Kiyotsugu Hirayama (平山 清次, Hirayama Kiyotsugu; October 3, 1874 – April 8, 1943) was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called "Hirayama families" in his honour.
Source: “Kiyotsugu Hirayama”, English Wikipedia (CC BY-SA 4.0), excerpt