Janssen, Pierre Jules César (pyĕr zhül sāzärˈ zhäNsĕnˈ) [key], 1824–1907, French astronomer. In 1857–58, in Peru, he worked on the determination of the magnetic equator; in the Azores (1867) he examined magnetic and topographical conditions; and in Japan (1874) and in Algeria (1882) he observed the transit of Venus. Janssen accompanied various solar eclipse expeditions, notably that to Guntur, India, in 1868, where he devised a new method of studying the solar prominences spectroscopically and discovered, almost simultaneously with J. N. Lockyer, the chemical constitution of the prominences. He was active in the establishment of the astrophysical observatory of Meudon in Paris and in 1876 became its director. There he gathered an important series of solar photographs included in his Atlas de photographies solaires (1904). He later became director of the observatory on Mont Blanc.
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