Geodesy in Japan : Legends and highlights
Kosuke Heki
Dept. Earth Planet. Sci., Hokkaido Univ.
In this talk, I briefly review early histories of geodesy and geodynamics in Japan, andintroduce a few highlights during the last two centuries.
(1) Dawn of geodesy: Meridional arc length measurement in 1801
The samurai government of Japan closed the country in 1639 and only allowed
contacts with the western world through Dutch traders using a tiny window at
Nagasaki, Kyushu. Even in this period, the Dutch version of the French book
Astronomie (Lalande, 1764) was imported to Japan as Astronomia of Sterrekunde(1773), and it drew attention of samurai scientists in the astronomical bureau of thegovernment. In the middle of the 18th century, geodetic expeditions by the Frenchacademy of science revealed the difference between the arc lengths between Ecuadorand Finland, and confirmed the Earth’s ellipticity. In Japan, Tadataka Ino (1745-1818),
a wealthy retired merchant in Kanto, studied modern geodesy to accurately map theJapanese Islands. He was also keen in measuring the meridional arc length in Japan,and determined it as 110.75 km by numbers of surveying campaigns early in 19thcentury in NE Japan. This is marginally different from the modern value 110.95 km,
and the overall accuracy of the maps made by his surveys amazed Europeans whoentered Japan 50 years later.
(2) Polar motion and the International Latitude Service: Z-term in 1900
Japan opened the country again in 1854, and the new government vigorously
imported up-to-date science at that time from Europe and America. The firstinternational geodetic observation campaign started in 1899 soon after Mizusawa, NE
Japan, was selected as one of the latitude observatories along 3908’. After analyzing
the data in 1900, the ILS central bureau at Potsdam, Germany, suggested to reduce theweight of the Japanese data due to its abnormally large residuals derived using theequation relating the latitude change  with the polar motion (X, Y), i.e.  = Xcos
+ Ysin . The young director of the observatory, Hisashi Kimura (1870-1943), foundthat a new longitude-independent annually-changing term could largely reduce theresidual (Kimura, 1902 Astron. J.). The new term (Z-term) was officially adopted byILS, and this became one of the earliest world-class achievements by a Japanesescientist. The Z-term is also the first signature of the fluid core of the Earth, detected
earlier than the seismological discovery of the shadow zone of the S wave in 1906.

(3) Geodetic campaigns to detect continental drift during 1920-1930s
Torahiko Terada (1878-1935), a professor of physics (also a member of theEarthquake Research Institute) in University of Tokyo, was a supporter of thecontinental drift hypothesis by Alfred Wegener. He hypothesized that the JapaneseIslands came apart from the continent and have been drifting away (similar to themodern back-arc opening concept). To verify this idea, Japanese national committee ofgeodesy conducted the first campaign in 1928 to detect the drift of the TobishimaIsland off the coast of the Japan Sea (Tobishima means “flying island”, just like
Laputa in Gulliver’s Travels) using both astronomical positioning and terrestrial
triangulation. Re-occupations in 1934 and in 1954 did not yield consistent coordinatechanges for different intervals and techniques. After all, the attempt was not successful(opening of the Japan Sea is not currently active anyway), but the practice of such an
ambitious project 90 years ago still impresses us.
(4) Last 70 years
Advance of geodesy after the World-War II in Japan and the world is so rapid, as
recognized in various papers presented in this meeting, and I will just mention a few
milestones. Yoshihide Kozai (1928- ) formulated how the orbital elements of satellites
change in time (e.g. 1959 Astron. J.), which paved way to the accurate measurement ofJ2 and the discovery of J3 of the Earth by periodic changes of the perigee height of theVanguard satellite. In 1987, a long history of earth rotation observations based on
optical telescopes ended, and Int. Polar Motion Service (central bureau at Mizusawa,Japan) was replaced by IERS. Space geodesy in Japan started in 1980s at Simosato(SLR) and Kashima (VLBI), with the Japanese participation in NASA/CrustalDynamics Project. Deployment of the dense GNSS array started in the middle of1990s, and resulted in many new discoveries such as slow fault movements. Newly
launched Japanese satellites for geodesy include Ajisai (SLR), ALOS-1&2 (SAR), andQZSS (GNSS). The current frontier seems to lie in the ocean floor positioning with theGNSS-Acoustic technique.