Heki Earth, Planets and Space (2020) 72:38 https://doi.org/10.1186/s40623-020-01164-8 FRONTIER LETTER Open Access Geodesy in Japan: legends and highlights Kosuke Heki* Abstract Here, I review modern history of geodesy and geodynamics in Japan, highlighting a few episodes during the last two centuries. The review starts with the frst measurement of the meridional arc length in Japan by Tadataka Ino (1745– 1818) early in the nineteenth century, done as a part of the mapping campaign of the country. Next, I mention the frst international recognition of Japanese geodesy realized by the discovery of a new term in the Earth’s polar motion by Hisashi Kimura (1870–1943) at the beginning of the twentieth century. Finally, I review an unsuccessful campaign to detect present-day continental drift in Japan shortly before World War II, conducted by the Geodetic Committee of Japan being inspired by the hypothesis of active opening of the Sea of Japan by Torahiko Terada (1878–1935). Keywords: Geodesy, History, Japan, Arc length, Polar motion, Continental drift, GNSS Dawn of geodesy in Japan: frst meridional arc the arc lengths were available in Chinese books, Ino and length measurement Takahashi recognized their inconsistency between lit- Figure 1 summarizes the geodetic episodes that I intro- erature and uncertainty in the conversion to the units of duce in this article, and frst of all, I focus on the frst length formerly used in Japan. An accurate value of the geodesist in Japan. Te last samurai government of Japan arc length was also necessary for astronomical calibra- (the Edo Bakufu in Japanese) closed the country in 1639, tion of latitudes in making precise maps. prohibiting most interactions with foreign countries, and Takahashi and Ino submitted a proposal to perform a the stable feudal regime lasted until the Meiji Restoration geodetic survey from Edo to the northernmost part of in 1868. Te government had an astronomical bureau the main island (Honshu) of Japan, and Ezo (Hokkaido). (the Tenmon-kata) responsible for compilation of calen- Te government approved their proposal, considering dars. People in the bureau acquired necessary expertise its importance from the viewpoint of foreign afairs. At in astronomy from literature published in seventeenth– that time, occasional interactions with Russians occurred eighteenth centuries in China based on knowledge in and around Ezo, and the government recognized it an brought by Jesuit missionaries from Europe. urgent need to have precise geographic knowledge of the Tadataka Ino (1745–1818) retired from his family busi- country. Such a social situation matched the personal ness as a merchant in Sawara (currently in the Chiba Pre- interest of people in the astronomical bureau. fecture, Kanto) at the age of 50. He moved to Edo (Tokyo) Ino and his team conducted the frst survey in 1800 and started to study astronomy and geodetic survey- and obtained a preliminary quantity for the meridian arc ing under Yoshitoki Takahashi (1764–1804), the leading length by comparing the astronomical latitude difer- astronomer in the bureau nearly 20 years younger than ences and the distances measured by ground surveying. T. Ino. Tey were interested in determining the merid- Next year, his mapping mission was upgraded to an of- ian arc length of the Earth because they needed accurate cial national project. His team fnished the frst compre- dimension of the Earth to compile a calendar predicting hensive surveys of NE Japan and submitted the maps to precise dates and times of lunar/solar eclipses. Although the government in 1804. By compiling the data observed until then, he determined the one-degree meridional arc length in Japan as 110.75 km (originally published in a *Correspondence: [email protected] Dept. Earth Planet. Sci., Hokkaido Univ., N10 W8 Kita-ku, Japanese unit of length). Tis is only ~ 0.2% shorter than Sapporo 060-0810, Japan the modern value 110.95 km (Fig. 2). © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Heki Earth, Planets and Space (2020) 72:38 Page 2 of 9 JAPAN WORLD Earth’s elliptical shape with the latitude dependence of Earth shape 1750 1750 the arc length. Takahashi passed away in 1804, but Ino and his group Metric system continued geodetic surveys, also in SW Japan, subse- 1800 Modern survey (1799) 1800 quent to those in NE Japan. Te prime meridian was (1800-1816) T. Ino defned at Kyoto, the ancient capital