Pierre-Jules-Cesar Janssen French astronomer Date of Birth: 22.02.1824 Country: France

Biography of Pierre Jules Cesar Janssen

Pierre Jules Cesar Janssen (1824-1907) was a French astronomer. He was born on February 22, 1824, in Paris. He graduated from the University of Paris in 1852. Janssen taught mathematics and physics at a high school and worked as a private tutor.

In 1862, he established his own small observatory and began observing the Sun. In 1865, he became a professor of physics at the School of Architecture. In 1868, Janssen participated in an expedition to India to observe a total solar eclipse.

In 1878, he became the director of the newly established Meudon Astrophysical Observatory, a position he held until his death. In 1893, he founded an observatory on Mount Blanc to solve problems in astrophysics, earth physics, and meteorology.

Janssen was one of the first astronomers to apply photographic and spectroscopic methods in astronomy, particularly in the study of the Sun. During the observation of a solar eclipse on August 18, 1868, he independently developed a new method of observing solar prominences outside of an eclipse. He concluded that they were gaseous in nature. Janssen also recorded the yellow line in the spectrum of prominences, which was later attributed by J. Lockyer to a new element called helium.

He compiled an atlas of the solar surface and discovered the chromosphere, a gaseous envelope around the Sun. Janssen proposed a method for determining the chemical composition of the atmospheres of different planets based on the absorption lines and bands in the spectrum of sunlight reflected by the planets. He attempted to detect water vapor in the atmosphere of Mars using this method.

Janssen was a foreign member of the St. Petersburg Academy of Sciences and a member of the Royal Society of London. The Paris Academy of Sciences established a medal in his name, and the French Astronomical Society awarded him a prize.

Pierre Jules Cesar Janssen passed away in Meudon on December 23, 1907.