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Years, months and days are based on obvious astronomic cycles, which makes them largely standardized and ubiquitous across all societies. Weeks, on the other hand, have no relationship to astronomical observations. Why do we have them and where do the seven days come from, when it just as well could have been more or fewer? A cursory read of the articles on Wikipedia, Britannica and several other sources reveals that weeks appear to originate in Mesopotamia, but none of them explain the number seven in particular. Is its origin unknown?

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    You should have a look at this related question.
    – Steve Bird
    Oct 13 at 14:48
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    discovermagazine.com/planet-earth/… gives a good rundown of why. and the 7 day week is based off of astronomical observations.
    – ed.hank
    Oct 13 at 15:51
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    @ed.hank: this is an unbelievably shoddy text. To begin with, they confuse sidereal and _ synodic_ months. Given that, I would not rely on it for anything.
    – sds
    Oct 13 at 16:22
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    @jamesqf: sidereal month is ~27.3 days. synodic month (lunation) is ~29.5 days. In the ancient past everyone knew that "a month is 29.5 days". The text starts with "The Moon cycle is 27 days and seven hours long, and there are 13 phases of the Moon in each solar year" This is like saying "Roman caravels were made of Damascus steel". Would you trust such a text?!
    – sds
    Oct 13 at 16:44
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    Doesn't Wikipedia's Week answer this? If not, can you clarify why not? Oct 14 at 2:09
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Having weeks of seven days is a direct consequence of the lunar cycle. Each 29 day lunar cycle comprises four equal phases of 7.25 days long each. Starting from New Moon:

  • Waxing crescent ending in First Quarter;
  • Waxing gibbous ending in Full Moon;
  • Waning gibbous ending in Third Quarter; and finally
  • Waning crescent ending in New Moon.

These seven and a quarter day cycles are readily apparent to all individuals, and comprise an obvious cycle greater than a single day and less than a full month.

In a society without electric street lights, this omnipresent cycle of bright nights under the gibbous moon alternating with much dark nights under the crescent moon is one that drives all evening activities. Groups would time their evening activities to occur under the gibbous moon, and as close as possible to Full Moon.

Note that the day of, and the two days just before and after, the Full Moon are nearly twice as bright as the evenings two days before and after the Full Moon. This albedo effect is nearly unique to dust covered surfaces, such as the lunar surface, and is how and why scientists were nearly certain the moon was dust covered long before the Apollo missions in the 1860's.

As for history: The Romans marked the days of each month relative to the Calends (day following New Moon), Nones (day following First Quarter); and Ides (day following Full Moon) of each lunar cycle.

Having these three brilliantly lit nights each month, under which one might even be tempted to try reading a newspaper, is absolutely unmissable to anyone who is frequently about at night away from streetlights.

For those city dwellers with no experience of this: treat yourself to a true nature experience. Attend a dark night sanctuary twice during the coming year, on a cloudless or nearly so night: once at or within a day of New Moon; and again at or within a day of Full Moon. The difference will astound you.

===

Objections ae raised below that the structure of four seven day weeks in a month is not natural because, at various times and places, other subdivisions lengths of the lunar cycle existed.

That's horse hockey. The seven day cycle is the most common; but that doesn't have to mean it's the only one that makes sense. If a society wished or needed to distinguish the brightest third of the lunar cycle, around Full Moon, it would be natural to do that as well (or instead of) with three nine or ten day cycles. That doesn't invalidate the lunar origin of the seven day week. Being one natural read for such a cycle doesn't require being the only possible such read.

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    A more nuanced approach.
    – Lucian
    Oct 14 at 2:18
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    [citation needed]. If weeks of seven days were a natural outcome of the lunar cycle, then why is there such a profusion of other sub-monthly periods?
    – Mark
    Oct 14 at 2:24
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    @PieterGeerkens, Rome had an eight-day market cycle. I don't know what sub-monthly period, if any, Sumer used, but Wikipedia has a list of sub-monthly cycles ranging in length from four days to thirteen days.
    – Mark
    Oct 14 at 2:34
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    @Mark: Not all weeks are moon-based, nor are all moon-based weeks related to moon-quarters; for instance, Egyptians divided the month into three decans, and the Dacians into five six-day weeks.
    – Lucian
    Oct 14 at 2:37
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    @Mark: Seven day weeks being a natural read of the moon's four quarters doesn't require that it be the only possible read. See my addenda. The existence of other reads just means not all societies were identical and with identical needs from a calendar. Oct 14 at 2:46
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Where did the idea of weeks originate and why do they have seven days?

The idea of weeks, not necessarily seven days in length, originated independently, across many cultures, and at various times, ultimately for purely utilitarian reasons, such as having a practical, medium-sized time unit, between a day (which is oftentimes too short) and a month (which is oftentimes too long).

Where did the idea of weeks originate and why do they have seven days?

