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Pierre Jules César Janssen (22 February 1824 – 23 December 1907), usually known in French as Jules Janssen, was a French astronomer who, along with the English scientist Joseph Norman Lockyer, is credited with discovering the gas helium.

Pierre Jules César Janssen
oil on canvas
44 × 32.5 cm (17.3 × 12.8 in)
Musée d'Orsay Paris


Life, work, and interests

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).

Pierre Jules César Janssen

Discovery of helium

In 1868 Janssen discovered how to observe solar prominences without an eclipse. While observing the solar eclipse of August 18, 1868 at Guntur, in Andhra Pradesh, British India, he noticed a bright yellow line with a wavelength of 587.49 nm in the spectrum of the chromosphere of the Sun. This was the first observation of this particular spectral line, and one possible source for it was an element not yet discovered on the earth. Janssen was at first ridiculed since no element had ever been detected in space before being found on Earth.

On 20 October of the same year, Joseph Norman Lockyer also observed the same yellow line in the solar spectrum and concluded that it was caused by an unknown element, after unsuccessfully testing to see if it were some new type of hydrogen. This was the first time a chemical element was discovered on an extraterrestrial body before being found on the earth. Lockyer and the English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios).[1][2]
Observatories

At the great Indian eclipse of 1868 that occurred in Guntur, Janssen also demonstrated the gaseous nature of the red prominences, and devised a method of observing them under ordinary daylight conditions.[3] One main purpose of his spectroscopic inquiries was to answer the question whether the Sun contains oxygen or not. An indispensable preliminary was the virtual elimination of oxygen-absorption in the Earth's atmosphere, and his bold project of establishing an observatory on the top of Mont Blanc was prompted by a perception of the advantages to be gained by reducing the thickness of air through which observations have to be made. This observatory, the foundations of which were fixed in the snow that appears to cover the summit to a depth of ten metres, was built in September 1893, and Janssen, in spite of his sixty-nine years, made the ascent and spent four days taking observations.

In 1875, Janssen was appointed director of the new astrophysical observatory established by the French government at Meudon, and set on foot there in 1876 the remarkable series of solar photographs collected in his great Atlas de photographies solaires (1904). The first volume of the Annales de l'observatoire de Meudon was published by him in 1896.
International Meridian Conference

In 1884 he took part in the International Meridian Conference.
Death, honors, and legacy
Janssen's grave in Paris

Janssen died at Meudon on 23 December 1907 and was buried at Père Lachaise Cemetery in Paris. During his life he was made a Knight of the Legion of Honor and a Foreign Member of the Royal Society of London. Craters on both Mars and the moon are named in his honor.
In popular media

In the Simpsons episode Bart's Comet after Bart Simpson sabotages Principal Seymour Skinner's weather balloon, Principal Skinner says, "Curse the man who invented helium! Curse Pierre-Jules-Cesar Janssen!"
Notes and references

^ Oxford English Dictionary (1989), s.v. "helium". Retrieved 16 December 2006, from Oxford English Dictionary Online. Also, from quotation there: Thomson, W. (1872). Rep. Brit. Assoc. xcix: "Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium."
^ For the name "helium" see also Jensen, William B. (2004). "Why Helium Ends in "ium"". Journal of Chemical Education 81 (7): 944. doi:10.1021/ed081p944.
^ Kochhar, R. K. (1991). "French astronomers in India during the 17th - 19th centuries". Journal of the British Astronomical Association 101: 95. Bibcode 1991JBAA..101...95K.

Further reading

Royal Astronomical Society (1908). Monthly Notices of the Royal Astronomical Society. Priestley and Weale., from Monthly Notices of the Royal Astronomical Society, 1908, vol. 68, pp. 245–249
Obituary, from Popular Astronomy, 1908, vol. 16, pp. 72–74
Obituary, from Astronomische Nachrichten, 1908, vol. 177, p. 63 (in French)
Obituary, from The Astrophysical Journal, 1908, vol. 28, pp. 89–99 (in French)
1911 Encyclopædia Britannica Eleventh Edition, entry for Janssen
Janssen statue, description and black-and-white picture from The Observatory, 1922, vol. 45, pp. 175–176
Brief biography, from the High Altitude Observatory at Boulder, Colorado
Janssen's chronophotography

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.

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