 12 months in the year than by the number of fingers on their hand.

[yor_on (OP)]

kind of cool, isn't it? Trying to understand where it comes from
A lunar month Cakus? you mean from it showing itself very small, to full, right?

That seems as a very interesting explanation to me. The moon has a strange attraction, not only for lovers
And it fits Pete's 'The day is based on the Earth's spin, the month on the moon's orbital motion about the earth, the year on the earth's orbital motion about the sun. ' very well it seems.

Think we solved it

[evan_au]

If you count a month by phases of the moon, the average length is 29.530589 days (29 d 12 h 44 min 2.9 s).
If you count a month by movement of the moon across the stars, the average length is 27.321661 days (27 d 7 h 43 min 11.5 s)
So you can't really align a 7-day week (or a 28-day, 4-week period) and a lunar month: you need extra special days, or have the weeks asynchronous to the phases of the moon.

You can't align the moon's rotation to the year, either, since it is not a simple ratio. So you need leap years, or have the months asynchronous to the Year.

Footnote: The decimal system was promoted during the French Revolution, including decimal time and a decimal week (with 10 days): http://en.wikipedia.org/wiki/French_Revolutionary_Calendar#Ten_days_of_the_week

A variety of other lengths of week have been used from time to time: http://en.wikipedia.org/wiki/Week

[yor_on (OP)]

Assuming you are right there Evan, you usually are we haven't solved it then?
Damn, it seemed to fit so well. The moon is a breathtaking thing.

Any more suggestions or ideas folks?

[CliffordK]

Footnote: The decimal system was promoted during the French Revolution, including decimal time and a decimal week (with 10 days): http://en.wikipedia.org/wiki/French_Revolutionary_Calendar#Ten_days_of_the_week

Most interesting.

Apparently at the time, people typically worked 6 days a week, with 1 rest day.
The French extended this to working 9 days a week, with 1 rest day.

Had the French also invented the 2 day weekend, I wonder if their calendar would have endured. 

Of course, it would have been a pain for international commerce unless they had convinced the USA, or other European countries to also adopt the calendar.