Pierre Janssen

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

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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).

Discovery of helium

In 1868 Janssen discovered how to observe solar prominences without an eclipse. On August 18 of that same year, while observing an eclipse of the Sun in 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 October 20 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 world 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, Janssen also demonstrated the gaseous nature of the red prominences, and devised a method of observing them under ordinary daylight conditions. 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.

Janssen died at Meudon and was buried at Père Lachaise Cemetery in Paris.

Craters on Mars and on the moon are named in his honor.

Appearances in Pop Culture

The Simpsons mention Janssen in the episode "Bart's Comet." When Bart makes an embarrassing floating blimp resembling Principal Skinner, Skinner yells "Curse the man who invented helium! Curse Pierre Jules César Janssen!"

Further reading

• Obituary, 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

• Janssen statue at Meudon (color photopgraph)

• Brief biography, from the High Altitude Observatory at Boulder, Colorado

• Janssen's chronophotography

Notes and references

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.