神武天皇の紀元前660年即位説（2）Emperor Jimmu's 660 BCE Accession: How the Date Was Calculated

Today, I offer further insight into the theory that Emperor Jimmu took the throne in 660 BCE, the basis of the emperor's “single unbroken line” (bansei ikkei).

In my previous article, I suggested that the prewar rise of the theory of the “unbroken line of the emperor” is not historical fact but a kind of ideology that took shape in the modern era. This time, I look at that part a little further.

The image of the emperor as a single unbroken line has long governed Japan, from before the war to today. Yet the theory that places the accession of Emperor Jimmu, known as Japan's first emperor, in 660 BCE has no basis in historical fact.

Working out such a date requires advanced technology, available only after the Japanese had mastered the Chinese calendrical techniques imported around 602 through a monk from Baekje. Personally, I think it was most likely calculated in the first half of the 7th century.

Involved in the background is the thirty-third sovereign, Empress Suiko (r. 593–628). But Suiko herself was no more than a screen for those in power; the ones who actually held the reins were the fixer Soga no Umako and Prince Shotoku. This triangle was the driving force behind Japan's first large-scale Sinicization. Without it, the spread of Buddhism in Japan would have stalled.

The three revered China, and to secure lasting political power they learned deeply from the Chinese imperial system. What drew their attention most was the craft of producing the “histories” of ancient China, and the great effect on governance that it brought.

In fact, the Nihon Shoki (completed in 720), one of Japan's oldest books, records that before the Kojiki (712) and the Nihon Shoki itself were produced as national projects led by the emperor, books equivalent to histories had already been produced in Japan. The first such project was carried out by Suiko, Soga no Umako and Prince Shotoku.

From this we can infer that in the 7th century many advanced Chinese technologies were imported into Japan, and that, with the help of naturalized people able to understand and handle them and of scholars invited from China and Korea, Japan's first history was produced as a major project.

Chinese histories in particular had long drawn actively on advanced astronomy and calendrical knowledge to calculate detailed chronologies, including those of successive emperors. The strongest influence was the Records of the Grand Historian (Shiji) by Sima Qian (145–86 BCE?), which was extremely attractive to Japan's rulers of the time.

In short, around the first half of the 7th century, with the help of naturalized people and scholars from China and Korea, and using Chinese chronological techniques, they worked out the accession date of Jimmu, the first emperor of the old traditions.

The Nihon Shoki records that Jimmu took the throne in the “year of Shin-yu” (辛酉). On this basis, when the Western Gregorian (solar) calendar was adopted nationally in 1872, in the Meiji era, the earlier lunisolar method of calculation was reviewed, and the date was eventually fixed at 660 BCE.

Even before that, however, there had been moves to convert the dates of Japan's lunisolar calendar, in use for over a thousand years, into the Gregorian calendar. Two figures of the Edo period had a strong influence on the Meiji-era 660 BCE theory.

One is Hakuseki Arai (1657–1725), who served as a Confucian official of the Tokugawa shogunate. Deeply learned in Confucianism, he also published a book titled Seiyo Kibun, which introduces Western knowledge learned from the Dutch, the only foreigners permitted to trade, at the port of Nagasaki, the one port left open during the era of national seclusion.

It is conceivable that Arai, drawing on knowledge from the Dutch (the Gregorian calendar in particular), was the first to propose 660 BCE for Jimmu's accession, which had been placed within the lunisolar calendar of the day.

The other is the National Learning scholar Mabuchi Kamo (1697–1769). But Kamo was a scholar of National Learning through and through, and how he came to propose the 660 BCE theory is unknown. He studied widely, however, so it is possible that he was influenced by Arai.

Seen in this way, through the flow of calendrical technology in Japanese history, it dimly emerges that the theory of 660 BCE for Jimmu cannot be accepted, at the least. Then, before the Second World War, when the ideology of the unbroken line was rising, a person appeared who, drawing on astonishing research, pointed out that 660 BCE may be not mere historical fact but a date derived from Chinese astronomical calculation.

I will look at this person in the next article.