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

Janssen was born and educated in Paris, and built an observatory on the flat roof of his house there. 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. Scientific expeditions took him to many parts of the world, including Peru and Japan; during the Franco-Prussian War he travelled to Algeria by balloon to observe an eclipse.
Having constructed a special spectroscope, Janssen showed that the dark bands in the solar spectrum can be resolved into rays, and, in 1864, that the origin of the phenomenon is terrestrial. He called the rays 'telluric rays'.
In 1868, Janssen went to India to observe a total eclipse of the Sun. He was unable to correlate certain lines in the solar spectrum with wavelengths of any known elements. English scientist Norman Lockyer made the same discovery of a new, unknown element and reported it simultaneously to the French Academy of Sciences.
Atlas de photographies solaires 1904 contained Janssen's photographs of the Sun, taken from 1876 onwards.