Ismaël Boulliau (; Latin: Ismaël Bullialdus; 28 September 1605 – 25 November 1694) was a 17th-century French

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...

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, structure, space, models, and change. History On ...

who was also interested in history,

theology Theology is the systematic study of the nature of the divine and, more broadly, of religious belief. It is taught as an academic discipline, typically in universities and seminaries. It occupies itself with the unique content of analyzing th ...

classical studies Classics or classical studies is the study of classical antiquity. In the Western world, classics traditionally refers to the study of Classical Greek and Roman literature and their related original languages, Ancient Greek and Latin. Classics ...

, and

philology Philology () is the study of language in oral and written historical sources; it is the intersection of textual criticism, literary criticism, history, and linguistics (with especially strong ties to etymology). Philology is also defined as ...

. He was an active member of the

Republic of Letters The Republic of Letters (''Respublica literaria'') is the long-distance intellectual community in the late 17th and 18th centuries in Europe and the Americas. It fostered communication among the intellectuals of the Age of Enlightenment, or ''phil ...

, an intellectual community that exchanged ideas. An early defender of the ideas of

Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulat ...

Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...

and

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

, 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 Calvinism (also called the Reformed Tradition, Reformed Protestantism, Reformed Christianity, or simply Reformed) is a major branch of Protestantism that follows the theological tradition and forms of Christian practice set down by John C ...

parents, Susanna Motet and Ismael Bullialdus. His father was a notary by profession and an amateur astronomer who made observations in

Loudun Loudun (; ; Poitevin: ''Loudin'') is a commune in the Vienne department and the region of Nouvelle-Aquitaine, western France. It is located south of the town of Chinon and 25 km to the east of the town Thouars. The area south of Loudun i ...

, France. His older brother was originally named after their father Ismael, but died shortly after birth. At the age of 21, Bullialdus converted to

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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

. He was one of the earliest members to be elected as a foreign associate into the

Royal Society of London The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...

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

, 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 The Bibliothèque nationale de France (, 'National Library of France'; BnF) is the national library of France, located in Paris on two main sites known respectively as ''Richelieu'' and ''François-Mitterrand''. It is the national repository ...

(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

Marin Mersenne Marin Mersenne, OM (also known as Marinus Mersennus or ''le Père'' Mersenne; ; 8 September 1588 – 1 September 1648) was a French polymath whose works touched a wide variety of fields. He is perhaps best known today among mathematicians for ...

Henry Oldenburg Henry Oldenburg (also Henry Oldenbourg) FRS (c. 1618 as Heinrich Oldenburg – 5 September 1677), was a German theologian, diplomat, and natural philosopher, known as one of the creators of modern scientific peer review. He was one of the fo ...

Christiaan Huygens Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists o ...

, and

Fermat Pierre de Fermat (; between 31 October and 6 December 1607 – 12 January 1665) was a French mathematician who is given credit for early developments that led to infinitesimal calculus, including his technique of adequality. In particular, he is ...

. 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 Nicolas-Claude Fabri de Peiresc (1 December 1580 – 24 June 1637), often known simply as Peiresc, or by the Latin form of his name, Peirescius, was a French astronomer, antiquary and savant, who maintained a wide correspondence with scientis ...

Principal works

Boulliau, Ismaël – De natura lucis, 1638 – BEIC 169735

*''De natura lucis'' (1638) *''Philolaus'' (1639) *''Expositio rerum mathematicarum ad legendum Platonem utilium'', translation of

Theon of Smyrna Theon of Smyrna ( el, Θέων ὁ Σμυρναῖος ''Theon ho Smyrnaios'', ''gen.'' Θέωνος ''Theonos''; fl. 100 CE) was a Greek philosopher and mathematician, whose works were strongly influenced by the Pythagorean school of thought. Hi ...

(1644) *''Astronomia philolaica'' (1645
e-rara.ch
*''De lineis spiralibus'' (1657) *''Opus novum ad arithmeticam infinitorum'' (1682) *''Ad astronomos monita duo'' (1667) Ismael Bullialdus' most famous work is ''Astronomia Philolaica''. Published in 1645, the book is considered by some modern-day

historians of science The history of science covers the development of science from ancient times to the present. It encompasses all three major branches of science: natural, social, and formal. Science's earliest roots can be traced to Ancient Egypt and Mesop ...

to be the most important book in astronomy between

and Newton. The book widened the awareness of

'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: # Planets have a simple motion in a simple line. # Planetary revolutions are equal, perpetual, uniform. # They should be regular revolutions or composed of regular revolutions. # They can only be circular. # Or composed of circles. # Motions should have a principle of equality. # Since they admit of a certain inequality, the center of the zodiac must be the reference point of inequality. # This point is in the sun. # Half of the inequality is attributed to eccentricity, the other to another cause which makes the planet slower at

aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

, less slow at perihelion, without disturbing the equality of motion or transposing it to some other place, whether the circle or the surface. # When the planet, moving from aphelion, comes to quadrature on the same surface, with equal motion, it should differ from the apparent motion of the first inequality completely or nearly so; but because the other half f the inequalityis due to the distance etweenthe circles, the center of planetary motion must be between the points of true and apparent motion. # Since the equal motion in the first quadrant is greater than the apparent motion, that part of apparent motion must be greater, hence from the first quadrant to

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

the arc described going to perihelion must be larger than the first. # All revolution is composed of circular parts; the same is true of each part. # Equal motion is uniform; thus, the motion in coming from aphelion corresponds to the larger parallel circles, which increase from aphelion to perihelion. This equal motion does not correspond to a single circle, but to several unequal circles to which the apparent motion also corresponds; the apparent motion includes all the circles on the same surface. The motion must lsobe eccentric and inclined. # These circles follow one another in a continuous series and are all parallel among themselves; they do not overlap or enclose one another; the apparent motion forms a solid surface containing larger and smaller circles.

Bullialdus' hypothesis

* Conical Hypothesis: "The Planets, according to that astronomer oulliau always revolve in circles; for that being the most perfect figure, it is impossible they should revolve in any other. No one of them, however, continues to move in any one circle, but is perpetually passing from one to another, through an infinite number of circles, in the course of each revolution; for an ellipse, said he, is an oblique section of a cone, and in a cone, betwixt the vertices of the ellipse there is an infinite number of circles, out of the infinitely small portions of which the elliptical line is compounded. The Planet, therefore, which moves in this line, is, in every point of it, moving in an infinitely small portion of a certain circle. The motion of each Planet, too, according to him, was necessarily, for the same reason, perfectly equable. An equable motion being the most perfect of all motions. It was not, however, in the elliptical line, that it was equable, but in any one of the circles that were parallel to the base of that cone, by whose section this elliptical line had been formed: for, if a ray was extended from the Planet to any one of those circles, and carried along by its periodical motion, it would cut off equal portions of that circle in equal times; another most fantastical equalizing circle, supported by no other foundation besides the frivolous connection betwixt a cone and an ellipse, and recommended by nothing but the natural passion for circular orbits and equable motions," (Adam Smith, History of Astronomy, IV.55-57).

References

* * * Hatch, Robert A. (1998). ''Boulliau Bibliography.'' University of Florida. Retrieved 2013-11-19.