**Charles-Eugène Delaunay** was a French mathematician and astronomer whose theory of lunar motion advanced the development of planetary-motion theories.

- When Delaunay graduated in 1836 he was ranked first among all students across all academic disciplines.
- Delaunay had graduated before the prize was instituted but Mme de Laplace requested that he become the first recipient of the prize which consisted of the complete works of Laplace.
- It turned out to be a decision which changed the course of Delaunay's career, for reading Laplace's great works gave him a passion for celestial mechanics and he decided that he would make a career in that subject.
- Arago suggested to Delaunay that he come to the Paris Observatory and train to become an astronomer but Savary advised against this course of action.
- Delaunay then entered the École des Mines and trained as an engineer.
- For his doctoral dissertation Delaunay undertook research on the calculus of variations and was awarded his doctorate for his thesis De la distinction des maxima et des minima dans les questions qui dépendent de la méthode des variations Ⓣ(Kinds of maxima and minima in questions that depend on the method of variations) in 1841.
- On the one hand Delaunay gained excellent experience teaching at the Sorbonne, but he was still enrolled as a student at the École des Mines and he had to take an extra year to complete his studies there as a consequence.
- Delaunay published further papers on astronomy, publishing several papers on perturbations of Uranus in 1842 and 1843 and after this his first work on the theory of tides.
- This paper contains what today is known as 'Delaunay's method' although several of his later papers contain generalisations of the method as it first appeared here.
- We should also note that when observations of the Moon's orbit showed that it was deviating from its theoretical path, Delaunay correctly suggested in 1865 that this could be due to the rotational period of the earth slowing due to tidal friction.
- In 1855 Delaunay was elected to the Astronomy Section of the Academy of Sciences.
- Delaunay found the longitude, latitude and parallax of the Moon as infinite series.
- We should mention the rivalry between Delaunay and Le Verrier.
- He was dismissed from his post as Director and in March 1870 Delaunay was appointed to the post to succeed his rival.
- However Delaunay's appointment came only weeks before France declared war on Prussia.
- This was a time of extreme difficulty for Delaunay who succeeded against all the odds to save the Paris Observatory.
- The Commune was suppressed after two months of bitter fighting during which time Delaunay had an equally difficult task keeping the Observatory safe from the riots and fighting in the city.
- In 1872 Delaunay and three companions drowned in a boating accident when the boat they were in capsized following a gust of wind.

Born 9 April 1816, Lusigny-sur-Barse, France. Died 5 August 1872, At sea (near Cherbourg, France).

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Astronomy, Physics

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive