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Japanese Days of the Week: the 'Seven Luminaries'

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The Japanese days of the week seem quite different from anything in Western languages:

Sunday Monday Tuesday Wednesday Thursday Friday Saturday
J orthography 日曜日 月曜日 火曜日 水曜日 木曜日 金曜日 土曜日
Hiragana にちようび げつようび かようび すいようび もくようび きんようび どようび
Romanisation nichiyōbi getsuyōbi kayōbi suiyōbi mokuyōbi kin'yōbi doyōbi
Meaning 'Sun day' 'Moon day' 'Fire day' 'Water day' 'Wood day' 'Gold day' 'Earth day'

The initial impression is one of primitivity and mystery. The first Chinese character in the name of each day is a simple concept linked with an elemental force of nature such as fire, wood, water, etc. The final character hi, here pronounced bi, means 'sun' or 'day'. The middle character 曜 yō, though familiar to Japanese speakers through its use in the days of the week, is relatively obscure. Yōbi (曜日) is, to all intents and purposes, a single expression meaning 'day of the week'.

Yet the key to understanding the names of the days of the week lies in that obscure second character 曜. Pronounced yào in Mandarin Chinese, 曜 means 'sunlight' or 'luminary; shining body'. The sun, the moon, and the five planets were called the 'seven luminaries' (七曜 qī yào) by the ancient Chinese, much as the ancient Greeks and Romans referred to them as the seven planets. It is from the 'seven luminaries' that the Japanese days of the week are derived.

The first two days of the week are named after the sun and the moon, which represent the male principle ( yáng, Japanese yō) and the female principle ( yīn, Japanese in) respectively. To understand the remaining five we must look at the ancient Chinese theory of the 'Five Elements' 五行 (wǔ-xíng). The Five Elements started out as a primitive system for explaining the universe but gradually developed to become an all-embracing cosmological system. Each element was equated to (among others) a direction, a colour, a season, a time of day, a planet, and a musical note in the pentatonic scale.

The first five planets of the solar system in Chinese, Japanese, and Vietnamese are still named after the Five Elements. In Chinese and Japanese, Mercury is called 水星 ('Water Star'), Venus is called 金星 ('Metal Star'), etc. Vietnamese places the word Sao ('star') in front of each element to form the name of the planet. (Note 2: Names of the outer planets & earlier planet names)

火星 水星 木星 金星 土星
Ch huǒxīng shǔixīng mùxīng jīnxīng tǔxīng
J kasei suisei mokusei kinsei dosei
V Sao Hỏa Sao Thủy Sao Mộc Sao Kim Sao Thổ
Eng Mars Mercury Jupiter Venus Saturn

So the Japanese days of the week are not a system of 'primitive elements' after all! In fact, they run in parallel with days of the week of the Ancient Greeks and Romans -- more closely in parallel, indeed, than modern English:

Japanese 日曜日 月曜日 火曜日 水曜日 木曜日 金曜日 土曜日
Meaning Sun-day Moon-day Mars-day Mercury-day Jupiter-day Venus-day Saturn-day
Latin dies solis dies lunae dies Martis dies Mercurii dies Jovis dies Veneris dies Saturni

The obvious question is: could there be a historical link between the 'seven luminaries' and the 'seven planets' of Western and Middle Eastern antiquity?

The answer is, yes. The most commonly accepted theory is that the use of the seven planets originated in ancient Mesopotamia and Egypt, was adopted by the Greeks and Romans, and then found its way to China. However, the specific route and timing is not clear. The Cihai (辞海), a Chinese encyclopaedia, carries the following entry for 七曜历 (七曜曆) qī yào lì, or 'seven luminaries calendar':

七曜历 qī yào lì, i.e., method of recording days according to the 七曜 qī yào. China normally observes the following order: sun, moon, Mars, Mercury, Jupiter, Venus, and Saturn. Seven days make one week, which is repeated in a cycle. Originated in ancient Babylon (or ancient Egypt according to one theory). Used by the Romans at the time of the 1st century AD, later transmitted to other countries. This method existed in China in the 4th century. It was also transmitted to China by Manichaeans in the 8th century from the country of Kang () in Central Asia (Note 3: The country of Kang[1]).

The earliest use of the 'seven luminaries' (七曜 qī yào) is attributed by Cihai to Fan Ning (範寧 / 范宁), a scholar who lived from AD 339-401. Tellingly, the Chinese 'seven luminaries' were arranged in the same order as the Middle Eastern planetary names for days of the week, and not in the classic order of the Chinese five elements, which put water before fire.

Besides the Manichaean route noted by the Cihai, there was also an Indian route of transmission in the 8th century. The Chinese Buddhist monk Yi Jing (義凈 / 义净) and the Ceylonese or Central Asian Buddhist monk Bu Kong (不空, Amoghavajra) are both credited with referring to the seven-day cycle of planetary names in their writings, drawing on Indian sources. The Indians, in turn, appear to have taken this from the West. (Note 4: The Buddhist route of transmission)

Although there were several routes of transmission into China, it appears that the Indian route was the direct source of the Japanese names for days of the week. In 806, the famous Japanese monk, Kobo Daishi (弘法大師) (Note 5: Kobo Daishi) brought Bu Kong's writings back to Japan along with a huge quantity of other Buddhist scriptures. Great interest was taken in Bu Kong's astrological work by Japanese astronomers, with the result that the planetary names found their way into Japanese calendars of the time. One such calendar was used by the Japanese statesman Fujiwara no Michinaga (藤原道長) for writing his diary in 1007, in which the present-day Japanese names for the days of the week can be found (Note 6: Fujiwara no Michinaga[2]).

