Why was the seven-day week favored? Weekday Connotations

- Connotations (the big table)
- Further Connotations (links)
- Open Questions

- Sevenfold Star
- Menorah
- Star of David
- Binary Tree
- Octahedron
- Geometry
- An approximation of pi?
- Concluding Remarks

There is an ongoing discussion about the origin of the seven-day week which gives interesting historical information and speculations. It seems that the week originated in Babylon or Egypt. Any new light shed on this question is appreciated.

The above explantion is an evolutionary (after-the-fact) argument: "the seven-day week just got the most followers, another week length could have gained world dominance ...". But why was the seven-day week chosen in the first place? In other words, did it mean anything to have seven days in a week?

According to this theory, if the time from new moon to new moon were e.g. 31.08 days, we'd have an eight-day week (one quarter = 31.0 / 4 = 7.77 days).

The four-quarter system is not the only way to divide a lunation. Historically, there was at least one other way of dividing a lunar month:

Joseph Needham reports that the Chinese divided day (obviously midnight to midnight) and **lunation into seven parts** (full moon to full moon). Because this system (like the new-waxing-full-waning system) is a generalization of certain natural cyclical processes, this is also applicable to the week - the descriptions and hexagram associations are given in the table below.

(Joseph Needham, Wang Ling, Science and Civilisation in China, Cambridge University Press, Cambridge 1956, vol.2 History of Scientific Thought, p. 332, 13. Fundamental Ideas of Chinese Science, Table 17, Association of the kua with the lunar and diurnal cycles in the Tshan Thung Chhi)

A calendar that is built on the seven is the Seven Space calendar.

It is obvious that 7 days * 13 weeks make a "season" or quarter year of 91 days. Both factors are prime numbers (odd numbers were considered "good luck"). To get the 365-day year, make one season a day longer (or two longer seasons in leap years). The card deck of 4 suits with 13 cards each (plus the occasional joker) is a model of this (see astonishing playing card math or Karl Palmen's playing card calendar).

- A normal human can keep about seven (±2) objects in conciousness simultaneously. The week is a natural consequence of this psychological fact: its number of days is just the number one can look at and keep separate easily. But 5-, 6-, 8-, and 9-day weeks also fit this criterion. And other week lengths have actually been used historically. Perhaps seven is the best compromise for short-term planning?
- There are seven main chakras in the human body (if you believe in this - and this is just the most popular number)
- Seven is the highest number that can be coded with three bits (including zero). This is a weak explanation, unless you are into Chinese I Ching kua or discover new facts about the working of the human brain.
- There were seven (astrological) planets known to the ancients: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn.
- There are seven musical tones in the Western chromatic scale (as well as five half tones). And Indonesian Gamelan music knows two scales; one of seven and one of five tones. Could this be a hint at a possible musicological / psychological / neurological explanation?

I think the "seven-planets" explanation is the most probable, since the week is so old and the planetary code is still in the names. The Babylonians may have wanted to code knowledge into the number or honor all celestial gods equally (see Sevenfold Star).

Other Calendrics at this site.

Essays on mathematical themes.

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