Rudolf Wolf, in full Johann Rudolf Wolf (born July 7, 1816, Fällenden, near Zürich, Switz.—died Dec. 6, 1893, Zürich), Swiss astronomer and astronomical historian.
Wolf studied at the universities of Zürich, Vienna, and Berlin and in 1839 went to the University of Bern as a teacher of mathematics and physics; he became professor of astronomy there in 1844. In 1855 he accepted a professorship of astronomy at both the University of Zürich and the Federal Institute of Technology in Zürich. At his instigation an observatory was opened at Zürich in 1864.
Wolf confirmed S.H. Schwabe’s discovery of a cycle in sunspot activity and by use of earlier records defined the cycle’s length more accurately, at an average of 11.1 years. Wolf also correlated this solar cycle with the observations of the Earth’s magnetism made by Johann von Lamont. In 1849 he devised a system, still in use, of gauging solar activity by counting sunspots and sunspot groups, which are known as Wolf’s sunspot numbers.