Hirayama, Kiyotsugu

(b. Miyagi prefecture, Japan, 13 October 1874; d. Tokyo, Japan, 8 April 1943)

Celestial mechanics.

Hirayama graduated in 1896 from the University of Tokyo, where he continued his graduate studies in astronomy. He subsequently became an assistant professor and later a full professor at the university and was simultaneously a staff member of the Tokyo Astronomical Observatory.

In 1915 Hirayama went to the United States and studied celestial mechanics under Ernet W. Brown at Yale and ephemerides at the U. S. Naval Observatory in Washington, D. C. At Brown’s suggestion that a key to the problems of celestial mechanics lies in the movements of the asteroids and satellites, Hirayama worked on an explanation of the condensations and gaps of the distribution of the mean motions of asteroids. He thought that the condensations were caused by the destruction of a planet. He called a condensation (similar group) a “family” and theorized that each member of a family would have similar eccentricity, inclination, and mean motion (or orbital semi-major axis).

Among the 790 orbits of asteroids presented in the Berliner astronomisches Jahrbuch for 1917, Hirayama in 1918 identified three asteriod families; the number later increased to five. In 1919 he identified thirty-one asteroids of the Themis family, thirty-eight of the Eos family, twenty-three of the Koronis family, sixteen of the Maria family, and eighty-one of the Flora family.

Based on statistics as well as on the known principles of celestial mechanics, Hirayama’s hypothesis was a rare theoretical accomplishment, considering the level of research in astronomy in Japan at the time. His other achievements are in latitudinal change, variable-star theory, and the history of Oriental astronomy.

BIBLIOGRAPHY

Articles by Hirayama are “Groups of Asteroids Probably of Common Origin,” in Astronomical Journal, 31 (1918), 185–188; and “Notes on an Explanation of the Gaps of the Asteroidal Orbits,” ibid., 38 (1928), 147–148.

On Hirayama and his work, see Yusuke Hagihara, “Hirayama Kiyotsugu sensei o shinobite,” in Tenmon Geppõ, 36 , no.6 (1943), 65–67; and “Hirayama Kiyotsugu sensi no omonaru kenyu ronbun,” ibid., 67–68.

S. Nakayama