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The seven days of the week seems to be commonly named after celestial bodies. What I find curious is that all seven days apparently share the same names in both West and East.

  • Sunday is, of course, Sun's Day. In East Asia, it is 日曜日 - Day of the Sun.
  • Monday comes from "Moon's Day". In East Asia, it is 月曜日 - Day of the Moon.
  • Tuesday is Mars / Týr's day. In East Asia, it is 火曜日 - Day of Mars.
  • Wednesday is Mercury / Odin / Wodin's Day. In East Asia, it is 水曜日 - Day of Mercury.
  • Thursday is Jupiter / Thor's Day. In East Asia, it is 木曜日 - Day of Jupiter.
  • Friday is Venus / Frigg / Freyja's Day. In East Asia, it is 金曜日 - Day of Venus.
  • Saturday is Saturn's Day. In East Asia, it is 土曜日 - Day of Saturn.

The choice of planets probably reflects their higher visibility. But this doesn't explain why they are ordered the aforementioned way. So, is there an astronomical reason for this specific order? And does it explain the apparent agreement between Europe's and the Sinitic cultural sphere in naming their weeks - or did it come about due to cultural diffusion / pure coincidence?

(Note: I used Japanese for the Asian examples, but Korea shares the same system. Traditional Chinese did too, but Monday through Saturday was simplified to "Star Day 1-6" in Modern Chinese while Sunday remained the Sun's.)


@LieRyan has raised some evidence[1] supporting the idea that the order became globalised due to cultural diffusion. So this just leaves the question of why the order is the way it is.

[1]: Interestingly, one of the linked sources noted that the order of the five planets differed from classical Chinese ordering of elements. While true, it is actually the reverse of the (different, but more common) traditional order.

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One theory says,

`If you order the "planets" according to either their presumed distance from Earth (assuming the Earth to be the center of the universe) or their period of revolution around the Earth, you arrive at this order: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn

Now, assign (in reverse order) these planets to the hours of the day:

1=Saturn, 2=Jupiter, 3=Mars, 4=Sun, 5=Venus, 6=Mercury, 7=Moon, 8=Saturn, 9=Jupiter, etc., 23=Jupiter, 24=Mars

Then next day will then continue where the old day left off:

1=Sun, 2=Venus, etc., 23=Venus, 24=Mercury
And the next day will go
1=Moon, 2=Saturn, etc.

If you look at the planet assigned to the first hour of each day, you will note that the planets come in this order:

Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus

This is exactly the order of the associated week days. Coincidence? Maybe.

Another article says

Ancient Mesopotamian astrologers linked a planet-god to each hour of the day and then arranged them to their correct cosmological order. They used a seven-sided figure to keep track of the proper names of the hours and days in relation to the planet gods where each vertex was marked with a planet’s name in the proper order.

So the conclusion is that the order of the week-days is supposed to be governed by the planetary positions of the planets of our solar system.

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  • $\begingroup$ That is a very interesting theory (+1). Though it does seem a little convoluted. $\endgroup$ – Semaphore Nov 11 '14 at 22:23
  • $\begingroup$ @Semaphore It is not far-fetched. This (the day of the week named after the celestial body that covers the hour of the day at sunrise) is quite explicitly the idea of hora in Indian astrology/astronomy (jyotiṣa) and this aspect was probably borrowed from the Greeks/Romans. $\endgroup$ – ShreevatsaR Feb 20 '16 at 2:34
  • $\begingroup$ The first theory ("Lords of the hours") is correct, the second is utter nonsense. The Ancient Mesopotamians did not even have a seven-day week. $\endgroup$ – fdb Nov 26 '16 at 20:05
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As Amit mentions, the naming probably originated from hellenistic astrology in Egypt, wherein each day would be associated with the influence of a particular celestial object. The origins are lost, but the writings of Roman historian Dion Cassius (AD 150-235) have survived; he describes the scheme as follows:

The celestial objects were ordered according to their orbital period:

  • Saturn 29 years
  • Jupiter 12 years
  • Mars 687 days
  • Sun 365 days
  • Venus 224 days
  • Mercury 88 days
  • Moon 29 days

Now assign the first hour of the first day to Saturn, the second to Jupiter, etc, and repeat the cycle. The 24th hour is then assigned to Mars, the 1st hour of the second day to the Sun, and so forth. The first hour of the 8th day will correspond with Saturn again, and the whole cycle is complete. Each day would then be named after the celestial object assigned to the first hour (which corresponds with 6 a.m.). In this way, we get the correct order

Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus.

The association between Saturday and the Sabbath may come from the Book of Genesis: the planets were created on the 4th day, which according to Jewish tradition starts on the first Tuesday, at 6 p.m. (13th hour). If we start our naming scheme at that point, with Saturn first, we indeed arrive at Mercury on Wednesday 6 a.m., Jupiter on Thursday 6 a.m., etc. Another theory states that Saturn is associated with bad luck, so one should refrain from working on this day. The Christian adoption of Sunday as the day of prayer was probably meant to stomp out competing religions such as the sun-worshiping Mithraism; also, Sunday was believed to be the day of Jesus' resurrection.

By the first century AD the astrological week had reached India, where the names of the seven days in Sanskrit follow the names of the same seven planets. From India, the weekdays spread to Tibet, Burma, Nepal, Thailand, and Ceylon; by the Song dynasty, at the end of the first millennium, it had reached China.

source: Mapping Time: The Calendar and Its History

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The choice of planets probably reflects their higher visibility.

