<p>The modern 7-day week was adopted in the late Roman Empire, replacing an 8-day "nundinal week" that had previously been in use mainly for market intervals. As far as I know, there was no particular reason for the 8-day length other than that it was a convenient size to separate market days.</p>

<p>In the early Empire, Christians and Jews both used a 7-day week for religious purposes and as they grew to be a major part of the Empire's population, it became convenient to link the market week with the religious week. This worked because 7 worked about as well as 8 for markets, and so by the late Empire, it was a done deal.  </p>

<p>See <a href="https://en.wikipedia.org/wiki/Nundinae" rel="noreferrer">https://en.wikipedia.org/wiki/Nundinae</a> for a good discussion.</p>

<p>Where did the Jewish 7-day cycle come from?  It's not certain.  A practical market cycle can be anywhere in the 5-10 day range -- it doesn't matter a lot -- so if early Jews preferred a 7-day religious week (from Genesis, Chapter 1, probably) then the market cycle would have synched up just like in the Roman Empire.  It's also possible they inherited it from some previous mid-Eastern culture.  I doubt anyone knows for sure any more, so it must remain to some degree speculation.</p>

CC BY-SA 4.0
2018-05-21T22:06:55.387Z
It answers, why were 7-day "week" used in the late Roman Empire, but it doesn't answer, what is the origin of the 7-day week.
peterh
CC BY-SA 4.0
2018-05-21T22:21:00.820Z
@peterh I added some material to cover that point, but it is necessarily speculative.
Mark Olson
CC BY-SA 4.0
2018-05-27T15:43:12.137Z
Does anyone know when Genesis 1:1 was actually written for the first time?
Jack R. Woods
CC BY-SA 4.0
2020-03-09T01:41:47.990Z
@Jack R. Woods The best current scholarship thinks that it was first *written down* in the current form around the time of the Babylonian Captivity, in the 500s BC. But, if so, that was certainly not a new composition, but was a final redaction of older texts and material passed down orally. When does the earliest version (now millennia lost) date from? Academics have lots of theories, each based on hints and tiny bits of data and a lot of wishful thinking. (The ancient tradition is that it was composed by Moses somewhere around 1300 BC.)  No one really knows.
Mark Olson
CC BY-SA 4.0
2018-05-21T20:39:36.047Z
2018-05-21T22:20:30.443Z
2018-05-21T22:20:30.443Z
Mark Olson
<p>The 7 days of the week certainly fit somewhat as a quarter of a month. But there was another argument for the ancients to use the number seven, because they knew about seven "planets", seven objects which wander across the sky relative to the thousands of fixed stars. The Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn. Earth wasn't a planet according to the IAU definition at the time. Earth was instead the center of the universe.</p>

<p>The weekly calendar of the ancients was pretty busy:</p>

<p><strong>Sunday</strong> - <a href="http://www.etymonline.com/index.php?term=Sunday&amp;allowed_in_frame=0" rel="nofollow">Sun</a> you're free, sober up and get a suntan</p>

<p><strong>Monday</strong> - <a href="http://www.etymonline.com/index.php?term=Monday&amp;allowed_in_frame=0" rel="nofollow">Moon</a> the day after being free</p>

<p><strong>Tuesday</strong> - <a href="http://www.etymonline.com/index.php?term=Tuesday&amp;allowed_in_frame=0" rel="nofollow">Jupiter</a> Zeus day, be clever and figure something out</p>

<p><strong>Wednesday</strong> - <a href="http://www.etymonline.com/index.php?term=Tuesday&amp;allowed_in_frame=0" rel="nofollow">Mercury</a> go somewhere and talk about it</p>

<p><strong>Thursday</strong> - <a href="http://www.etymonline.com/index.php?term=Thursday&amp;allowed_in_frame=0" rel="nofollow">Mars</a> fight</p>

<p><strong>Friday</strong> - <a href="http://www.etymonline.com/index.php?term=Friday&amp;allowed_in_frame=0" rel="nofollow">Venus</a> make love</p>

<p><strong>Saturday</strong> - <a href="http://www.etymonline.com/index.php?term=Saturday&amp;allowed_in_frame=0" rel="nofollow">Saturn</a> please take a bath!</p>

