Sydney Chapman, FRS, (29 January 1888 - 16 June 1970) was a mathematician and geophysicist. He was born in Eccles, near Salford in England. He attended the Royal Technical Institute, Salford (now the University of Salford). In 1904 he entered Manchester University, where he obtained a B.Sc. in Engineering (1907) and a B.Sc. in Mathematics (1908).

Chapman persued his study of mathematics at Trinity College, and in his final year he was appointed Senior Assistant at the Greenwich Observatory, where he remained until 1914.

A pacifist, Chapman was exempted from military service, and returned to Cambridge as a college lecturer in 1914. He was asked to return to the Observatory in 1916, and he remained until 1918.

In 1919 Chapman was appointed Professor of Mathematics at Manchester University, the same year he was made a Fellow of The Royal Society. In 1922 he married Katherine Nora (they subsequently had three sons and a daughter).

In 1924 Chapman became Professor of Mathematics at Imperial College, London, where he remained until 1946. During this time he co-authored Geomagnetism with Julius Bartels and with T.G. Cowling The Mathematical Theory of non-Uniform Gases. No longer a pacifist, he undertook civilian war work, and in 1943-1945 worked for the army counil on problems of military operational research.

From 1946-1953 Chapman was Sedleian Professor of Natural Philosophy at Oxford University and a Fellow of Queen's College. After his retirement from Oxford he divided his time between the Geophysical Institute in Alaska and the High Altitude Observatory in Boulder, Colorado. He died in Boulder in 1970 after a short illness.