He was born in Osaka, the eldest son of Takahashi Yoshitoki, who would later become a shogunate astronomer. His childhood name was Sakusuke. His father was a scholar devoted to making calendars with Western astronomy, and his younger brother Kagesuke later became an astronomer too as heir of the Shibukawa family. He grew up surrounded by stars and calculations.
He moved to Edo to join his father, who had been summoned there as a shogunate astronomer. Under his father he studied astronomy, calendar science and land surveying, and he also studied Chinese learning at the Shōheikō, the shogunate's academy. His father's pupils included Inō Tadataka, so Kageyasu grew up seeing the world of surveying up close.
His father Yoshitoki died, and while still in his teens he succeeded him as shogunate astronomer (tenmonkata). Because he was young and inexperienced, his father's friend the astronomer Hazama Shigetomi became his guardian and taught him many branches of astronomy. He worked on observing the stars, the calendar and translating books.
The shogunate ordered him to make a map of the world. He collected Dutch books, but since his knowledge of geography and Dutch was still limited, he brought the Nagasaki Dutch interpreter Baba Sajūrō to Edo to help. Comparing Western maps and Chinese books, he strove to draw the world accurately.
Drawing on Western maps, Chinese books and Mamiya Rinzō's report of his exploration of Sakhalin, he wrote a study of northern geography. Trial maps made during the project were printed from copper plates by Aōdō Denzen and are said to have been the first copperplate maps in Japan.
With help from Hazama Shigetomi and others, he completed a new world map. Adding the results of Mamiya Rinzō's Sakhalin exploration to the latest Western maps, it was cutting-edge for its time, and its accuracy kept it in use until the early Meiji era. Around the same time he also translated a Russian state letter written in Manchu script.
He became head of the Office for Translating Barbarian Books, set up by the shogunate to translate Western books, and began translating a French encyclopedia from its Dutch version into Japanese. This office, which gathered Dutch-learning scholars such as Ōtsuki Gentaku and Baba Sajūrō, later grew into the center of the shogunate's Western studies.
He became the shogunate's librarian and chief astronomer, also taking charge of the shogunate's library. He continued studying Manchu and wrote several books on it. He also pursued a plan to translate related documents into Dutch and publish them, to correct errors in an account of Japan published in Russia.
As shogunate astronomer he had supervised and supported Inō Tadataka's nationwide survey. After Tadataka's death, he kept the death secret and continued the work with Inō's pupils, completing the great coastal map of Japan, which accurately drew the coastline of the whole country, and presented it to the shogunate. It carried on the aims of his father and Tadataka.
He met the German physician Siebold, who had come to Edo with the head of the Dutch trading post, and they became close. Kageyasu wanted new Western geography books and maps, and in return he is said to have given Siebold Japanese maps that were forbidden to be taken abroad.
Forbidden maps of Japan and other items were found in the luggage of Siebold as he prepared to go home, setting off the Siebold Incident. It emerged that Kageyasu had given him the maps, and he was arrested and imprisoned. Many others involved were also questioned.
While the investigation continued, he fell ill in prison and died in his mid-forties. Even after his death he was formally sentenced and severely punished. It was a sad end for a scholar who had left great achievements such as the world map and Inō's map. His grave in Ueno stands beside those of his father Yoshitoki and Inō Tadataka.
Inō TadatakaŌtsuki GentakuTakahashi YoshitokiMamiya RinzōPhilipp Franz von Siebold
Mamiya RinzōRai San'yōHirose TansoAizawa SeishisaiNinomiya SontokuŌshio HeihachirōWatanabe KazanTōyama Kagemoto
People of the Edo period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.