A shop, the planets and the days of the week

In Tokyo there is a shop called Sui Kin Chi Ka Moku Do Ten Mei Kai, written 水金地火木土天冥海. At first glance this seems a very long, hard to pronounce and difficult to remember name, so why name a shop like that?

The names of the planets in the solar system

The explanation is that the name refers to a mnemonic for the planets in the solar system: each kanji in the name of the shop is the first kanji of the name of a planet, starting from the one closest to the sun. So most Japanese people will recognise and remember the name. And because in Japanese everything gets abbreviated if at all possible, in practice everyone calls the shop “Suikin”.

One oddity about the name is that most people will in school have learned the sequence Sui Kin Chi Ka Moku Do Ten Kai Mei ( 水金地火木土天海冥), so the last two names are swapped. This is because when the shop was founded (in 1984), Pluto (冥王星, meiōsei) was closer to the earth than Neptune (海王星, kaiōsei), but because of Pluto’s strongly elliptical orbit, this is only the case for two periods of about 20 years in the total of 248 year it takes to complete one orbit around the Sun. The last time this happened was between 1979 and 1999, and it won’t happen again until 2227.

What puzzled me is the connection between the order of the planets and the order of the days of the week in Japanese. The names of the planets are:

Table 1. Names of the planets in English and Japanese

The suffix -sei (星) actually means “star”. The names of the last three planets are Japanese descriptions of the roles of the gods Uranus, Neptune and Pluto. The other five planets are named after the Five Elements (五大, godai) used in Chinese philosophy: water (水), metal (金), fire (火), wood (木), and earth in the sense of soil (土).

The days of the week are listed in Table 2. The suffix -yōbi (曜日) means “weekday”. I’ve added the names used in the Roman calendar for comparison.

Table 2. Names of the days of the week in English and Japanese and their Roman name

The order of the days of the week

What I found intriguing is that the days of the week occur in the same order in our calendar and in the Japanese calendar. My first assumption was that the Japanese simply borrowed the Western convention after contact with the West, which would have been in the 16th century at the earliest, but it turns out that this is not the case: the adoption of a 7-day week and the naming of the weekdays was introduced in China in the 4th century (from either ancient Mesopotamia or Egypt, and likely via Greece) and from there in Japan in the 9th century. I found the answer on the website of the Sendai Astronomical Observatory (Sendaishi Tenmondai, 仙台市天文台). What follows is a rewording of that explanation.

In ancient times, people read the time and season from the stars in the night sky. They discovered that five of these “stars” were moving in a strange way, overtaking each other and sometimes appearing to move backwards. They called these wakusei (惑星), literally “perplexing stars”. These are the five planets Mercury, Venus, Mars, Jupiter and Saturn that can be seen with the naked eye.

The ancients thought that planets, the sun and the moon were turning around the earth. As a result of their observations they thought the moon was closest to the earth followed by Mercury, Venus, the Sun, Mars, Jupiter, and Saturn:

Furthermore, the belief was that the sun, the moon, and the five planets were “star gods” (星の神, hoshi no kami) who dominated time and space. And the astronomers observed the moon cycle of 28 days, which was split into four 7-day units, with one day dedicated to each star god. This was the basis for the notion of the ​”week”. The division of the day into 24 hours had already evolved based on the concept of the twelve constellations of the zodiac. Each hour was believed to be controlled by a specific star god, and each day was believed to be dominated by the specific star god controlling the first hour of that day. And the order in which the star gods were assigned to the hours and days was based on the order in which the “stars” were observed.

If we start on the first hour of the first day with Saturn, the planet furthest away from the earth, this is Saturn’s day (土曜日). Each hour we move to the next closer planet in the geocentric model: Jupiter, Mars, the Sun,Venus, Mercury and the Moon, and we repeat this process, so after the Moon we start again with Saturn. Doing so, after 24 hours, we arrive at the Sun (日曜日), and 24 hours after that, the Moon (月曜日), then Mars (火曜日), Mercury (水曜日), Jupiter (木曜日), Venus (金曜日), and after one week, this pattern repeats itself and we’re back at Saturn:

This is how the ancient astronomers in Mesopotamia or Egypt related the days of the week to the planets, and from there it was adopted in ancient Greece and Rome but also in ancient China and Japan. To my mind it’s really quite amazing to learn that the order of the days of the week relates in this way to the ordering of the “stars” in the geocentric model.

Bonus track

Here is a small Haskell program that creates the week day sequences from the sequence of the “star gods”:

  starGodSequence = reverse ["月","水","金","日","火","木","土"]
  starGodsAllHours = concat ( repeat starGodSequence )
  starGodsDaysFrom s = (head s):starGodsDaysFrom (drop 24 s)
  weekDays = take 7 ( starGodsDaysFrom starGodsAllHours )

The program performs the operations explained above: it takes the list of “star gods” in geocentric order and reverses them so that “土” (Saturn) is first, this is starGodSequence. It then repeats this list forever (in Haskell you can do this). The concat flattens this list of lists into a single list, starGodsAllHours.

The function starGodsDaysFrom is recursive, what it does is take the first element in a list, then drops 24 elements (including this first one), and repeats the process, again potentially forever. We apply this function to starGodsAllHours. This produces a new list which repeats the days of the week forever.

Finally, to get the seven days of the week, we take the first seven elements of that new list.


The banner photo shows the moon-viewing room (shōgetsurō, 嘯月楼) of the Shisen-dō (詩仙堂) temple in Kyoto.