What is the tenmonkata (天文方)?
— the shogunate's bureau of sky and calendar
Meaning
The tenmonkata was the office of the Edo shogunate in charge of astronomical observation and calendar-making.
Today we buy calendars; in Edo-period Japan, making the calendar was the government's job. The calendar even announced eclipses in advance. So a calendar that drifted from the sky had to be corrected — and that was the tenmonkata's work.
How it worked
- Making the calendar — revising its underlying computation was called kaireki (calendar reform). The Kansei reform (in force from 1798) was carried out by Takahashi Yoshitoki, recruited into the bureau.
- Checking against the sky — eclipses were tests of the calendar's accuracy. After a partial lunar eclipse in 1798, Yoshitoki himself recorded in a letter that his own calendar's prediction had run about half a koku late.
- Surveying and maps — according to the Geospatial Information Authority of Japan, Yoshitoki hoped to measure the length of one degree of latitude, petitioned for a survey to Ezo (Hokkaidō), and recommended his student Inō Tadataka for the job.
When outsiders walk in
At the time of the Kansei reform, no one inside the bureau could actually do it. In 1795 the shogunate summoned Takahashi Yoshitoki, a private scholar in Osaka, to Edo and appointed him to the tenmonkata.
That same year, Inō Tadataka, a merchant of Sawara who had just retired at fifty-one, became Yoshitoki's student. His teacher was nineteen years younger. For a moment, a government bureau had become a place people went to learn. After five years of study, Tadataka started walking — to Ezo, at fifty-six.
When Yoshitoki died in 1804, his eldest son Takahashi Kagayasu succeeded him at twenty. Tadataka's map stayed with this bureau into the son's generation.
Fragments where this word appears
- Inō Tadataka (survey diary / Takahashi Yoshitoki's 1798 letter on the late eclipse) — Japanese archive page: 伊能忠敬 超詳細版
(As more fragments touch the tenmonkata, they will be added here.)
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