Although not in widespread use except for astrological purposes, this system of names was nevertheless maintained by the Japanese right through to the modern era. At one stage the days got out of kilter in eastern Japan and had to be rectified by a calendar reform in 1685. When they came under pressure to harmonise their working calendar with the West in the latter half of the 19th century, the Japanese turned to this old system to name the days of the week, officially adopting them in 1876. After this the names gradually came into general use in Japan (Note 7: The crucial step[3]).

In China, on the other hand, the planetary names largely died out. When the seven-day week was adopted under Western influence in the modern era, the Chinese turned to a completely different system to name the days of the week.

Incidentally, the Japanese word for 'week', shū, is etymologically derived from Chinese roots and has the meaning of 'cycle'. It has since been borrowed into Chinese as one of the alternatives for 'week'.


  1. The country of Kang ( kāng) was one of a group of small states in Central Asia. It was located in the area near Samarkand (present-day Uzbekistan) and came under the control of the Tang dynasty in the 8th century. Manichaeism was founded in Persia in the 3rd century AD by Mani, who drew on several religions, including Zoroastrianism and Christianity, in creating it. Manichaeism was characterised by a dualistic view of the world as a struggle between the forces of light (the spiritual world) and darkness (the material, physical world). It reached the western regions of present-day China by the 6th-7th century and China proper by the end of the 7th century. It was eventually banned in China but continued to exist for several centuries, becoming a force behind peasant uprisings in the 10th-12th centuries. Most information on the web concerning Manichaeism is related to its influence on Christianity through St Augustine. For some additional information, see Zoroaster and Mani (by H. G. Wells).
  2. Fujiwara no Michinaga (藤原道長 966-1027), a member of the powerful Fujiwara family in the Heian period (794-1185), became a major behind-the-scenes power by judiciously marrying his daughters to succeeding Emperors. His period in power saw the greatest flowering of Japan's aristocratic culture. For more information on Fujiwara no Michinaga, see the Britannica article. Fujiwara no Michinaga's diary is known as the 'Mido Kanpakuki' (御堂関白記). Fourteen out of 36 volumes are still extant. The diary was written in what was known as a 具注暦 (guchūreki or 'annotated calendar'), a type of calendar widely used in the Heian period. Under the date for each day, the guchureki carried detailed astrological information on the constellations, the Chinese zodiac, and lucky and unlucky days, followed by several lines of space for writing, making it popular for diary-writing among the court nobles.
  3. Up until the moment that the Japanese government decided to adopt them for the new Western-style week, the planetary names had little more than astrological significance in Japan. They were in no way comparable to the Western days of the week, which had had official status in the calendar since AD 321. After the guchureki-style calendars (具注暦 guchūreki, see note) went out of use, for much of the second millennium the planetary names in Japan were mainly used in kanagoyomi (仮名暦, kanagoyomi), calendars in the kana script used by women. The kanagoyomi recorded only the planetary name for the first day of each month. It was not until 1873, with the adoption of the solar calendar from the West, that every day of the month came to be marked with the name of the day. In March 1876, the Western-style week was decreed for public use, starting the following month. This meant that the seven-day week became a part of everyday life rather than just an astrological curiosity. However, it was many years before industry moved to the custom of taking Sunday as the day of rest. While the adoption of the seven-day week is understandable from the point of view of modernisation (or Westernisation), it's not clear why the old planetary names were adopted to name them. Perhaps it was the contemporary practice of turning to classical written Chinese to render the new concepts that were being imported wholesale from the West. The adoption of the planetary names, taken from Chinese tradition, was very much in tune with the age. While it may have been standard practice, however, the adoption of the planetary names was by no means a foregone conclusion. As the Chinese example shows, there were alternatives. Indeed, were this decision to be taken today, more than likely the English names would be adopted. The Japanese week would be very different if the days were named 'Sandē' 'Mandē' 'Chūzudē' 'Wenzudē' 'Sāzudē' 'Furaidē' and 'Satādē'! At the end of the day, the decision to use old astrological categories to render the new-fangled calendar from the West should be regarded as something of a 'creative leap'. Without this step, the long journey from the West through India, China, and one thousand years of Japanese tradition would have ended as a mere footnote to history. By having taken this step, the Japanese can claim a place alongside the languages of the West as heirs to the ancient day names. The concept of a direct Japanese link to Western tradition is presented most aggressively at the site, where the writer, identified only as 'Lumi', dismisses the idea that the Japanese are endebted to Chinese precedents. The thrust of her argument is that the planetary names came to Japan almost directly from the West, with India and China serving only as stepping stones. Bu Kong, author of the Xiuyaojing, merely relayed what he had heard from monks in the 'far west'. The use of the Chinese planetary names is simply a case of clever adaptation to Chinese culture by Bu Kong (a foreigner) and his disciple. The planetary names never really took hold in China and soon faded with the decline of Buddhism. Lumi thus implies that the planetary names never really belonged to either the Indians or the Chinese. Bu Kong's theories, fresh from the 'far west', were picked up from his disciples within 40 years (virtually 'hot off the press') by the famous Japanese monk Kobo Daishi and brought back to Japan, where they were adopted eagerly, maintained without error for a thousand years, and are still in use today. Borrowing from the West at an early stage where the Chinese failed to do so is almost a badge of pride. This view is understandable given that the Lumi site is about fortune-telling, for which the astrological day names are essential. However, it glosses over the historical context. Kobo Daishi went to China because it was the font of learning and civilisation at the time. Part of China's flowering was its openness to outside influences (including ideas brought in by people like Bu Kong) in a way seldom found later in history. It is strange to pretend that China is only tangentially related to the process by which the planetary names were brought back to Japan.