Achille-Pierre Dionis du Séjour's father was Louis-Achille Dionis du Séjour and his mother was Geneviève-Madeleine Héron. Louis-Achille Dionis du Séjour held a legal position in the Cour des Aides, the board of excise, in Paris.
Dionis du Séjour attended the Collège Louis-le-Grand in Paris and the studied at the Faculté de Droit. He published a treatise on the analytic geometry of plane curves in 1756 Traité des courbes algébrique. As a result of this work he was elected associé libre of the Académie des Sciences in the same year as its publication.
In 1758 he was appointed as a member of the parliament in Paris. He combined this political career with his research in mathematics and astronomy which, although of high quality, was no more than a hobby for him. However, he wrote extensively on applications of mathematics to astronomy, in particular planetary orbits, and his work was highly regarded by Lagrange, Laplace and Condorcet.
Dionis du Séjour applied the latest analytic mathematical methods to the study of problems in astronomy. Over a period of almost 20 years from 1764 he wrote a series of memoirs on eclipses, occultations (when one astronomical body comes in front of another), calculating orbits, and other such topics, and these were brought together in a two volume work Traité analytique des mouvements apparents des corps célestes which he published, volume one in 1786 and volume two in 1789.
He published two other volumes on mathematical astronomy. The first was Essai sur les comètes en général; et particulièrement sur celles qui peuvent approacher de l'orbite de la terre (1775) which, as the title suggests considers comets and, in particular, shows that the probability of a collision between a comet and the earth is very low. It is interesting, particularly given the interest in this topic today, to see the subject being explored 225 years ago. The second volume is Essai sur les phémomènes relatifs aux disparitions périodique de l'anneau de Saturne was published in 1776 and, again as suggested by the descriptive title, it explains the variation in the appearance of the rings of Saturn.
Dionis du Séjour also worked on the theory of equations, not attaining the depth of results of Bézout or Lagrange. With Condorcet and Laplace he undertook a determination of the population of France :-
Utilising the list of communes appearing in the Cassini map of France and the most recent information furnished by the civil engineers, this enquiry 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 French Revolution caused problems for Dionis du Séjour, both in his political roles and in his scientific work. He was elected as a deputy of the Paris nobility on 10 May 1789. This gave him a seat in the National Assembly. The storming of the Bastille on 14 July 1789 marked the start of the French Revolution and the National Assembly then became the Constituent Assembly. On 30 September 1791 the Constituent Assembly was replaced by Legislative Assembly and Dionis du Séjour's role came to an end at this time.
On 31 November 1791 Dionis du Séjour was appointed as a judge of the Paris tribunal but later he resigned and retired to his estate in Argeville. However from 5 September 1793 to 27 July 27 1794 there was the Reign of Terror during which 17,000 were officially executed. Dionis du Séjour was in fear of his life throughout this period and the anxiety he experienced may well have hastened his death which was less than a month after the Reign of Terror ended.
As to Dionis du Séjour's character, he is described as :-
... appreciated for his simplicity, his liberalism, and his humanity.
Article by: J J O'Connor and E F Robertson