The idea of seven-day weeks, specifically, seems to have originated in ancient Sumer, during the first half of the third millennium BCE.

Weeks, on the other hand, have no relationship to astronomical observations.

Some don't, others do; thus, the Roman nundinal cycle constitutes a clear example of the former, whereas the Egyptian decan or the Dacian six-day week1 are regarded as instances of the latter, representing either a third or a fifth of a lunar month, respectively.

1 Yes, I am well aware that the linked post nowhere explicitly states that the 180 lanky pillars, corresponding to a semester, are periodically subdivided into groups of six, by thirty short, sturdy ones, (also) representing the number of days in a month.

Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?

As already stated above, weeks containing both a greater and a smaller number of days did exist throughout history, and across the globe.

Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?

A cursory read of [encyclopedic] articles [...] reveals that weeks appear to originate in Mesopotamia, but none of them explain the number seven in particular. Is its origin unknown?

That is indeed correct; specifically, in Lower Mesopotamia (particularly, Sumer), during the early dynastic period, as already stated above.

Where did the idea of weeks originate and why do they have seven days?

Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?

A cursory read of [encyclopedic] articles [...] reveals that weeks appear to originate in Mesopotamia, but none of them explain the number seven in particular. Is its origin unknown?

Though initially stemming from ancient Sumer, as already noted twice above, the seven-day week only came into widespread use due to the (direct or indirect) influence of the famous Babylonian empire on the Greek-Roman world of classical antiquity; thus, while not its original inventors, the Babylonians were ultimately responsible for not only preserving its continuous usage, but also facilitating it unto others; but why would they do that in the first place ?

Of course, part of the reason would be that humans, in general, perpetuate ancestrally inherited customs through the generations, unless they prove either completely unnecessary, or seriously detrimental, or until a significant improvement appears somewhere along the way, thereby altering the long chain of transmission. So, while tradition does certainly play an important part, what other reasons could there have possibly been, to help explain not merely its adoption and preservation, but also its proliferation ?

To better understand this, let us take the following factors under more careful consideration:

Babylonian mathematics

  • Before asking ourselves why or how Babylonians counted days, in particular, let us inquire as to how they counted in general; thus, they are reputed to have employed a rather robust sexagesimal counting system:
    • Human beings have five fingers on each of their four members, hence the use of decimal and vigesimal counting systems, such as that of the Maya, for instance.
    • Apart from this, they also customarily divide things into halves, thirds, and quarters, hence the occasional use of a dozenal base.
    • The smallest numerical value whose mathematical properties satisfy all aforementioned divisibility requirements is sixty, which, rather unsurprisingly, and as a direct consequence of the above, possesses a relatively large number of divisors:
      • 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60.
    • Notice that the first or smallest number absent from the above list is none other than seven.

Babylonian astronomy

  • The shape and position of the moon in the (night) sky repeat periodically, with a mean value of about 291/2 days for a synodic month, and of 271/3 days for a sidereal month, respectively.
  • Throughout this time, it passes through four easily discernible lunar phases: empty, half-full, full, and half-full again, averaging between 65/6 days and 73/8 days, or, in other words, about seven days.

But how about other celestial bodies ?

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Probably from the Middle East

The Iranian calendars or Iranian chronology (Persian : ﮔﺎﻩﺷﻤﺎﺭﯼ ﺍﯾﺮﺍﻧﯽ , Gāh-Šomāri-ye Irāni) are a succession of calendars invented or used for over two millennia in Iran , also known as Persia. One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes. The most influential face in laying the frameworks for the calendar and its precision was the 11 century Persian polymath , hakim Omar Khayyam . The modern Iranian calendar is currently the official calendar in Iran. It begins at the midnight nearest to the instant of the vernal equinox as determined by astronomic calculations for the Iran Standard Time meridian (52.5°E or UTC+03:30 ). It is, therefore, an observation-based calendar, unlike the Gregorian , which is rule-based.

Following the rise of extremist Christianity in Europe, Sunday was declared a holiday instead of a Friday.

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    Welcome to HSE. What has your answer to do with the fact that weeks have seven days? Oct 13 at 17:02
  • They observed 365 days, the rest is estimates and math. @José Carlos Santos 365÷12≈30 and 30÷4≈7
    – user52467
    Oct 13 at 19:55
  • It turned out later that a year is 365 days and six hours. So they added a leap year. (A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year)
    – user52467
    Oct 13 at 20:00
  • @user52467: Friday is a sacred day for both Jews and Christians; for the former, it represents the day of preparation before the Sabbath; for the latter, it is the day of Christ's crucifixion. Islam diplomatically chose to adopt this day, so as to express both kinship with, as well as independence from other Abrahamic faiths.
    – Lucian
    Oct 13 at 21:42
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    @user52467 Why 30÷4? Why not 30÷5=6 or 30÷3=10 exactly? If this reasoning were true, 6 or 10-day week would be more logical.
    – Zeus
    Oct 14 at 5:43

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