The Tenpo Calendar, the Most Precise Lunisolar Calendar, and the Meiji Government's Turn to the Gregorian Calendar to Save Money

These days I have become absorbed in calendrical science. I am working to decode the Tenpo calendar, the most precise lunisolar calendar ever made and the one Japan used before the Meiji government, for reasons of its own finances, brought in the Gregorian calendar we now call the “new calendar.”

The Tenpo calendar was a calendar of Japan’s own. The shogunate’s official astronomers read Dutch books for themselves and absorbed Western methods of calculation, and they blended those methods, with real skill, into the long history of Chinese-derived calendars cultivated in Japan and into a calendrical art that had become thoroughly Japanese during the Edo period.

Yoshitoki Takahashi wore down his life deciphering the Dutch edition of Lalande’s treatise on astronomy, known in Japan as the Rarande Rekisho. His son, Kagesuke Shibukawa, and Nobuakira Adachi took up that work and gathered it into the Shinko Rekisho, and the Tenpo calendar was built on that foundation. With the earlier Kansei calendar, Japanese scholars had learned Western astronomy through Chinese translations that reached them by way of China. With the Tenpo calendar, they read the Dutch sources directly, and the calendar stands at the very point where Japanese calendrical science took that step forward.

Personally, I see the adoption of the Gregorian calendar as the moment when Japan, under the banner of what it called “civilization and enlightenment,” showed the first signs of coming apart. That is why I have been thinking about a project centered on the time “before Gregory.”

What is generally called the old calendar is modeled, in essence, on the Tenpo calendar’s package of methods, but strictly speaking it is not the Tenpo calendar. Its calculations were so difficult, and so much of it could never be worked out, that the old calendar in use today has been considerably simplified.

So, working on my own with the help of AI, I have been reading the old documents left by Kagesuke Shibukawa, who made the Tenpo calendar. I am pinning down his particular methods of calculation by mathematical means, and I am trying to restore, with those methods built in, a calendar from before Gregory. That calendar is the Tenpo calendar itself, which may fairly be called the most “Japanese” calendar of all.

Let me say something here about the sensibility I am calling “Japanese.” The Tenpo calendar grew out of the Chinese calendar, which descends from the wisdom of antiquity. Japan honed that inheritance for a thousand years, made it fully its own, and only at the very end took in Western techniques of calculation. No other calendar in the world was made this way, and it was a groundbreaking achievement. Its precision, moreover, is the highest of any lunisolar calendar in history, and it is known to be more precise even than the Gregorian calendar. It can be called, in other words, the finest calendar humanity has ever produced.

Even more than its precision, though, I believe the character of the Tenpo calendar strikes the balance that sits most comfortably with what Japan is. Japan took possession of a technology created by China, the great giant of ancient wisdom, and made it Japanese in Japanese places. After nurturing it for a thousand years, it took in just a little from Western Europe, that frontier at the western edge of the world.

If this is the balance that suits Japan best, then we Japanese ought, in the distance we keep from the West, to go all the way back to that point. The modernity forced on Japan by Western encroachment after the Meiji era was clearly excessive, and it has left us with something like a metabolic disorder. Just as many young people today long for the world before smartphones, drawn to the feel of 1990s Hong Kong or of Japan in the early 2000s, I have begun to give weight to the sensibility of the time “before Gregory.”

For me, then, the interest in calendrical systems is not only about calendars. It touches something fundamental, namely how far back we are willing to take history itself when we think about it. This is because, in every age, the calendar has been fated to change at the convenience of those in power.

The most recent example is Ukraine. Under President Volodymyr Zelenskyy, and in line with the wishes of the United States, Ukraine declared its break with Russia and set out to join NATO and the EU. Along the way, a bill was submitted to move the Christmas holiday from January 7, the date it had shared with the Russian Orthodox Church, to December 25. The president signed it, and it became law on July 28, 2023.

On May 24 of that same year, the Orthodox Church of Ukraine had officially announced that it would move to the Revised Julian calendar from September 1. As a result, January 7, which corresponds to December 25 in the Julian calendar, ceased to be a public holiday, and only December 25 in the Gregorian calendar remained an official one. The year 2023 was the first time the country celebrated Christmas as a nation on December 25. Ukraine is now retracing, in vivid detail, the nightmare that Japan lived through in the Meiji era.

Calendars change with the designs of those who govern. On the Japanese side, the emblem of this was the Meiji government.

The Tenpo calendar, in fact, remained in use until the fifth year of Meiji. Under the old calendar, however, the following year, Meiji 6, would have had an intercalary month, a second sixth month. The government had only just moved to monthly salaries, the system that came with modern industry, and it would have had to pay its officials thirteen times that year.

If it adopted the new calendar, the Gregorian one, the intercalary month would disappear, and every year would need only twelve months’ pay. Better still, because the twelfth month of Meiji 5 would last only two days, the government could use that as grounds to skip the payment and get by with eleven months. Short of money, the government used this trick, adopted the Gregorian calendar, and escaped its financial crisis.

To put it plainly, Japan did not adopt the Gregorian calendar because it was superior. It was a historical moment in which a calendrical system changed because of the government’s own purse and its cunning. The Gregorian calendar has been in use ever since, and it remains in use today because it fit so well with the system of industrial society.

Until the Edo period, by contrast, the Japanese had no notion of working five days out of seven and resting over a two-day weekend. Days off differed by occupation, and working hours were incomparably shorter than ours. Compared with the people of Edo, we have lost a great deal to this industrial social system, the Gregorian calendar.

One could say that the industrial system has robbed us of chances for people to meet, connect, and spend time with one another. To win them back, each of us will need, from now on, to live by two calendars, carrying both the Gregorian calendar that industrial society has put in place and the Japanese sensibility from before Gregory, at the same time, in a single body. This is not merely a matter of how we sense time.

By each of us living in these parallel worlds, I believe, we can encourage a rethinking of our excessive relationship with the West, and this will prepare us for an important turning point over the next twenty years, when the relationship at the center of it all, the one between Japan and the United States, is renewed.

As the brutal heat of summer comes to an end, I feel that what we must do now lies not in making demands of politics, which, given our demographics, will never change, but in the calendar.