HISTORY OF SCIENCE AND TECHNOLOGY

PIERRE-JULES-CESAR JANSSEN (1824 – 1907)

Pierre Janssen was born in Paris, France. An accident when he was young left him extremely lame and it is for this reason that he was unable to go to school. He studied at home and, at the age of sixteen, found employment as a bank clerk. Janssen continued studying mathematics in his spare time and he eventually entered the Sorbonne. He received his degree in mathematics and physical sciences in 1852 and obtained his doctorate in 1860.

Janssen remained at the Sorbonne, working on the surgical instrument design in the Faculty of Medicine, until he obtained a post as a teacher in the Lycée Charlemagne. He eventually worked at the School of Architecture in Paris. Despite his handicap, Janssen was an avid traveller; always in pursuit of scientific knowledge. He went to Peru in 1857 to find the magnetic equator. Ten years later he was experimenting in the Azores followed closely by a visit to India to study the solar eclipse in 1868. To get to the Algers in 1870 he had to escape from Paris, a city in siege during the Franco-Prussian war, by balloon! In 1974 he travelled to Japan to photograph the transit of Venus across the surface of the Sun.

His work on the solar eclipse in 1868, using a spectroscope, had shown that solar prominences are made of gas. It was Janssen who identified the layer of gas that surrounds the sun, that he named the chromosphere. He also realised that some of the dark lines in a solar spectrum were created by water vapour in the Earth’s atmosphere.

Janssen pioneered, at the same time as the English scientist Joseph Lockyer, a method of photographing the spectra of the Sun’s prominences without needing to wait for an eclipse. It was Janssen who first noticed the lines in the spectrum from solar prominences that did not correspond to any known element at that time. Lockyer proved that this was, in fact, a new element that he named helium.

In 1874 the French government asked Janssen to be the director of a new observatory at the top of Mont Blanc. This observatory had been built in this particular location because the Earth’s atmosphere was less thin and there would be less atmospheric interference. Janssen’s mission was to find out if the oxygen found in the solar spectrum was actually due to oxygen present in the sun or contamination from the Earth’s atmosphere. Janssen was sixty five years old when he made the journey to the top of Mont Blanc where he stayed four days to conclude his investigation.

The following year, in 1875 he was made the director of the New Astrophysical Observatory at Meudon. It was Janssen himself who had chosen the location for this new observatory so that it was well away from the pollution of Paris. Janssen worked at the observatory for the rest of his life and took some amazing photos of the Sun that he published in 1904 in his book Atlas de Photographies Solaires.

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