CC BY-SA 4.0
2018-05-22T17:47:19.073Z
You got Jupiter and Mars mixed up.
Florin Andrei
CC BY-SA 4.0
2018-09-14T04:08:40.377Z
Was *die mercuri* the 'market day'?
amI
CC BY-SA 3.0
2015-04-17T14:09:11.673Z
2015-04-17T14:09:11.673Z
LocalFluff
<p>The synodic period of the moon is $29.53$ days, a little shorter than a calendar month, which is on average about $30.4$ days. This is slightly longer than its orbital period, but corresponds to the periodic visual appearance of the moon as viewed from Earth. I mention this to make it clear that we should be forgiving of a little imprecision.</p>

<p>Conventionally, the moon's appearance is divided into four phases: first quarter, full, last quarter, new. That means that on average, each phase lasts about $7.4$ days. Since calendars count days in integer amounts, a $7$-day period seems to be a natural choice.</p>

<p>The social importance of the seven-day period in Western cultural probably has much more to do with its religious significance in Abrahamic religions than astronomy per se (although certainly not unique to it). But its ultimate origin probably does lie in the natural division of the moon's appearance into four phases, which correspond to an apparent geocentric celestial latitude difference between the Moon and Sun of $0^\circ$, $90^\circ$, $180^\circ$, and $270^\circ$.</p>

<p>That, the explicit answer to your question is</p>

<ul>
<li>1 week = 7 days = one lunar phase.</li>
</ul>

CC BY-SA 3.0
2017-11-18T21:00:06.437Z
A couple of supportive diagrams: https://www.universetoday.com/wp-content/uploads/2008/10/moondiagram.png  https://upload.wikimedia.org/wikipedia/commons/4/46/Moon_phases_en.jpg
wim
CC BY-SA 4.0
2018-05-21T22:13:13.293Z
It is not an exact match. The Babylonians had "leap weeks", the last week of all months were elongated to 8 or 9 day to match the month to the lunar cycle.
peterh
CC BY-SA 3.0
2015-04-17T05:54:32.150Z
Seven day week took a while to spread east: "The earliest known reference in Chinese writings to a seven-day week is attributed to Fan Ning, who lived in the late 4th century in the Jin Dynasty" http://en.wikipedia.org/wiki/Week#Adoption_in_Asia
Wayfaring Stranger
CC BY-SA 3.0
2015-04-17T01:19:05.237Z
2015-04-17T01:19:05.237Z
Stan Liou
<p>Why do we divide time into weeks?  Is there any celestial reason why humans do this?  </p>

<ul>
<li>one year: earth revolution around the sun </li>
<li>one month: moon revolution around the earth </li>
<li>one week: 7 days = ??? </li>
<li>one day: earth rotation about its axis</li>
</ul>

CC BY-SA 4.0
2018-05-21T22:11:09.280Z
@user6760 The Babylonians used a lunar calendar, a month was three 7-day weeks and a 8 or 9 day one. However, there is no answer, why do the Babylonians used the 7-week cycle. They could have used, for example, 3 10-day weeks, it hadn't be lesser rational (they used a base-6 and a base-10 number system together).
peterh
CC BY-SA 3.0
2015-04-17T01:07:22.293Z
This is probably Babylonian's fault in the first place.
user6760
<planet><time>
Years, months, days, and ... weeks?
wim
# Years, months, days, and ... weeks?

(Asked by: username_0 on 2015-04-17)

Why do we divide time into weeks? Is there any celestial reason why humans do this?

* one year: earth revolution around the sun
* one month: moon revolution around the earth
* one week: 7 days = ???
* one day: earth rotation about its axis

**Tags:** planet, time

> **Comments:**
> **username_2**: This is probably Babylonian's fault in the first place.  
> **username_1**: @username_2 The Babylonians used a lunar calendar, a month was three 7-day weeks and a 8 or 9 day one. However, there is no answer, why do the Babylonians used the 7-week cycle. They could have used, for example, 3 10-day weeks, it hadn't be lesser rational (they used a base-6 and a base-10 number system together).  

---

## 3 answers

### Answer by username_8 on 2015-04-17. Score: 20 | Accepted answer

The synodic period of the moon is $29.53$ days, a little shorter than a calendar month, which is on average about $30.4$ days. This is slightly longer than its orbital period, but corresponds to the periodic visual appearance of the moon as viewed from Earth. I mention this to make it clear that we should be forgiving of a little imprecision.

Conventionally, the moon's appearance is divided into four phases: first quarter, full, last quarter, new. That means that on average, each phase lasts about $7.4$ days. Since calendars count days in integer amounts, a $7$-day period seems to be a natural choice.

