February 22, 1824, Paris (France) - December 23, 1907, Meudon (France)
Pierre Jules César Janssen graduated in mathematics from the University of Paris in 1852 and initially worked as a bank clerk although his passion was observing eclipses.
A year later he became a teacher at the Lycée Charlemagne and then at the École Speciale d'Architecture between 1865 and 1871. During this time, he was not only involved in teaching but also several scientific missions such as those to:
- Peru (1857). To determine the magnetic equator.
- Italy and Switzerland (1864). To study telluric absorption in the solar spectrum.
- Azores in Portugal (1867). To carry out optical and magnetic experiments.
- Trani in Italy (1867).
- Algiers (1870).
- Japan (1874) and Oran in Algeria (1882). Successfully observe the Transit of Venus.
- Thailand (1875).
- Caroline Islands (1883).
- Alcoceber in Spain (1905).
The mission to Guntur in India was of great significance because it was undertaken to study the total eclipse of the Sun that occurred on 18 August 1868. In this way, he was able to demonstrate the gaseous nature of the prominences and red flames of the Sun. To do this he devised a method of observing them under ordinary daylight conditions without the use of eclipses. One of the main objectives of his spectroscopic investigations was to answer the question of whether the Sun contains oxygen or not.
During the experiment, he observed a bright yellow line with a wavelength of 587.49 nanometres in the spectrum of the Sun's chromosphere, indicating the existence of an unknown element. When he reported this fact, he was ridiculed, as no element had ever been detected in space before it was found on Earth. On 20 October of the same year, the English astronomer Lockyer also observed the same yellow line in the solar spectrum and concluded that it was caused by an unknown element after he unsuccessfully tried to prove that it was a certain type of hydrogen. He later proved that it was a hitherto new element: helium. Janssen was therefore its discoverer.
At the summit of Mont Blanc, he was able to virtually eliminate the light absorption of oxygen in the earth's atmosphere.
In 1874 he invented a photographic revolver with a focal plane shutter for the chronophotographic study of the passage of Venus. Subsequently, this invention was of great use to scientists such as Étienne Jules Marey for observations and research.
In 1875 he was appointed director of the Paris Observatory in Meudon, where he presented his photographic revolver to the Société Francaise de Photographie and, a year later, to the Académie des Sciences.
In 1877 he photographed the sun, in 1881 the great comet, and in 1885 sunspots. In 1880, he worked on solarisation, and in 1881, Julius Scheiner, Pierre Janssen and Karl Schwarzchild developed sensitometry.
In 1904, he published an atlas of numerous solar images known as the ‘Atlas of Solar Photographs’, as well as the first volume of the ‘Annales de l'Observatoire de Meudon’.
In 1935, the International Astronomical Union (IAU) decided to name a lunar astroblem after him. A lunar crater and a marker are also named after him, as is an asteroid named after his wife, Henrietta.
