Ismaël Boulliau (Latin: Ismaël Bullialdus; 28 September 1605 – 25 November 1694) was a 17th-century French astronomer and mathematician who was also interested in history, theology, classical studies, and philology. He was an active member of the Republic of Letters, an intellectual community that exchanged ideas. An early defender of the ideas of Copernicus, Kepler and Galileo, Ismael Bullialdus has been called "the most noted astronomer of his generation". One of his books is Astronomia Philolaica (1645).

Life and career

Ismael Bullialdus was the second-born to his Calvinist parents, Susanna Motet and Ismael Bullialdus. His father was a notary by profession and an amateur astronomer who made observations in Loudun, France. His older brother was originally named after their father Ismael, but died shortly after birth. At the age of 21, Bullialdus converted to Roman Catholicism and was ordained at age 26. One year later, in 1632, he moved to Paris. Enjoying the patronage of the de Thou family, Bullialdus worked for 30 years in Paris as a librarian associated with the brothers Jacques and Pierre Dupuy, who were working on the Bibliothèque du Roi (Bibliothe), France's first royal library. After the death of his employers, the brothers Dupuy, Bullialdus became secretary to the French ambassador of Holland. After a dispute with him in 1666, however, he once again moved, this time to the Collège de Laon, where he worked again as a librarian. Bullialdus published his first work De Natura Lucis in 1638, which he followed with many more published works, ranging from books to published correspondence during his time involved with the Republic of Letters. He was one of the earliest members to be elected as a foreign associate into the Royal Society of London on April 4, 1667, only seven years after the Society was founded. He spent the last five years of his life as a priest, the same occupation in which he started his career. He retired to the Abbey St. Victor in Paris, where he died at the age of 89.

Involvement in the Republic of Letters

Bullialdus was an active member of the Republic of Letters, the long-distance intellectual correspondence network that had emerged as an international community of self-proclaimed scholars and literary figures. Bullialdus was a prolific correspondent, with around 5,000 letters that have survived to this day. His letters demonstrate the geographic reach of the Republic of Letters; he corresponded with scholars not only in nearby countries like Holland and Italy, but also in Scandinavia, Poland, and the Near East. Around 4,200 of them are in the Collection Boulliau of the Bibliothèque nationale de France (formerly the "Bibliothèque du Roi") with another 800 to or from him that are outside the collection in 45 different archives in nearly a dozen countries. Unfortunately, many of his manuscripts are lost; shortly after his death, his entire library—books, manuscripts and correspondence—was dispersed. The most famous of the known letters included in the original Archive Boulliau include correspondence with notable thinkers, including Galileo, Marin Mersenne, Henry Oldenburg, Christiaan Huygens, and Fermat. In addition to his own letters, Bullialdus contributed to "The Archives of the Scientific Revolution". Among Bullialdus' papers were notes and examinations of rare manuscripts. Also found among his letters were copies of his contemporaries' manuscripts which he had preserved. Arguably most notable were the ten volumes of original autographs addressed to Nicolas-Claude Fabri de Peiresc.

Principal works

Ismael Bullialdus' most famous work is Astronomia Philolaica. Published in 1645, the book is considered by some modern-day historians of science to be the most important book in astronomy between Kepler and Newton. The book widened the awareness of Kepler's planetary ellipses, however, whereas Kepler used a physical cause to explain planetary motion, and called on math and science to support his theory, Bullialdus offered an entirely new cosmology, the "Conical Hypothesis".

Assumptions of Philolaic Astronomy

Bullialdus' Philolaic Astronomy consists of 14 main assumptions:

Bullialdus' hypothesis