b o u l l i a u
An early Copernican,
Keplerian, and defender of Galileo, Boulliau was the most noted astronomer
of his generation. Although his career reflects many of the movements
associated with the Scientific Revolution, Boulliau was widely known in
the Republic of Letters as an historian, classical scholar, and
philologist. Arguably his correspondence network (which rivals the
combined efforts of Mersenne and Oldenburg) marks the transition from érudit
to
homme
de science, from intelligencer to statesponsored science.
Born to Calvinist parents in Loudun (28 Sept 1605), Boulliau converted to Catholicism and moved to Paris in the early 1630s. During the next thirty years he enjoyed the patronage of the family de Thou and assisted the Brothers Dupuy at the Bibliothèque du roi, home of their famous ‘Cabinet.' Here Boulliau extended the humanist ideal of intelligencer to matters of science. Boulliau's first book, De natura lucis (1638), grew out of an ongoing conversation on the nature of light with his friend Pierre Gassendi (15921655). Against Gassendi's atomist claims, Boulliau defended Kepler's punctiform analysis but argued, against Kepler, that light behaved three dimensionally (Prop. 7) and could be understood as a mean proportional ‘between corporeal and incorporeal substance' (Theorem I). With a smile, Descartes somehow missed the point by reading ‘between substance and accident.' Later in the volume Boulliau provided one of the first statements of the law of illumination (Prop. 27). In the following year Boulliau published his Philolaus (1639), which had circulated in manuscript in the years following Galileo's condemnation. Thoroughly Copernican, there was perhaps little remarkable about the book except, as Descartes noted, that it was published at all. Boulliau's purpose was to provide new geometrical and optical arguments for the motion of the earth. Although he was attacked by JB Morin (15831656) and several Italian astronomers, Boulliau continued to embrace Kepler's central claim, that nature ‘loves simplicity, she loves unity...she uses one cause for many effects' (Mysterium cosmographicum, 1596, Chpt. 1). Struck by the elegance of Galileo's ‘Platonic Cosmogony' and Kepler's ‘Platonic Solids,' Boulliau sought a single solution to two longstanding problems: Accelerated circular free fall (Galileo's Problem) and planetary motion (Kepler's Problem). The solution, Boulliau suggested, would be found in the ‘secret that lay hidden in uniform acceleration' and in the rule that governed the attenuation of light. Alas, a solution proved no mean feat, and Boulliau was not equal to the task. But his Philolaus received wide notice. It underscored his belief that a deeper unity  simple and elegant  would unite heaven and earth. The clearest expression of these commitments came in his Astronomia philolaïca (1645), arguably the most important book in astronomy between Kepler and Newton. Without doubt, this work extended awareness of Kepler's planetary ellipses. But where Kepler sought a physical cause for planetary motion  and called on astronomers and mathematicians for assistance  Boulliau provided an entirely new cosmology, the ‘Conical Hypothesis.' Arguing that planets orbit the sun in an elliptical path, Boulliau again sought a single solution to two problems. For astronomers, the problem was to locate the planet at a given time; for cosmologists, the problem was to explain orbital motion. Because circles and ellipses are conic sections, Boulliau imagined each planet moving on the surface of an oblique cone in an elliptical orbit with the sun at the lower focus. By construction, the axis of the cone bisected the base, which simultaneously defined the empty focus of the ellipse and the centers of circular motion. The position of a planet on the ellipse at a given time (Kepler's Problem) was thus defined by the intersecting circles (parallel to the base of the cone) where the planet's motion, at any instant, was uniform and circular around its center (Plato's Dictum). But where Kepler invoked analogies of the lever and magnetic attractions and repulsions, Boulliau explained acceleration and deceleration along the ellipse as the natural motion of the planet from smaller or larger circles. The result was elegant and practical. If Kepler's ‘area rule' was suspect on physical and geometrical grounds, it was also difficult to apply. By contrast, Boulliau provided a model of simplicity: Planetary motion was not caused by external forces but by reason of geometry. Tedious trialanderror calculation was now simple and direct. Arguably  in context  Kepler's construction was ingenious but useless. The foundations of Boulliau's cosmology, however, were soon called into question. In 1653 Seth Ward attacked his hypothesis claiming to provide a more simple and accurate model. In his published response (1657), Boulliau acknowledged the difficulty (noted in his Philolaïca) but showed that Ward's alternative (the ‘simple elliptical' model) was not equivalent in fact. If they were observationally equivalent, the Conical Hypothesis would show an error of 8' of maximum heliocentric longitude, not 2.5'. Ward failed to note the difference. Boulliau nevertheless supplied a new ‘modified elliptical' model. Compared to Kepler's calculations (using the same Tychonic data) the new model was slightly more accurate for several of the planets. Although the BoulliauWard debate ended abruptly and while Boulliau's tables were widely copied in England and Italy the ‘problem of the planets' was far from resolved. Boulliau published a number of works that blurred the distinction between ‘science and humanism,' among them editions of Theon of Smyrna's Platonici (1644), Manilius' Astronomicon (1655), Ptolemy's De judicandi facultate (1663), and De lineis spiralibus (1657), inspired by Archimedes and Pappus. In his Ad astronomos monita duo (1667) Boulliau employed his scientific as well as his historical skills to establish the period of Mira Ceti, a longperiod variable star. Boulliau died in Paris 25 November 1694. Robert A. Hatch  University of
Florida


rah.feb.1998
