Georges Marc Marie Sagnac (14.10.1869 — 26.02.1928) — French experimental physicist. Born in Périgueux. Graduated from the University of Paris (1894), where he worked as an assistant until 1900. From 1900 he was a professor at the University of Lille, and from 1911 at Paris.
His research concerned the optics of moving bodies, the interference of light, X-rays and radioactivity. He discovered the secondary radiation produced by cathode rays and X-rays when they fall on metals. He determined the charge of the beta particles of radioactive radiation. In 1913 he proved experimentally that an observer within a rotating system can determine its speed of rotation (the Sagnac experiment).