Born Paris, France, 24 July 1853
Died Paris, France, 15 January 1948
French solar astronomer Henri Deslandres carried out intensive studies of the behavior of the various layers of the atmosphere of the Sun – photosphere, chromosphere, and corona – and their changes through the solar cycle. He graduated from the École Polytechnique in Paris in 1874 and began a career in the army, rising to the rank of captain in the engineers, but resigned in 1881 to begin research in ultraviolet spectroscopy with Alfred Cornu at the École Polytechnique. Later at the Sorbonne, Paris, he received his doctorate in 1888, for work on arithmetic laws that describe the wavelengths of various bands in molecular spectra (Delandres’s third law).
Like many French physical scientists of the nineteenth century, Deslandres had a strong interest in physical and instrumental optics, and their application to a variety of fields, including astronomy, meteorology, astrophysics, and spectroscopy. He was appointed to the Paris Observatory in 1889 and put in charge of setting up a spectroscopic department there by director Admiral Ernest Mouchez . In 1897, Deslandres was appointed assistant astronomer at the Meudon Observatory, an observatory created by Jules Janssen in 1876 specifically for astrophysical work. There, Deslandres rapidly rose through the ranks, becoming astronomer in 1898, assistant director in 1906, and (upon Janssen’s death), in 1908, director of the Meudon Observatory. When the Paris and Meudon observatories were united in 1926, he also directed the new institution until his retirement in 1929.
Deslandres and his contemporary George Hale represent the second generation of solar physicists. From the 1880s, both men (often in competition with each other) furthered the knowledge of the constitution and circulation of the solar atmosphere, notably through their introduction of photography to record the appearance of prominences, and their nearly simultaneous invention in 1894 of a new instrument, the spectroheliograph, with which the spectra of selected parts of the solar atmosphere could be photographed and studied, giving clues about their composition. Always an experimentalist and an instrument-designer rather than a theorist, Deslandres later created another device, the spectro-enregistreur des vitesses, to monitor the radial velocities of solar gas clouds using the Doppler effect. From extensive investigations with increasingly sophisticated versions of these two instruments, Deslandres was able to conclude that the chromosphere does not vary much during the sunspot cycle, whereas the areas associated with faculae display variations similar to those shown by the faculae during this cycle. He also showed that plages (a term he coined) have the same structure as prominences. Convinced of the magnetic nature of solar spots, Deslandres further carried out, with the assistance of Louis d’Azambuja , an ambitious program of daily photographing of the Sun.
Deslandres employed his spectrographic devices for measuring stellar radial velocities, and the rotational velocity of Jupiter and Uranus as well as of Saturn and its ring, showing that Uranus was retrograde. He also looked into the spectra of comets and their tails. Deslandres further participated in several eclipse expeditions: to Fundium, Senegal, in 1893; to Japan in 1896; and to Spain in 1900 and 1905. His spectra of Arcturus and Aldebaran provided the earliest evidence for the existence of chromospheres in red giants.
Deslandres played a major role in international astronomical organizations, representing the Société Astronomique de France at the 1904 conference where the International Union for Co-operation in Solar Research was founded, and serving on its committees on solar research with the spectroheliograph to investigate the spectra of sunspots and solar rotation. He was the delegate of the French Academy of Sciences to the 1919 meeting in Brussels, where both the International Research Council (now [ICSU]) and the International Astronomical Union [IAU] were established, and served as vice president of the IAU from 1922 to 1928. The French Academy of Sciences elected Deslandres to membership in 1902 and to its presidency in 1920, and the corresponding academies in Belgium, Italy, the United Kingdom, and the United States elected him in later years. He received medals from the United States National Academy of Science, the Royal Astronomical Society, and the Astronomical Society of the Pacific. The French Académie des sciences named a prize for Deslandres shortly after his death. He and his wife had one son, Philippe.
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