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Lyot, Bernard

Lyot, Bernard

(b.Paris, Francee,27 February 1897; d Cairo, Egypt, 2 April 1952)

solar and planetary astronomy, optics.

Lyot, whose father, a surgeon, died when the boy was seven, studied to be an engineer. After graduating from the École Supérieure d’Électicité in 1917, he obtained a post at the École Polytechnique as assistant to the physicist A. Pérot, with whom he studied maritime and aerial radio guidance devices for the army. Pérot put him in contact with the observatory at Meudon, where he himself was working. Lyot joined this observatory in 1920 and, having earned his licence és sciences,successively became assistant (1925), astronomer (1930), and chief astronomer (1943).

The first problem that Lyot studied—which also was the subject of his doctoral dissertation in 1929—concerned the polarization of light reflected from the surfaces of the planets. In order to detect this polarization he had to construct a polariscope ten times more sensitive than the existing instruments. From 1923 to 1930 he determined, for each planetary surface, the polarization curve as a function of the angle of vision and, by comparison with laboratory measurements of various substances, he obtained information on the structure of the surfaces observed. He showed, notably, that the lunar soil behaves like volcanic dust and that Mars experiences sandstorms.

The polarization of the solar corona could not be studied during eclipses because the latter’s duration was too short. The observation of the corona when there was no eclipse—conducted with the aid of a screen masking the image of the disk—was thought to be impossible because of the light diffused by the atmosphere. Lyot showed that the “parasite” light was caused by the apparatus and could be eliminated. Thus in 1930 he invented the coronograph, which has become the classic instrument for observing the solar corona.

In order to be able to use all the information contained in the image given by the coronograph, it was necessary to work in strictly monochromatic light and to avoid introducing absorption. These two apparently contradictory conditions were satisfied in the monochromatic filter that Lyot devised in 1933. He employed a property of birefringent crystalline laminae, which in polarized light produce interferences, the periods of which vary with the thickness of the lamina: by stacking up a suitable series of laminae, one obtains well-separated passbands, the width of which does not exceed one angstrom. This filter is used in the network of automated heliographs that today assure the permanent observation of solar eruptions and rapidly evolving chromosphericphenomena.Conjoined with the coronograph, the filter has permitted the taking of films showing the movements of solar prominences; they are among the most popular astronomical documents. Lyot himself took the first of these films in 1939.

Lyot spent his life studying phenomena which seemed to be impossible to detect. His skill as a physicist was such that no major advance has been made in the types of instruments that he invented. His work was conceived in accordance with an initial overall plan; for instance, as early as 1923 he stated the principle of the photoelectric polarimeter, which the electron multiplier enabled him to realize in 1950. This instrument, which can detect polarized light when it constitutes as little as 1/10,000 of the total flux, has made possible the observation of coronal rays without the coronograph and without the need for high-altitude stations.

Lyot was elected to the Académie des Sciences in 1939 and in the same year received the gold medal of the Royal Astronomical Society. He was awarded the gold medal of the Astronomical Society of the Pacific in 1947. A pleasant and unpretentious person, Lyot divided his time between his laboratory at Meudon, mountain observations, and his family. He enjoyed skiing, swimming, and sailing and appeared astonishingly youthful. He died of a heart attack upon his return from an exhausting mission to Khartoum to observe a solar eclipse.

BIBLIOGRAPHY

I. Original Works. Lyot’s investigations were primarily in four fields.

On polarization he wrote“Recherches sur la polarisation de la lumiere des planetes et de quelques substances terrestres,” in Annales de l’Observatoire, Section de Meudon,8 (1929), 1–161; and six notes in Comptes rendus… de l’Academie des sciences,189 (1929), 425–426;191 (1930), 703–705, 834–836; 198 (1934), 249–251, 774–777.

On the coronograph and its applications, see three notes, ibid.,191 (1930), 834–836; 193 (1931), 1169–1173;194 (1932), 443–446; two articles in Astronomie:45 (1931), 248–253; 46 (1932), 272–287; ’Études de la couronne solaire en dehors des –clipses,” in Zeitschrift für astrophysik, 5(1932), 73–95; and “A Study of the Solar Corona and Prominences Without Eclipses,” in Monthly Notices of the Royal Astronomical Society,99 (1939), 578–596.

The monochromatic filter is the subject of two notes in Comptes rendus,197 (1933, 1593–1595, 212 (1941, 1013-1017; and “Le filtre monochromatique polarisant et ses applications en physique solaire,” inAnnales d’astrophysique,7 (1944), 31–79.

On the photoelectric polarimeter, see a note in the Comptes rendus,226 (1948), 25–28.

Three other papers should be mentioned: “line nouvelle méthode d’observation de la couronne solaire,” ibid., 231 (1950), 461–464; “Étude spectroscopique de la rotation de la couronne solaire,” ibid., 233 (1951), 1529–1532, written with A. Dollfus; and “Étude des défauts d’homogénéité de grands deques de verre,” inRevue d’optique théorique et insmentale29 (1950), 499-512, written with M. Françon.

II. Secondary Literture. See the following, listed chronologically: F. Robbins, “The Gold Medallist of the Royal Astronomical Society,” in Journal of the British Astronomical Association,49 (1939), 259–263; A. Chevalier, “Notice neerologique,” inCompws rendm,234 (1952), 1501–1505; A, Danjon, “Bernard Lyot,” in Annals dastrophysique15 (1952), 75–78; and L. d Azambuija, L oeuvre de B, Lyot l’ inAstronomie,66 (1952), 265–277.

Jacques R. LÉvy

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