Mme la Marquise de Laplace donated a new annual prize, the Laplace Prize, to be given to the student who was ranked top in his year at the École Polytechnique. 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. Mme la Marquise de Laplace was delighted with the first winner of the prize and she called him "her eldest son". 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 Ⓣ in 1841. He published his first paper on astronomy submitting Note sur la précession des équinoxes Ⓣ to the Academy of Sciences in the same year. Biot chose him, later in 1841, to substitute for him at the Sorbonne in giving the course in physical astronomy. 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. He completed his studies and qualified as an engineer in 1843.
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. He was already working on his lunar theory by this time and published Mémoire sur une Méthode nouvelle pour la détermination du mouvement de la Lune Ⓣ in 1846. 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. Among his other works were Cours élémentaire de mécanique Ⓣ (1850) and Traité de mécanique rationnelle Ⓣ (1856), and Cours élémentaire d'astronomie Ⓣ (1853). 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. He published his theory in Ralentissement de la rotation de la terre Ⓣ (1866). We mention also his report on the progress of astronomy Rapport sur les progrès de l'astronomie Ⓣ (1867).
From 1845 to 1850 he taught courses at the École des Mines; these were descriptive geometry, stereotomy, mechanical drawing, analytical mechanics, and elementary physics. He suffered a great tragedy in 1849 when his young wife died and it was largely due to Liouville's efforts to support him at this time that he was able to continue with his academic work. He taught mechanics at the École Polytechnique from 1850 being named Professor of Mechanics there in 1851. He also held a chair of mechanics at the Sorbonne from 1850. In 1855 Delaunay was elected to the Astronomy Section of the Academy of Sciences. He continued his association with the École des mines, being named Engineer in Chief in 1858, then being raised to Engineer First Class in 1867. He published, in 1860 and 1867, two volumes on lunar theory La Théorie du mouvement de la lune Ⓣ which contained the results of twenty years work. Of course the theory of the moon is an important case of the three body problem. Delaunay found the longitude, latitude and parallax of the Moon as infinite series. These gave results correct to 1 second of arc but were not too practical as the series converged slowly. However this work was important in that it contained the beginnings of functional analysis. From 1862 he was a member of the Bureau des Longitudes.
We should mention the rivalry between Delaunay and Le Verrier. This evolved into a full scale argument and around 1860 it was most certainly a very public argument. We should remark that Le Verrier was a difficult man and fell out with many of his colleagues. Le Verrier was the Director of the Paris Observatory and by 1869 he had become very unpopular with his colleagues at the Observatory following his drive for efficiency. Attempts were being made to have him removed which were eventually successful. 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.
The popularity of Napoleon III, the French emperor, had declined and he decided that a war with Prussia might change his political fortunes. His advisers had told him that the French Army could defeat Prussia with ease, so the plan looked like a winner. Bismarck, the Prussian chancellor, saw a war with France as an opportunity to unite the South German states so he too saw the political advantage of war. With both sides actively seeking a war, the Franco-Prussian War became inevitable. On 14 July 1870, Bismarck sent a telegram aimed at infuriating the French government. He succeeded, for on the 19 July France declared war on Prussia. The German offensive met with only an ineffective French reply. In August, the German army trapped part of the French army in Metz and they surrendered on 1 September. On 19 September the German army began to blockade Paris which surrendered on 28 January 1871. This was a time of extreme difficulty for Delaunay who succeeded against all the odds to save the Paris Observatory. The French government was threatened by an uprising in Paris in March 1871, in which radicals established their own short-lived government, the Paris Commune. 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. He largely succeeded but the Observatory did sustain some damage.
In 1872 Delaunay and three companions drowned in a boating accident when the boat they were in capsized following a gust of wind.
Article by: J J O'Connor and E F Robertson
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