Sydney Chapman FRS was the Sedleian professor between 1946 and 1953. He was also one of the outstanding geophysicists of the twentieth century. His achievements cover a wide range of topics and include pioneering theory of transport in non-uniform gases, the first proposal of the nature of the ozone layer, and a comprehensive early theory of the nature of geomagnetic storms. The last of these forms the foundation of present-day studies of the Earth’s magnetosphere, and the impact of solar activity on society, referred to as space weather.