Born Lusigny, Aube, France, 9 April 1816
Died at sea near Cherbourg, France, 5 August 1872
Charles-Eugène Delaunay was a professor, director of the Paris Observatory, mathematician, and a significant contributor to lunar theory. The son of Jacques-Hubert Delaunay, a mathematics teacher, and Catherine Choiselat, Delaunay entered the École Polytechnique in 1834. Ranked first in his class 2 years later, he received the first Laplace Prize, a copy of the astronomer’s complete works that is said to have prompted his interest in celestial mechanics. After turning down an offer from Dominique Arago to join the Paris Observatory after his mentor Félix Savary , from the Bureau des longitudes, said that this amounted to forfeiting his independence, Delaunay attended the École des mines, with which he stayed closely associated through the early part of his career. He married Marie-Olympe Millot in 1839, and they had a son the following year; after her untimely death in 1849, he raised his son alone and devoted himself to the pursuit of lunar motion theory.
In November 1838, Delaunay was hired by the École Polytechnique as répétiteur(teaching assistant), for the course on geodesy and machines, later nominated répétiteurof mechanics to replace Urbain Le Verrier , and was made professor in 1851. From 1841, he was Jean-Biot ’s suppléantfor the course of physical mechanics at the University of Paris (Sorbonne), whose chair Delaunay occupied in 1848.
Despite Le Verrier’s opposition, Delaunay was elected at the Paris Académie des sciences in 1855. He was later nominated to the Bureau in 1867, became a fellow of the Royal Society in London in 1867, and was appointed director of the Paris Observatory after Le Verrier’s dismissal on 2 March 1870. That same year, Delaunay was awarded the Gold Medal of the Royal Astronomical Society.
Above all, Delaunay was an indefatigable analytical computer. His first paper was a short note published in 1838 in Joseph Liouville’s Journal de mathématiques pures et appliquées. Although astronomy seems to have been his early passion, Delaunay’s 1841 doctoral thesis at the Sorbonne was also concerned with mathematics, namely the calculus of variations (“De la distinction des maxima et des minima dans les questions qui dépendent de la méthode des variations”). In the 1840s, he also worked on Uranus inequalities and tide theory.
“Baron of the Moon,” according to Biot, Delaunay dedicated 20 years to the painstaking computations involved in the theory of its motion. Starting in 1846, he developed an original method for this problem that involved canonical equations in what are today called Delaunay variables. In 1860 and 1867, he published the two volumes on his monumental Théorie des mouvements de la lune, where, in this important case of the three-body problem, Delaunay expressed the longitude, latitude, and parallax of the Moon as infinite series, his results being correct to 1″ but not very practical because of slow convergence. In the language of modern-day nonlinear dynamics, he replaced the actual chaotic (nonintegrable) Hamiltonian by a nonchaotic (integrable) approximation designed to give good agreement with the real dynamics. The 461 terms in the perturbating function, sometimes developed to the ninth order, take up more than 100 pages. Delaunay’s contemporaries and followers, Simon Newcomb and Henri Poincaré among others, praised this work in the highest terms. The basis for theoretical developments in analytical mechanics by Poincaré, George Hill , and Anders Lindstedt, Delaunay’s theory introduced methods that are still in use today for the computation of artificial satellite motions.
An intense and bitter rivalry developed between Delaunay and Le Verrier. After having presented the academy with preliminary results concerning inequalities in Uranus’ motion in 1842, Delaunay was criticized by Le Verrier. Because of discrepancies between Peter Hansen ’s tables of the Moon and Delaunay’s theoretical predictions, Verrier alleged to have found errors in the theory. In 1865, Delaunay suggested that they arose from a slowing of the Earth’s rotation due to tidal friction, an explanation today believed to be correct.
The opposition was mostly rooted in personal resentment and struggle for the control over French astronomy. The author of two successful textbooks on mechanics and machines, Cours élémentaire de mécanique théorique et appliquée(1851) and Traité de mécanique rationnelle(1856), Delaunay had caught the attention of Emperor Napoléon III, who sought his support in rejuvenating the moribund Bureau des longitudes as counterpower to Le Verrier’s observatory. Delaunay was thereby instrumental in Le Verrier’s fall from grace in 1870 and was appointed in his place, despite having no experience in astronomical observation.
As director of the Paris Observatory, Delaunay was keen to transfer it outside of the city to the suburban town of Fontenay-aux-Roses, or to keep it in Paris only if Louis XIV’s building were leveled. During the unrest caused by war and insurrection in 1870–1871, Delaunay courageously preserved the integrity of the institution. He had set out to reorganize the conduct of astronomical research and observation in France when he lost his life in a shipwreck as he was surveying the fortifications of Cherbourg’s harbor.