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JANSSEN, PIERRE JULES CESAR (1824–1907)

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Originally appearing in Volume V15, Page 155 of the 1911 Encyclopedia Britannica.
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JANSSEN, See also:PIERRE JULES CESAR (1824–1907) , See also:French astronomer, was See also:born in See also:Paris on the 22nd of See also:February 1824, and studied See also:mathematics and physics at the See also:faculty of sciences. He taught at the lycee See also:Charlemagne in 1853, and in the school of See also:architecture 1865–187i, but his energies were mainly devoted to various scientific See also:missions entrusted to him. Thus in 1857 he went to See also:Peru in See also:order to determine the magnetic See also:equator; in 1861–1862 and 1864, he studied telluric absorption in the See also:solar spectrum in See also:Italy and See also:Switzerland; in 1867 he carried out See also:optical and magnetic experiments at the See also:Azores; he successfully observed both transits of See also:Venus, that of 1874 in See also:Japan, that of 1882 at See also:Oran in See also:Algeria; and he took See also:part in a See also:long See also:series of solar See also:eclipse-expeditions, e.g. to See also:Trani (1867), Guntoor (1868), See also:Algiers (187o), See also:Siam (1875), the See also:Caroline Islands (1883), and to Alcosebre in See also:Spain (1905). To see the eclipse of 187o he escaped from besieged Paris in a See also:balloon. At the See also:great See also:Indian eclipse of 1868 he demonstrated the gaseous nature of the red prominences, and devised a method of observing them under See also:ordinary daylight conditions. One See also:main purpose of his spectroscopic inquiries was to See also:answer the question whether the See also:sun contains See also:oxygen or not. An indispensable preliminary was the virtual elimination of oxygen-absorption in the See also:earth's See also:atmosphere, and his bold project of establishing an See also:observatory on the See also:top of Mont See also:Blanc was prompted by a See also:perception of the advantages to be gained by reducing the thickness of See also:air through which observations have to be made. This observatory, the See also:foundations of which were fixed in the See also:snow that appears to See also:cover the See also:summit to a See also:depth of ten metres, was built in See also:September 1893, and Janssen, in spite of his sixty-nine years, made the ascent and spent four days taking observations. In 1875 he was appointed director of the new astrophysical observatory established by the French See also:government at See also:Meudon, and set on See also:foot there in 1876 the remarkable series of solar photographs collected in his great See also:Atlas de photographies solaires (1904). The first See also:volume of the Annales de l'observatoire de Meudon was published by him in 1896. He died at Paris on the 23rd of See also:December 1907. See A.

M. See also:

Clerke, Hist. of Astr. during the 19th See also:Century (1903) ; H. See also:Macpherson, Astronomers of To-See also:Day (1905).

End of Article: JANSSEN, PIERRE JULES CESAR (1824–1907)

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