紀元前660年と日本書紀の成立The Myth of 660 BCE: How China's Influence Shaped Japan's Earliest History

Today, I look at the particular date of 660 BCE, so important to Japanese history, and at how the Nihon Shoki came to be written, shaped by the designs of the ancient imperial house and by overwhelming Chinese influence.

Japan still has an emperor, and his line is said to go back to 660 BCE. Every year February 11, the day that celebrates 660 BCE, is a national holiday, and this 660 BCE theory is widely known among the Japanese. Yet very few people know the history behind how this particular date was fixed.

How Could an Event from 660 BCE Be Determined?

First, how could anyone know of an event from so long ago as 660 BCE? Moreover, there is something unnatural in the way the date was worked out: it follows the Western Gregorian calendar.

Until the early Meiji period (1868–1912), Japan used a lunisolar calendar developed domestically, which was fundamentally different from the solar-based Gregorian calendar. The Gregorian calendar was adopted in Japan only in 1872, quite recently.

To put it plainly, 660 BCE has no basis and was produced by a very rough method of calculation. But before the Second World War in particular, a current of emperor worship arose to lift the national spirit, and the date drew great attention and could no longer be revised.

On the strength of this ancient date, the idea of the emperor's “single unbroken line” (万世一系, bansei ikkei) took shape: the belief that the imperial house has continued as one lineage forever, from the first emperor, Jimmu. This is entirely ideology.

In this period, not only criticism of the emperor, set as the spiritual pillar of the Empire of Japan, but any inquiry into, research on, or remark about the roots of the imperial house was forbidden. For that reason, for a long time no one could voice opposition to the theory that Japan began when Emperor Jimmu took the throne in 660 BCE. Strangely, this influence continued after the war and is unchanged today.

The Origins of the 660 BCE Date: Nihon Shoki and Ancient Chinese Astronomy

So, what was the actual basis for the 660 BCE calculation?

The answer lies in one of Japan’s oldest historical texts, Nihon Shoki (compiled in 720 CE). According to the Nihon Shoki, Jimmu, the first emperor, took the throne in the “year of Shin-yu” (辛酉). This will be unfamiliar to Western readers, but it is an ancient Chinese method of calendrical calculation.

As a premise, for a long time Japan had neither astronomy nor a calendrical system. The calendar was first brought around 602 by a monk from Baekje. But the Korean Peninsula too was under Chinese influence, so what he brought was in fact a Chinese calendar.

In this age, Chinese influence reached Japan by way of Korea. Buddhism and Chinese characters came the same way.

After that, Japan went on importing the calendars as they were updated in China, for more than 1,000 years, until the first domestic calendar, the Jokyo calendar (貞享暦, in use 1685–1755), was invented.

Could Anyone in Ancient Japan Have Known the Year 660 BCE?

For Japan, then, calendrical technology was a new technology that arrived by the 7th century at the latest, accepted naturally as one of the many cultures imported from China, the strongest influence of the day. No one could possibly know of 660 BCE, more than a thousand years before. Naturally, the Japanese before the calendar arrived did not know how to calculate one.

The Nihon Shoki, the basis of the date, like the oldest Kojiki (completed in 712), came out of a national project that gathered old traditions handed down in various places and compiled them under the emperor into a single book. History in Japan could be said to begin at this point.

It is clear that those old traditions had no technology for precise dating. But in other books there are traces of a quite different way of speaking figuratively of a far-off age, which I will introduce separately. Old traditions around the world probably share this character.

Why Was Nihon Shoki Different?

Yet the Nihon Shoki has an important character unlike the Kojiki: it aimed to set up an advanced system like that of China, which it revered.

For example, to establish an emperor and form a unified system of rule centered on him was a long-cherished wish of the rulers of the time, the imperial house and the clans. For that, they had to prove that the emperor was the only historical sovereign. And they had to justify that story absolutely.

So, learning much from China, they took as their reference the many “histories” China had written down the ages, and began the project of compiling Japan's first systematic history.