Achille Pierre Dionis du Séjour (January 11, 1734 – August 22, 1794) was a French

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

and

mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...

. Electronic Enlightenment Project (2008) ''Achille Pierre Dionis Du Séjour correspondence.'' Dionis du Séjour was born in

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

and was a distant relative of surgeon and anatomist

Pierre Dionis Pierre Dionis (1643 in Paris – 11 December 1718 in Paris) was a French surgeon and anatomist, First surgeon of the '' Enfants de France''. Biography Pierre Dionis was trained at the in Paris where he obtained the rank of master surgeon. He was ...

. He served as an advisor to parliament before he was appointed to the

French Academy of Sciences The French Academy of Sciences (, ) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific method, scientific research. It was at the forefron ...

in 1765. In 1775 he was elected a

Fellow of the Royal Society Fellowship of the Royal Society (FRS, ForMemRS and HonFRS) is an award granted by the Fellows of the Royal Society of London to individuals who have made a "substantial contribution to the improvement of natural science, natural knowledge, incl ...

. He was appointed to the Paris group for the

Estates-General of 1789 The Estates General of 1789 () was a general assembly representing the French estates of the realm: the clergy (First Estate), the nobility (Second Estate), and the commoners (Third Estate). It was the last of the Estates General of the Kingdom ...

and was one of the 47 who joined the

Estates of the Realm The estates of the realm, or three estates, were the broad orders of social hierarchy used in Christendom (Christian Europe) from the Middle Ages to early modern Europe. Different systems for dividing society members into estates developed a ...

that year. Dionis du Séjour died in

Vernou-la-Celle-sur-Seine Vernou-la-Celle-sur-Seine () is a commune in the Seine-et-Marne department in the Île-de-France region in north-central France. Vernou-la-Celle-sur-Seine is the twin town to Manorbier, in Pembrokeshire, South West Wales, and was twinned in ...

Publications

* ''Traité des courbes algébriques'' (Treatise on Algebraic Curves), with Mathieu-Bernard Goudin (1756) * ''Recherches sur le gnomonique et les rétrogradations des planètes'' (Research on

sundials A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

and

retrograde motion Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession ...

s of Planets), (1761) * ''Traité des mouvements apparents des corps célestes'' (Treatise on

apparent retrograde motion Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as ot ...

of celestial bodies), (1774) * ''Essai sur les comètes'' (Essay on

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

s) (1775) * ''Essai sur les disparitions périodiques de l'anneau de Saturne'' (Essay on the periodic disappearance of the ring of Saturn), (1776)

References

External links

Achille Pierre Dionis du Séjour biography
via University of St. Andrews 1734 births 1794 deaths 18th-century French astronomers 18th-century French mathematicians Members of the French Academy of Sciences Fellows of the Royal Society Scientists from Paris {{france-mathematician-stub