Janssen, (Pierre) Jules César (1824–1907)

French astronomer. He studied the solar spectrum. In 1867 he concluded that water vapour was present in the atmosphere of Mars; he developed a spectrohelioscope in 1868.

Janssen was born and educated in Paris, and built an observatory on the flat roof of his house there. Scientific expeditions took him to many parts of the world, including a trip to Peru 1857 in order to determine the magnetic equator; during the Franco-Prussian War he travelled to Algeria by balloon to observe an eclipse. He was made professor of physics at the Ecole Spéciale d'Architecture 1865, established an observatory on Mont Blanc in the Alps, and in 1875 became director of the new astronomical observatory at Meudon, near Paris. Atlas de photographies solaires 1904 contained Janssen's photographs of the Sun, taken from 1876 onwards.