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Dionis Du Séjour, Achille-Pierre

Dionis Du Séjour, Achille-Pierre

(b. Paris, France, 11 January 1734; d. Vernou, near Fontainebleau, France, 22 August 1794)

astronomy, mathematics, demography.

The son of Louis-Achille Dionis du Séjour, counselor at the cour des aides in Paris, and of GenevièveMadeleine Héron, Achille-Pierre studied in Paris at the College Louis-le-Grand and then at the Faculté de Droit. A counselor at the Parlement of Paris in 1758, he sat as a member of the Chambre des Enquêtes beginning in 1771 and in 1779 moved to the Grand Chambre, where he was appreciated for his simplicity, his liberalism, and his humanity. He devoted the bulk of his leisure time to mathematical and astronomical research, which brought him election as associé libre of the Académie des Sciences on 26 June 1765. (Dionis du Séjour maintained this title at the time of the reorganization of 1785 but resigned it on 14 July 1786 in order to be eligible for election as associate member of the physics section.) His cordiality, his devotion to the cause of scientific research, and his philosophic spirit brought him many friendships; and the quality of his writings earned the respect of Lagrange, Laplace, and Condorcet, among others.

With his friend and future colleague Mathieu Bernard Goudin, Dionis du Séjour published a treatise on the analytical geometry of plane curves (1756) and a compendium of theoretical astronomy (1761). From 1764 to 1783 he wrote a series of important memoirs on the application of the most recent analytic methods to the study of the principal astronomical phenomena (eclipses, occultations, reductions of observations, determination of planetary orbits, etc.). Revised and coordinated, these memoirs were reprinted in the two-volume Traité analytique des mouvements apparents des corps célestes (1786–1789), of which Delambre gives a detailed analysis. The Traité is completed by two works, one on comets (1775), in which he demonstrates the near impossibility of a collision of one of these heavenly bodies with the earth, and the other on the varying appearance of the rings of Saturn (1776). All these works are dominated by an obvious concern for rigor and by a great familiarity with analytical methods; if the prolixity of the developments and the complexity of the calculations rendered them of little use at the time, their reexamination in the light of present possibilities of calculation would certainly be fruitful.

In pure mathematics, beyond the study of plane curves, Dionis du Séjour was interested in the theory of the solution of equations, an area where his works have been outclassed by those of his contemporaries Bézout and Lagrange. Finally, in collaboration with Condorcet and Laplace, he undertook a systematic inquiry to determine the population of France. Utilizing the list of communes appearing in the Cassini map of France and the most recent information furnished by the civil registers, this inquiry was based on the empirical hypothesis that the annual number of births in a given population is approximately one twenty-sixth of the total of that population.

The Revolution interrupted Dionis du Séjour’s scientific activity. Elected a deputy of the Paris nobility on 10 May 1789, he sat in the National (later Constituent) Assembly until its duties were completed on 30 September 1791. Resigning later from the office of judge of a Paris tribunal, to which post he had been elected on 30 November 1791, he retired to his rich holdings in Argeville, a commune in Vernou, near Fontainebleau, where he died without issue almost a month after 9 Thermidor, having experienced, it seems, a period of difficulties and quite justifiable anxiety.


I. Original Works. A list of Dionis du Séjour’s papers is in Table générale des matières contenues dans l’Histoireet dans les Mémoires de l’Académie royale des sciences, VII-X(Paris, 1768–1809).

His books are Traité des courbes algébriques (Paris, 1756), written with Goudin; Recherches sur la gnomonique, les rétrogradations des planétes et les éclipses du soleil (Paris, 1761), written with Goudin; Recueil de problemes astronomiques résolus analytiquement, 3 vols. (Paris, 1769–1778), a collection of his papers on astronomy published in the Histoire de l’Académie royale des sciences; Essai sur les cométes en général; et particulièrement sur celles qui peuvent approcher de l’orbite de la terre (Paris, 1775); Essai sur les phénoménes relatifs aux disparitions périodiques de l’anneau de Saturne (Paris, 1776); Traité analytique des mouvements apparents des corps célestes, 2 vols. (Paris, 1786–1789); and Traité des proprétés communes à toutes les courbes, suivi d’un mémoire sur les éclipses du soleil (Paris, 1788), written with Goudin.

II. Secondary Literature. It should be noted that in any alphabetical listing Dionis du Séjour’s name sometimes appears as Dionis, sometimes as Du Séjour, and sometimes as Séjour. On Dionis du Séjour or his work, see the following (listed chronologically): J.S. Bailly, Histoire de l’astronomie moderne, III (Paris, 1782), index under Séjour; J. de Lalande, articles in Magasin encyclopédique ou Journal des sciences, des lettres et des arts, 1 (1795), 31–34; in Connaissance des temps.... pour l’année sextile VIIe de la République (May 1797), 312–317; and in Bibliographic astronomique (Paris, 1803), pp. 750–752 and index; Nicollet, in Michaud, ed., Biographic universelle, XI (1814), 401–403, and in new ed., XI (1855), 90–91; J. B. Delambre, Histoire de l’astronomie au XVIIIe siécle (Paris, 1827), pp. xxiii-xxiv, 709–735; R. Grant, History of Physical Astronomy (London, 1852), pp. 232, 267; J. Hoefer, in Nouvelle biographie générale, XV (Paris, 1858), 295–296; Poggendorff, I, 574–575; A. Maury, L’ancienne Académie des sciences (Paris, 1864), see index; J. Bertrand, l’Académie des sciences et les académiciens de 1666 à 1793 (Paris, 1869), pp. 311–312; J.C. Houzeau and A. Lancaster, Bibliographie generate de l’astronomie, 3 vols. (Brussels, 1882–1889; repr. London, 1964) I, pt. 2, 1301, 1313, 1341, II, cols. 385, 483. 1078, 1083, 1150, 1207; J. F. Robinet, A. Robert, and J. le Chapelain, Dictionnaire historique et biographique de la Revolution et de l’Empire, I (Paris. 1899), 643–644; F. Matagrin, Vernouet le château d’Argeville (Melun, 1905), pp. 128–129; A. Douarche, Les tribunaux civils de Paris pendant la Révolution, 2 vols. (Paris, 1905–1907), see index; N. Nielsen, Géométres français sous la Révolution (Copenhagen, 1929), pp. 73–79; and Roman d’Amat, in Dictionnaire de biographie française, XI (1967), 390–391.

RenÉ Taton

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