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