of the country. Ino 1745-1818 T. Terada 1850 Meiji Restoration 1878-1935 1850 conducted astronomical measurements to calibrate lati- tudes in mapping NE Japan. However, SW Japan elon- Chandler wobble (1891) gates in east–west, and he needed to calibrate the ground 1900 Z-term (1902) ILS (1899) 1900 Continental drift (1912) Tobishima geodetic survey results by performing astronomical lon- survey (1928-) gitude measurements. An accurate chronometer was 1950 End of WWII 1950 Sputnik (1957) necessary for precise longitude measurements, but it VLBI/SLR H. Kimura was not available then in Japan. Instead of it, Ino and his GPS array 1870-1943 GNSS 2000 Satellites for time- 2000 group tried to determine the longitude diferences in SW variable gravity Japan in 1805 by comparing the occultation times of the Fig. 1 History of geodesy in Japan. The three pioneers in Japanese Galilean satellites of Jupiter at remote points. In doing geodesy and geodynamics, Ino, Kimura, and Terada, and related that, they employed the procedure explained in Lalande events in Japan and the world mentioned in this article (1771), brought to Japan in a Dutch version only 2 years before. Tey continued observations of these Jovian satel- lites for 6 years but were not able to obtain enough data Neither Ino nor Takahashi was aware of the elliptic- to calibrate longitudes in SW Japan (Kazu 2005). ity of the Earth when they started the survey, i.e., they Tey continued the survey in SW Japan, and the fnal assumed the spherical Earth. In the closed Japan, the gov- version of the whole map of Japan, known as Dai-nihon- ernment only allowed direct contacts with Dutch traders enkai-yochi-zenzu, was submitted to the government through a tiny window at Nagasaki, Kyushu. For exam- in 1821, 3 years after Ino passed away. Japan resumed ple, the Dutch version of the French book Astronomie foreign diplomacy shortly before the Meiji Restoration (ver.2) (Lalande 1771) was imported to Japan as Astro- in 1868, the turning epoch to a modern country, and nomia of Sterrekunde, published in 1773. Y. Takahashi the maps made by Ino served as the basis of a series of obtained the book in 1803 and immediately understood national maps that the new government started to issue the importance of its contents. Over subsequent years, in 1877. Te quality and technical aspects of these maps his group translated it into Japanese and shared the latest are discussed in detail in Oda (1974). Ino is also known astronomical and geodetic knowledge in Europe (includ- in Japan as a hero who lived two lives, as a merchant and ing the shape of the Earth) within their community. It is a geodesist, proving that 50-year old is not too late to important to note that Takahashi and Ino indeed meas- become a geodesist. ured the arc length in Japan but not for studying the First international recognition of Japanese geodesy: Z‑term of polar motion 112 Lapland It is common nowadays that Japanese earth scientists publish papers in English journals, some of which may become internationally recognized and frequently cited. France A paper written by Hisashi Kimura (1870–1943) in 1902 111 would be one of the earliest examples. It was written Japan in response to unduly low rating to Japanese data in an Ecuador Arc length (km) international observing campaign of latitude variations. 110 In the last decade of the nineteenth century, Frie- 0306090 drich Küstner (1856–1936) in Germany and Seth Chan- Latitude (degree) dler (1846–1913) in America independently discovered Fig. 2 Meridian arc length measurements in Japan and the world. the Earth’s polar motion, which was theoretically pre- Comparison of the meridian arc length measurements in eighteenth dicted by Leonhard Euler (1707–1783) back in 1756. century by Europeans (expedition to Ecuador and Lapland, northern First, Küstner reported a quasi-annual latitude variation Finland, in early eighteenth century and measurements by the of 0.2 arcseconds in 1888. Te International Associa- French government to establish the metric system in late eighteenth century), and around 1802 by Ino and his team in NE Japan tion of Geodesy (IAG) coordinated an observing session in 1891 and confrmed that this variation was a polar Heki Earth, Planets and Space (2020) 72:38 Page 3 of 9 motion from the phase reversal of the latitude variations misconduct then). Te ILS central bureau admitted and observed in Hawaii and Germany.