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Pierre Jules César Janssen (22 February 1824 – 23 December 1907) was a French astronomer who, along...

Pierre Jules César Janssen (22 February 1824 – 23 December 1907) was a French astronomer who, along with the English scientist Joseph Norman Lockyer, is credited with discovering the gas helium. Janssen was born in Paris and studied mathematics and physics at the faculty of sciences. He taught at the lycée Charlemagne in 1853, and in the school of architecture 1865 – 1871, but his energies were mainly devoted to various scientific missions entrusted to him. Thus in 1857 he went to Peru in order to determine the magnetic equator; in 1861 – 1862 and 1864, he studied telluric absorption in the solar spectrum in Italy and Switzerland; in 1867 he carried out optical and magnetic experiments at the Azores; he successfully observed both transits of Venus, that of 1874 in Japan, that of 1882 at Oran in Algeria; and he took part in a long series of solar eclipse-expeditions, e.g. to Trani (1867), Guntur (1868), Algiers (1870), Siam (1875), the Caroline Islands (1883), and to Alcosebre in Spain (1905). To see the eclipse of 1870 he escaped from besieged Paris in a balloon (that eclipse was obscured by cloud cover, however). In 1868 Janssen discovered how to observe solar prominences without