Nice catch, but it's not perfect. I checked the Wikipedia pages for Mercury, Jupiter, and Saturn, and found that they're not in that order for apparent magnitude. You've also probably heard (or perhaps even observed for yourself) how bright Venus is - there's no reason for it to be that far down the list.

There aren't any other astronomical patterns that I can see.

  • Size: The ancients wouldn't have been able to properly figure out the sizes of the planets, given their limited technology. There's also no pattern in the week.
  • Distance from Earth: Nope. Once again, Venus is far, far away, while the Sun is number 1. Jupiter and Saturn are farther off, which would make sense, but the pattern isn't right.
  • Importance of their namesakes vs. importance of the bodies: This is a weird one, but hey, it's a shot. I thought that perhaps the ancients would have given their most important deities the first spots in the week, but that's clearly not the case. Look how far down Jupiter is. The same goes for Venus. The bodies do seem to be listed in their physical importance - after all, the Sun is a lot more important to us than Jupiter.
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  • $\begingroup$ Is not it funny that modern people check planets visibility with Wikipedia, instead of looking at the sky? If you ever try, you will conclude that seeing Mercury is very difficult, this is not because of magnitude but because it is close to Sun. It helps to know exactly where it is and stare at this very point in the sky. Binocular also helps. $\endgroup$ – Alexandre Eremenko Nov 11 '14 at 22:16
  • $\begingroup$ @AlexendreEremenko Oh, come on. I live in a suburban area, where it's pretty hard to see any planets, even with binoculars. I even sometimes have trouble finding Orion. But yeah (sorry, I missed your point at first!), I suppose that is ironic. $\endgroup$ – HDE 226868 Nov 11 '14 at 22:17
  • $\begingroup$ Yeah, that was just me speculating on why these were the planets chosen; I didn't mean to suggest it was how they were ordered on the week too (edited question to be clearer on this). Though @AlexandreEremenko raises a good point about how visible they might be to early astronomers vs their apparent magnitude. +1 for aiding the process of elimination btw. $\endgroup$ – Semaphore Nov 11 '14 at 22:19
  • $\begingroup$ Yeah, it is a good point. I hadn't considered that. I suppose that their best shot might be during an eclipse, when the Sun would be partially blotted out. $\endgroup$ – HDE 226868 Nov 11 '14 at 22:21
  • $\begingroup$ @Semaphore: no I do not worry about early astronomers. Unlike the modern amateurs, they had perfectly dark nights, and no light pollution at their disposal. $\endgroup$ – Alexandre Eremenko Nov 11 '14 at 23:08
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I came across a rather in-depth treatment of the order in The Seven Day Circle: The History and Meaning of the Week by Eviatar Zerubave, which supports the explanations given in Pulsar and Amit Tyagi's answers are correct.

If we examine how the sequence of the seven days of the astrological week might have derived from the order in which the Hellenistic astronomers arranged the seven planets, we are immediately struck by one obvious overall pattern ... one must proceed by "planetary leaps" that consist of skipping two planets each time.

[...]

Unfortunately, a piece written around A.D. 100 by the Greek essayist Plutarch and bearing the suggestive title "Why Are the Days Named After the Planets Reckoned in a Different Order form the Actual Order?" has been lost forever, and our earliest source on this matter is the third-century Roman historian Dio Cassius.

The book recounts two explanations given by Cassius Dio on why this is. Since he is much closer to the origins of the seven day week as we know it, presumably he had a more complete understanding of the motivations behind the order of the days. Nonetheless, the first theory appears rather batshit crazy:

One explanation [Cassius Dio] claims to have heard relates to the tetrachord, a diatonic series of four tones with an interval of a perfect fourth between the first and the last tone. The principle underlying the process of constructing intervals of perfect fourths happens to be identifiable to the one we have just discovered in the case of the days of the astrological week, namely, proceeding through the cycle of seven tones by skipping two tones each time.

The second theory however is presented as much more sensible and likely to be correct:

Dio Cassius claimed to have also head a second explanation of the order of the planetary days. This explanation is much more convincing, since it takes into account not only the traditional Hellenistic arrangement of the planets in a sequence corresponding to their orbital periods or assumed geocentric distances, but also the Hellenistic doctrine of chronocratories, which provides the astrological context within which the planetary seven-day week actually evolved.

According to this explanation, one would astrologically "assign" the first hour of the first day to the most distant planet, Saturn, and then proceed to assign each following hour to the following planet in the traditional sequence ...The twenty-fifth hour of the first day would have been assigned to the sun, yet the daily cycle was divided into only twenty-four hours, so it would be the first hour of the second day. Since the controller of the first hour of each day was also supposed to dominate that entire day as a whole, the entire first day came to be astrologically assigned to Saturn, the second one to the sun ...

So in conclusion, it appears that Pulsar and Amit are correct. Namely, that the order of the days comes from assigning the seven planets to the hours in sequence, and then taking first hour as the name of the day.

Unfortunately I can't accept both answers, so I'll pick the older one.

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Another answer is in https://astronomy.stackexchange.com/questions/10213/naming-of-the-planets-of-the-solar-system

Except for Venus the order is with increasing distance from the Sun. Why Venus is the odd man out is not clear.The names are in Sanskrit and mean something in Sanskrit.

Chandra is the Moon

Surya is the Sun

Mangala is Mars

Bhuda is Mercury

Jupiter is Guru (weighty in Sanskrit as its size was known from Surya Siddhanta)

Shukra is Venus

Saturn is Shanischara (slow-moving in Sanskrit)

The week days are ordered in that order of increasing distances except for Venus (I do not know why.) All planets are considered gods!

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