He was born into a family of dōshin, low-ranking samurai who guarded Osaka castle. The family's stipend was small, but from childhood he took a keen interest in sangaku, the study of calculation and numbers. This curiosity would later lead Yoshitoki to become Japan's foremost astronomer.
He succeeded his father as a dōshin in Osaka. While carrying out his duties, he kept studying mathematics in his free time. He is said to have bought books out of his small stipend and worked on calculations late into the night. Days of studying while working continued for a long time from here.
He married Shime, daughter of another dōshin family. The next year his eldest son Kageyasu was born, and two years later his second son Kagesuke. Shime managed the household budget to find money for observing instruments, steadily supporting her poor husband's research. Her devotion as a wife is still told today.
To study calendrical science he became a pupil of Asada Gōryū, a famous astronomer in Osaka. There he met Hazama Shigetomi, who had joined around the same time and became his lifelong friend. He was so devoted that, it is said, the day after his house burned down he was discussing calendars with Gōryū and Shigetomi in the ruins.
Around this time he worked on a Chinese book, the Lixiang kaocheng houbian, that Shigetomi had obtained. It described Kepler's theory that the sun and moon move in elliptical rather than circular orbits. The theory was difficult for Japanese of the time, but Yoshitoki mastered it together with his colleagues.
His ability was recognised by the shogunate, and he was summoned to Edo with Shigetomi to reform the calendar and appointed an official astronomer. It was an extraordinary rise for a low-ranking samurai. But the same year Shime, his wife who had supported him for so long, died at 29. Yoshitoki never remarried.
A few months after he came to Edo, Inō Tadataka, nineteen years his senior, visited asking to become his pupil. Yoshitoki taught Tadataka astronomy and calendrical science, and called him "Master Calculator" for his eager daily observation of the stars. It was the start of an unusual relationship between a younger teacher and an older pupil.
After repeated observations in Kyoto, he completed the Rekihō shinsho setting out the method for a new calendar. He had to reconcile views with the Tsuchimikado family, who handled the court calendar, and with other astronomers, which caused much hardship. Though working in poor health, he accomplished the great task of calendar reform.
The new calendar created mainly by Yoshitoki and Shigetomi came into use as the "Kansei calendar." Incorporating ideas from Western astronomy, it was more accurate than previous calendars. This achievement raised Yoshitoki's reputation further and deepened the shogunate's trust in him.
Finding the exact length of one degree of latitude to learn the size of the earth was also Yoshitoki's wish. He advised Tadataka that "a distance like that from Edo to Ezo must be measured" and worked hard to obtain the shogunate's permission. This became the start of Tadataka's survey of all Japan.
He obtained a Dutch translation of the astronomical work of the French astronomer Lalande and read it intensively despite illness. Astonished by contents far beyond previous knowledge, he left studies summarising its key points. His study is said to have been so intense that it wore away his life.
His chronic tuberculosis worsened and he died in Edo, still in the prime of life. His eldest son Kageyasu succeeded him as astronomer and completed the map of Japan after Tadataka's death. His second son Kagesuke also led the next calendar reform as astronomer. By his own will, Tadataka was buried beside his teacher Yoshitoki.
Santō KyōdenSakai HōitsuKobayashi IssaTani BunchōJippensha IkkuKyokutei BakinTakadaya KaheiSatō Issai
People of the Edo period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.