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.
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. 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.