Siméon Denis Poisson

Baron Siméon Denis Poisson (, US also ; French: [si.me.ɔ̃ də.ni pwa.sɔ̃]; 21 June 1781 – 25 April 1840) was a French mathematician and physicist who worked on statistics, complex analysis, partial differential equations, the calculus of variations, analytical mechanics, electricity and magnetism, thermodynamics, elasticity, and fluid mechanics. Moreover, he predicted the Arago spot in his attempt to disprove the wave theory of Augustin-Jean Fresnel. While his interpretations of physical phenomena were often proven wrong by later researchers, his contributions to mathematics have stood the test of time. He demonstrated that physical problems could suggest new mathematical ideas.

Early life and education Poisson was born in the small town of Pithiviers, now in Loiret, France, about 50 miles (80 km) south of Paris. His father was Siméon Poisson, a retired soldier in the French Army who had served in the Seven Years' War. In 1798, Poisson, then seventeen years of age, matriculated at the École Polytechnique in Paris after scoring in first place in the highly competitive entrance examination. Even as a first-year student, he immediately began to attract the notice of the professors of the school, who left him free to make his own decisions as to what he would study. In his final year of study, less than two years after his entry, he published two memoirs: one on Étienne Bézout's method of elimination, the other on the number of integrals of a finite difference equation. This was so impressive that he was allowed to graduate in 1800 without taking the final examination. The latter of the two memoirs was examined by Sylvestre-François Lacroix and Adrien-Marie Legendre, who recommended that it should be published in the Recueil des savants étrangers, an unprecedented honor for an eighteen-year-old. This success at once gave Poisson admittance into scientific circles. Joseph-Louis Lagrange, whose lectures on the theory of functions he attended at the École Polytechnique, recognized his talent early on and became his friend. Meanwhile, Pierre-Simon Laplace, in whose footsteps Poisson followed, regarded him almost as his son. The rest of his career until his death in Sceaux, near Paris, was occupied by the composition and publication of his many works and in fulfilling the duties of the numerous educational positions to which he was successively appointed.

Career Immediately after finishing his studies at the École Polytechnique, he was appointed répétiteur (teaching assistant) there, a position which he had occupied as an amateur while still a pupil in the school; for his schoolmates had made a custom of visiting him in his room after an unusually difficult lecture to hear him repeat and explain it. He was made deputy professor (professeur suppléant) in 1802, and, in 1806 full professor succeeding Jean Baptiste Joseph Fourier, whom Napoleon had sent to Grenoble. In 1808 he became astronomer to the Bureau des Longitudes; and when the Faculté des sciences de Paris was instituted in 1809 he was appointed a professor of rational mechanics (professeur de mécanique rationelle). He went on to become a member of the Institute in 1812, examiner at the military school (École Militaire) at Saint-Cyr in 1815, graduation examiner at the École Polytechnique in 1816, councillor of the university in 1820, and geometer to the Bureau des Longitudes succeeding Laplace in 1827.

As a teacher of mathematics Poisson is said to have been extraordinarily successful, as might have been expected from his early promise as a répétiteur at the École Polytechnique. Despite his many official duties, Poisson still found the time and energy to publish over 300 works on a variety of mathematical topics. Arago attributed to him the quote, "Life is good for only two things: doing mathematics and teaching it." A list of Poisson's works, drawn up by himself, is given at the end of Arago's biography. His greatest services to science were performed in the application of mathematics to the study of physics. Some of the most influential were his memoirs on electricity and magnetism. Also of great importance were his memoirs on celestial mechanics, in which he proved himself a worthy successor to Laplace. The most important of these are his memoirs Sur les inégalités séculaires des moyens mouvements des planètes (On the Secular Inequalities for the Means of Planetary Motion), Sur la variation des constantes arbitraires dans les questions de mécanique (On the Variation of Arbitrary Constants in Questions of Mechanics), both published in the Journal of the École Polytechnique (1809); Sur la libration de la lune (On the Libration of the Moon), in Connaissance des temps (1821), etc.; and Sur le mouvement de la terre autour de son centre de gravité (On the Earth's Movement About Its Center of Gravity), in Mémoires de l'Académie (1827). In the first of these memoirs, Poisson discusses the famous question of the stability of the planetary orbits, which had already been settled by Lagrange to the first degree of approximation for the disturbing forces. Poisson showed that the result could be extended to a second approximation, and thus made an important advance in planetary theory. The memoir is remarkable inasmuch as it roused Lagrange, after an interval of inactivity, to compose in his old age one of the greatest of his memoirs, entitled Sur la théorie des variations des éléments des planètes, et en particulier des variations des grands axes de leurs orbites (On the theory of variations in the elements of the planets, and in particular the variations in the major axes of their orbits). So highly did he think of Poisson's memoir that he made a copy of it with his own hand, which was found among his papers after his death.

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