Background
Jacques de Billy was born on March 18, 1602, in Compiegne, Oise, France.
Astronomer mathematician scientist teacher
Jacques de Billy was born on March 18, 1602, in Compiegne, Oise, France.
After studying humanities, Billy entered the Jesuit order in 1619 and completed his divinity studies, equivalent to a doctorate, in 1638.
A Jesuit, Billy spent his teaching career in the collèges of the society’s administrative province of Champagne - Pont à Mousson, Rheims, and Dijon. He taught either theology or mathematics, depending on which was needed. In 1629 - 1630 he taught mathematics at Pont à Mousson while he was still a theology student and not yet ordained a priest. Billy taught mathematics at Rheims from 1631 to 1633. Around this time he became a close friend of Claude Gaspar Bachet de Méziriac, the commentator on Diophantus who introduced him to indeterminate analysis.
Billy became master of studies and professor of theology at the Collège de Dijon, where one of his students was Jacques Ozanam, whom he taught privately because there was no chair of mathematics at the collège, and in whom he instilled a profound love for calculus. Finally, a professorship having been created in mathematics, he taught his favorite subject from 1665 to 1668. He also served as rector of a number of Jesuit Colleges in Châlons-en-Champagne, Langres and in Sens.
An active correspondence between Billy and Fermat began before 1659, of which one letter remains. Some of Billy’s writings originated in this exchange, including parts of the Doctrinae analyticae inventum novum, through which his name is still known to number theorists. It is an elaborate study of the techniques of indeterminate analysis used by Fermat and, on the whole, it explains them correctly. From it, one can guess at Fermat’s general line of activity in a field in which there are few pertinent documents.
In astronomy, Billy published numerical tables applicable to the three important theories (Ptolemy, Brahe, Copernicus) of the time. There is also a study on comets and several critiques against forensic astrology.
From 1619, Jacques de Billy belonged to the Society of Jesus.