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Wherever the Common Era Calendar (a.k.a. the Gregorian Calendar) is used — and it is now used by the governments of all countries — a week of seven days is also used in conjunction with it. But there is no 7‑day cycle in Nature from which this could have been derived, so why a week of seven days?
People use a 7‑day week because they have been born into a world where this is customary. In other words, the 7‑day week has been received from earlier generations. It has a long history. When the Roman emperor Constantine made Christianity the state religion early in the 4th Century CE the 7‑day week was officially associated with the Julian Calendar, and the association remained after the Julian Calendar was replaced by the Gregorian Calendar in the 16th Century CE.
The Christians received the 7‑day week from the Jews (in fact, the original Christians were Jews). The Jewish explanation for its use is that this was commanded by their god, named by them YHWH (using the Hebrew letters Yod-He-Vav-He). The Jewish Pentateuch (incorporated into the Old Testament of the Christian Bible) contains several injunctions attributed to YHWH which mention "a seventh day", upon which no "work" is to be done.
Whether or not a 7‑day week was in use by the Jews at the time of Moses in the middle of the 2nd millennium BCE is highly debatable, since YHWH's commands to Moses are not preserved in any contemporary records but only in documents which were composed around the middle of the first millennium BCE.
But in any case the 7‑day week is much older than the time of Moses, since it was also used by the Sumerians and Babylonians. Kerry Farmer remarks that "Some Historians believe that around 2350 BC Sargon I, King of Akkad, having conquered Ur and the other cities of Sumeria, instituted a seven-day week, the first to be recorded."
In many European languages the names of the days of the week are derived from the names of planets/gods. The table below (adapted from a web page by Dr Kelley Ross) gives the names for the planets/gods in various languages and the English name of the corresponding day of the week.
| The Planets | |||||
|---|---|---|---|---|---|
| Sumerian | Babylonian | Greek | Latin | English | Day name |
| Nanna | Sin | Selenê | Luna | Moon | Monday |
| Enki | Nabû | Hermes | Mercurius | Mercury | Wednesday |
| Inanna | Ishtar | Aphroditê | Venus | Venus | Friday |
| Utu | Shamash | Helios | Sôl | Sun | Sunday |
| Gugalanna | Nergal | Ares | Mars | Mars | Tuesday |
| Enlil | Marduk | Zeus | Iuppiter | Jupiter | Thursday |
| Ninurta | Ninurta | Kronos | Saturnus | Saturn | Saturday |
It is plausible to suppose that the association of planets and days of the week arose in prehistoric times as follows:
At some point in the evolution of humans, perhaps as far back as 100,000 years ago, they acquired sufficient intelligence to observe their environment and start to think about it. Obviously the night sky would have been of interest to early humans. The more intelligent among them would have observed that all of the luminous objects in the night sky maintained their positions relative to each other except for a few. Those that did not appeared to wander across the night sky (relative to the fixed stars), and thus eventually came to be called "wanderers". (The English word "planet" is derived from the Greek "planetes", which means exactly "wanderers".)
We may assume that tens of thousands of years ago humans did not think of the physical world as we do today, and in particular did not have an idea of the Earth as a large spherical object within a vast 3-dimensional space in which other large spherical objects moved. For them the nature of the luminous objects which they observed to wander along a band of the night sky, and the cause of their movement, was unknown. But since (by observation of the natural world) it was only living things which moved of themselves, it would be reasonable for early humans to assume that the wanderers, the planets, were living beings of some kind — beings of a very unusual nature, what we might now call "gods".
So for early humans the planets were gods. And obviously the Sun and the Moon belonged to their company. So how many gods were there? As many as could be observed (perhaps more). In addition to the Sun and the Moon there were five others (what we now call Mercury, Venus, Mars, Jupiter and Saturn). If days somehow became associated with these gods then we have the basis for a period of seven days. Perhaps a particular god was venerated each successive day without a break, which would give rise to repeated periods of seven days.
It is plausible to suppose that the earliest calendars were simple tallies of days from one new moon to the next (where "new moon" means the reappearance of the moon after two or three days of invisibility). Bones with 29 and 30 scratches have been found which are at least 40,000 years old, suggesting (since a lunation is approximately 29.5 days) that the scratches were a record of days (or nights) in a lunation. This was probably the first attempt by humans to divide the sequence of days into periods. They would quickly have noted that four successive 7‑day periods were almost, but not quite the number of days from one new moon to the next. This might have given rise to a calendar (such as is known to have been used by the Sumerians and Babylonians) in which the days of a lunation (a "month") were divided into four 7‑day periods beginning with a new moon, followed by one or two days (not part of any 7‑day period) until the next new moon.
The origin of the 7‑day week is sometimes attributed to dividing the 29 or 30 days of a lunation by four, to get a number close to seven. But a concept of division, which we find easily understandable, is not a concept that we can attribute to the earliest thinking humans. Counting and addition may have been the most advanced mathematical concepts for many thousands of years before the idea of division (as a numerical operation) was discovered.
On the basis of this explanation of the development of the idea of the week it is obvious why there are seven days in a week: This is the number of visible planets plus the Sun and the Moon.
If, instead of an asteroid belt between Mars and Jupiter, there had been a planet, then there would have been six visible planets, not five, so the number of celestial entities would have been eight, not seven. In that case humans would have developed a week of eight days, not seven.
The planet Uranus was first observed by telescope in 1690 (by Flamsteed) but was recognized as a planet (by Herschel) only in 1781. Had the solar system formed in such a way that Uranus came close enough to Earth to be observable with the naked eye (actually it can be observed with a very acute naked eye, but only when it is closest to Earth and one knows where to look for it) then the number of celestial entities would have been eight, and we would have an 8-day week, as the Etruscans — and following them, the Romans — actually did (until it was supplanted sometime after the 1st Century by the 7-day week).
The fact that humans have long used a week of seven days is thus the result of accident, namely, the fact that the solar system is the way it is, with five of the nine planets being sufficiently close to Earth to be normally visible with the naked eye.
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