Pierre Jules Cesar Janssen (1824-1907)

Rajesh Kochhar

Janssen was born in Paris on 22 February 1824. His maternal grandfather was the architect Paul-Fuillaume Le Moyne. An accident in early childhood left him permanently lame. He was thus kept at home and never attended school. Janssen attended the University of Paris, receiving his licencees sciences in 1852. In 1857 he became a tutor for the Schneider family, which owned iron and steel mills in Le Cruesot.

Janssen’s first scientific work was a study of the absorption of radiant heat in the medium of the eye. It earned him a doctorate of science in 1860.

At that time, astronomy was beginning to be merged with physics. In October 1859, the theoretical work of Gustav Kirchhoff (1824–87) solved the enigma of the spectrum and it became possible to establish the chemi­cal composition of the Sun and the stars. The physicists and astronomers then awaited a total solar eclipse to learn about solar prominences.

The eclipse of 18 August 1868, visible from India, the Malayan peninsula and Thailand, attracted a large number of astronomers. Janssen with his spectroscope was stationed at Guntoor (in Andhra Pradesh) near the Bay of Bengal.

All the observers on the morning of 18 August 1868 found out that the prominences were made of highly­ heated masses of gas, mostly hydrogen. But Janssen went further.  The  emission  lines  he  saw  during  the eclipse were so brilliant that he felt certain that he did not have to wait for the darkness of the eclipse to see them. He was prevented by clouds from trying the experiment the same afternoon after the eclipse was over. But the next morning, he directed the slit of his spectroscope to the portion of the sun’s limb where the day before the most brilliant prominences had appeared. He had no difficulty in spotting the emission lines again, this time in full daylight.

Visible spectrum of helium; the bright yellow line was the one observed by Janssen and Lockyer in 1868, and by Ramsay in 1895 (Wikimedia commons)
Visible spectrum of helium; the bright yellow line was the one observed by Janssen and Lockyer in 1868, and by Ramsay in 1895 (Wikimedia commons)

Janssen made a significant discovery. The yellow line which everybody had mistakenly thought to be that of sodium had to belong to some other element. Edward Frankland named this new element helium. It was discovered on the Earth in 1895, by William Ramsay. From 18 August to 4 September ‘during a period equivalent to an eclipse of 17 days’, Janssen worked at Guntoor, and then continued his observations at Simla in the Himalayas, where he created the first-ever spec­ trohelioscope. This discovery made it possible to observe the Sun’s chromosphere every day. (Janssen’s elementary spectrohelioscope was superseded by George Ellery Hale’s (1868-1938) invention of spectro­heliograph in 1889.)

Unknown to Janssen, Joseph Norman Lockyer (1836-1920) succeeded on October 1868 in observing solar prominences without an eclipse. By a strange coincidence, Janssen’s report from India and Lockyer’s from England were received at the French Academy on the same day and the event was commemorated by the French government in 1872 with the issue of a gold medal bearing the likeness of both Lockyer and Jenessen.

In 1876, Janssen became the founder-director of the Meudon Observatory, where he continued till his death on 23 December 1907. Janssen was the last Frenchman in India to contri­bute to the astronomical sciences It may not however be out of place to mention a couple of instruments that came from Paris.

Rajesh Kochhar is an astronomer and a historian of sciences. He is also the advisor of this project.

References:

  1. Dictionary of Scientific Biography.
  2. See Annual Report of Madras Observatory, 1868. There were also two British teams at Guntoor. One led by Maj. James Francis Tennant was sent out on the recommendation of the Royal Astronomical Society. The other team, led by Captain Bramfill, was sent by the Great Trigonometrical Survey of India aided by the Royal Society. Lts. John Herschel and Campbell were at Belgaum and Capt Haig at Bijapur (both now in Kamataka). Madras Observatory had two teams, one at Masulipatam (now in Andhra Pradesh) led by the Astronomer Norman Robert Pogson and the other at a small village Vunpurthy (in Andhra Pradesh) under the first assistant C. Ragoonathachary. There was a Prussian team at Moolwar near Bijapur. (The other team of Dr Vogel was at Aden). The second French team observed the eclipse from Wah-Tonne in Malacca. It was led by M. Stephan and included Rayet as a member.
  3. Young, C. A. The Sun, D. Appleton & Col., New York, 1895.

This article is an excerpt from the original article by the author in the Journal of the British Astronomical Association (vol.101, no.2, p.95-100), reproduced here with permission from the author.

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