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Pierre Jules César Janssen (22 February 1824-23 December 1907), usually known as Jules Janssen, was a French astronomer who, along with English scientist Joseph Norman Lockyer, is credited with discovering the gaseous nature of the solar chromosphere, and with some justification the element helium.
Janssen was born in Paris into a cultivated family. His father, Antoine César Janssen was a well known clarinettist from Dutch/Belgian descent, his father emigrated from Walloon Brabant to Paris. His mother Pauline Marie Le Moyne was a daughter of the architect Paul Guillaume Le Moyne.
Pierre Janssen 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 the Siege of Paris in a balloon. Unfortunately the eclipse was obscured from him by cloud.
More information: Wired
In the year 1874, Janssen invented the Revolver of Janssen or Photographic Revolver, instrument that originated the chronophotography. Later this invention was of great use for researchers like Etienne Jules Marey to carry out exhibitions and inventions.
In 1868, Janssen discovered how to observe solar prominences without an eclipse.
While observing the solar eclipse of 18 August 1868, at Guntur, Madras State (now in Andhra Pradesh), British India, he noticed bright lines in the spectrum of the chromosphere, showing that the chromosphere is gaseous. Present in the spectrum of the Sun, though not immediately noticed or commented upon, was a bright yellow line later measured to have a wavelength of 587.49 nm. 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. From the brightness of the spectral lines, Janssen realized that the chromospheric spectrum could be observed even without an eclipse, and he proceeded to do so.
On 20 October, Joseph Norman Lockyer in England set up a new, relatively powerful spectroscope. He also observed the emission spectrum of the chromosphere, including the same yellow line. Within a few years, he worked with a chemist and they concluded that it could be 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 after the Greek word for the Sun, ἥλιος (helios).
Janssen died at Meudon on 23 December 1907 and was buried at Père Lachaise Cemetery in Paris, with the name J. Janssen inscribed on his tomb. 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 honour. The public square in front of Meudon Observatory is named Place Jules Janssen after him. Two major prizes carry his name: the Prix Jules Janssen of the French Astronomical Society, and the Janssen Medal of the French Academy of Sciences.
Janssen named minor planet 225 Henrietta discovered by Johann Palisa, after his wife, Henrietta.
More information: SciHi Blog
We couldn't make people out of hydrogen and helium.
So we're made out of exploding stars.
John C. Mather
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