The seven-day week must go down as being one of the most “unnatural” yet persistent creations of man. It is very practical of course, but why do we have it?
There are no discernible periodicities that we have been able to find outside ourselves which take 7 days. There are no periodicities within ourselves either that are 7 days or multiples of 7 days. There are no celestial or astronomical cycles in tune with 7 days. There are no movements of the sun or the moon or the stars that give rise to a 7-day period. There are no weather or climate phenomena that repeat with a 7-day period. There are no human behavioural patterns that dance to a 7-day tune. There are no living things that have a 7-day life cycle. (There is a branch of pseudoscience which claims that living cells may be associated with a weekly or a half-weekly cycle – a circaseptan or a circasemiseptan rythm – but this is still in the realms of fantasy).
It would seem logical that our ancestors must have first noted the daily cycle long before they were even recognisable as human. As humans they probably then noted the lunar cycle of about 29 days and the yearly cycle of about 365 days. Our distant ancestors would also have noted that the period of the yearly cycle was a little more than 12 lunar cycles. By about 35,000 years ago we have evidence that the lunar cycle was known and was being tracked. This evidence is in the form of a tally stick with 29 marks – the Lebombo bone.
The development of beliefs in gods of light and separate gods of darkness is not so difficult to understand. The gods of winds and fires and mountains and rivers are equally understandable. The fact that the lunar cycle was rather badly synchronised with the annual solar cycle could well have led to the concept of sun-gods and moon-goddesses, each with their own areas of influence. (In fact, considering the imperfection of the design of the universe which is manifested in the lack of synchronisation between the various celestial cycles, it is difficult to understand how a concept of a single all-powerful creator ever arose. Why would such a poor design be the product of an all-powerful creator? Surely he could have managed the simple 3-body problem to synchronise the various rotations of the earth, the moon and the Sun?)
The invention of the seven-day week can best be dated to be at least 5,000 years ago to the time of the Babylonians. It was certainly long before the Old Testament came to be written to fit with the 7-day week which had already been invented and established. The story goes that
the seven-day week was actually invented by the Assyrians, or by Sargon I (King of Akkad at around 2350 B.C.), passed on to the Babylonians, who then passed it on to the Jews during their captivity in Babylon around 600 B.C. The ancient Romans used the eight-day week, but after the adoption of the Julian calendar in the time of Agustus, the seven-day week came into use in the Roman world. For a while, both the seven and eight day weeks coexisted in the Roman world, but by the time Constantine decided to Christianize the Roman world (around A.D. 321) the eight-day weekly cycle had fallen out of use in favor of the more popular seven-day week.
The idea that the 7-days originates from a division of the lunar cycle into 4 seems improbable. The lunar cycle (synodic period) is 29.5305882 days long. Three weeks of 10 days each or five 6 day weeks would fit better. That the annual cycle of 365.2425 days comes to dominate is not so surprising. Our calendar months are now attuned to the annual cycle and have no direct connection to the lunar cycle. But it is our 7 – day weeks which remain fixed. We adjust the length of our months and have exactly 365 days for each of our normal years. We then add an extra day every 4 years but omit 3 such extra days in every 400 years to cover the error. We make our adjustments by adding a day to the month of February for the identified leap years but we do not mess with the 7 days of the week.
It is far more likely that the 7 days comes from the seven celestial objects visible to the naked eye from earth and probably known to man some 5,000 to 10,000 years ago. They were familiar with the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn by then. Naturally each was a god in his own heaven and had to have a day dedicated just to him/her/it. The same 7 celestial objects are used for the days of the week not only in the Greek/Roman Western tradition, but also in Indian astrology. The Chinese /East Asian tradition uses the Sun, Moon, Fire, Water, Wood, Gold and Earth to name the seven days of the week. But this must have come after the 7 day week had already been established elsewhere. (For example, to name up to 10 days they could just have chosen to add days named for the Air, Beasts, Birds ….). Some languages use a numbering system and some use a mixture of all of the above. Rationalists and philosophers and dreamers have tried to shift to 5 and 6, and 8 and 10 day weeks but none of these efforts has managed to challenge the practicality or to dislodge the dominance of the seven-day week.
And now the whole world lives and marches – socially, culturally, politically – to the inexorable beat of the 7-day week.
Just because some long-forgotten astrologer/astronomer decided that he would dedicate each day to one of his seven known celestial gods (and he only had seven)! Even if he (unlikely to have been a “she”) worshipped an Earth-goddess, she must have been considered inferior to the celestial gods. Otherwise we would have had an 8-day week!
An alien race could be excused for concluding that humans must have evolved from once having seven fingers on each limb. Or that we once had seven limbs and have lost 3. Or that humans have an innate circaseptan rythm requiring extra rest and sleep every 7 days. Or that humans have a physiological need to go binge drinking on the sixth day and need the seventh day to recover!
But if the 7-day week is a Divine creation then the aliens will also have a 7-day week and will not be in the least surprised.
The number seven does have a few special properties:
- Seven, the fourth prime number, is not only a Mersenne prime (since 23 − 1 = 7) but also a double Mersenne prime since the exponent, 3, is itself a Mersenne prime. It is also a Newman–Shanks–Williams prime, a Woodall prime, a factorial prime, a lucky prime, a happy number (happy prime), a safe prime (the only Mersenne safe prime) and the fourth Heegner number.
- Seven is the lowest natural number that cannot be represented as the sum of the squares of three integers
- Seven is the number of notes in the traditional Western diatonic scale (major or minor)
- Indian Music has “Saptak Swaras,” seven octats (sa re ga ma pa dha ni) which are basics of music.
- Seven is the number of notes in the dastgah system of Persian traditional music
Not forgetting the Seven Wonders of the Ancient World. But they only came after the seven day week had been invented and introduced.