The social importance of the seven-day period in Western cultural probably has much more to do with its religious significance in Abrahamic religions than astronomy per se (although certainly not unique to it). But its ultimate origin probably does lie in the natural division of the moon's appearance into four phases, which correspond to an apparent geocentric celestial latitude difference between the Moon and Sun of $0^\circ$, $90^\circ$, $180^\circ$, and $270^\circ$.

That, the explicit answer to your question is

* 1 week = 7 days = one lunar phase.

> **Comments:**
> **username_9**: Seven day week took a while to spread east: "The earliest known reference in Chinese writings to a seven-day week is attributed to Fan Ning, who lived in the late 4th century in the Jin Dynasty" http://en.wikipedia.org/wiki/Week#Adoption_in_Asia  
> **username_0**: A couple of supportive diagrams: https://www.universetoday.com/wp-content/uploads/2008/10/moondiagram.png  https://upload.wikimedia.org/wikipedia/commons/4/46/Moon_phases_en.jpg  
> **username_1**: It is not an exact match. The Babylonians had "leap weeks", the last week of all months were elongated to 8 or 9 day to match the month to the lunar cycle.  

---

### Answer by username_3 on 2018-05-21. Score: 6

The modern 7-day week was adopted in the late Roman Empire, replacing an 8-day "nundinal week" that had previously been in use mainly for market intervals. As far as I know, there was no particular reason for the 8-day length other than that it was a convenient size to separate market days.

In the early Empire, Christians and Jews both used a 7-day week for religious purposes and as they grew to be a major part of the Empire's population, it became convenient to link the market week with the religious week. This worked because 7 worked about as well as 8 for markets, and so by the late Empire, it was a done deal.

See <https://en.wikipedia.org/wiki/Nundinae> for a good discussion.

Where did the Jewish 7-day cycle come from? It's not certain. A practical market cycle can be anywhere in the 5-10 day range -- it doesn't matter a lot -- so if early Jews preferred a 7-day religious week (from Genesis, Chapter 1, probably) then the market cycle would have synched up just like in the Roman Empire. It's also possible they inherited it from some previous mid-Eastern culture. I doubt anyone knows for sure any more, so it must remain to some degree speculation.

> **Comments:**
> **username_1**: It answers, why were 7-day "week" used in the late Roman Empire, but it doesn't answer, what is the origin of the 7-day week.  
> **username_3**: @username_1 I added some material to cover that point, but it is necessarily speculative.  
> **username_4**: Does anyone know when Genesis 1:1 was actually written for the first time?  
> **username_3**: @username_4 The best current scholarship thinks that it was first *written down* in the current form around the time of the Babylonian Captivity, in the 500s BC. But, if so, that was certainly not a new composition, but was a final redaction of older texts and material passed down orally. When does the earliest version (now millennia lost) date from? Academics have lots of theories, each based on hints and tiny bits of data and a lot of wishful thinking. (The ancient tradition is that it was composed by Moses somewhere around 1300 BC.)  No one really knows.  

---

### Answer by username_5 on 2015-04-17. Score: 1

The 7 days of the week certainly fit somewhat as a quarter of a month. But there was another argument for the ancients to use the number seven, because they knew about seven "planets", seven objects which wander across the sky relative to the thousands of fixed stars. The Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn. Earth wasn't a planet according to the IAU definition at the time. Earth was instead the center of the universe.

The weekly calendar of the ancients was pretty busy:

**Sunday** - [Sun](http://www.etymonline.com/index.php?term=Sunday&allowed_in_frame=0) you're free, sober up and get a suntan

**Monday** - [Moon](http://www.etymonline.com/index.php?term=Monday&allowed_in_frame=0) the day after being free

**Tuesday** - [Jupiter](http://www.etymonline.com/index.php?term=Tuesday&allowed_in_frame=0) Zeus day, be clever and figure something out

**Wednesday** - [Mercury](http://www.etymonline.com/index.php?term=Tuesday&allowed_in_frame=0) go somewhere and talk about it

**Thursday** - [Mars](http://www.etymonline.com/index.php?term=Thursday&allowed_in_frame=0) fight

**Friday** - [Venus](http://www.etymonline.com/index.php?term=Friday&allowed_in_frame=0) make love

**Saturday** - [Saturn](http://www.etymonline.com/index.php?term=Saturday&allowed_in_frame=0) please take a bath!

> **Comments:**
> **username_6**: You got Jupiter and Mars mixed up.  
> **username_7**: Was *die mercuri* the 'market day'?  

---

