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Astronomy / Biography

Vol. 2, NO. 4 / December 2016

Montaigne, Peiresc, Gassendi, and Cassini

The Provençal Humanists and Copernicus

Jean-Pierre Luminet

Letters to the Editors

In response to “The Provençal Humanists and Copernicus

In the first book I set forth the entire distribution of the spheres together with the motions which I attribute to the earth, so that this book contains, as it were, the general structure of the universe.
Nicolaus Copernicus, Preface to Pope Paul III, On the Revolution of the Heavenly Spheres, 1543.1

Nicolaus Copernicus wrote De revolutionibus orbium cœlestium (On the Revolutions of the Heavenly Spheres) over the course of many years, but the book was published following his death.2 With this book, historians believe, one sees the origin of a distinctively modern vision of the universe. The radical ideas presented in De revolutionibus first appeared in a brief manuscript, the Commentariolus, published some three decades earlier, but with a circulation limited to a small group of intellectuals.3

Claudius Ptolemy’s Almagest had since the second century CE been the apotheosis of Greek observational and mathematical science; it was the Greeks who championed a geocentric model of the universe. This view of the universe dominated European and Arab astronomy throughout the medieval period.

Aware of imperfections in Ptolemy’s system, and eager to discern a new order in the cosmos, Copernicus proposed an astronomical model in which the sun was at the geometric center of the universe, while the earth orbited it over the course of a year, and rotated on its own axis over the course of a day. A very similar model had been suggested by Aristarchus of Samos in antiquity. Heliocentrism demoted the earth to the status of a planet, another wandering star, just like Mercury, Venus, Mars, Jupiter, and Saturn.

Our planet no longer occupied a cosmologically privileged position.

In the second half of the sixteenth century, Omer Talon, a disciple of Petrus Ramus, was the first scholar in France to mention Copernicus. His Academicae questiones (1550) expressed a position quite favorable toward Copernicus; followers of Ramus were, generally speaking, hostile to Aristotle.4 Few of his contemporaries took Copernicus seriously, and most viewed heliocentrism negatively. One can even detect some mockery of Copernicanism in the poets of La Pléiade.5 What follows is an example from Guillaume de Salluste du Bartas:

Even so some brain-sicks live there now-a-days
That lose themselves still in contrary ways,
Preposterous wits that cannot row at ease
On the smooth channel of our common seas.
And such are those, in my conceit at least,
Those clerks that think—think how absurd a jest!—
That neither heavens nor stars do turn at all,
Nor dance about this great, round earthly ball,
But th’earth itself, this massy globe of ours,
Turns round about once every twice twelve hours.

The apparent absurdity in claiming that the earth was not, in fact, immobile meant that the Copernican doctrine was slow to spread and slower still to gain acceptance. Although now widely termed a scientific revolution, this description did not appear until the twentieth century, when it was coined by Thomas Kuhn.7

Skepticism and Astronomy

Michel de Montaigne was an exception to the general view that Copernicanism was absurd. In his Essais, he not only supported the heliocentric theory, but also perceived that the work of Copernicus was indeed a scientific revolution in the making. To understand the reasoning behind his embrace of heliocentrism, one must recall Montaigne’s fundamentally skeptical position with respect to the philosophy of knowledge.

Montaigne received a humanist education from a very young age. He forged a career as a magistrate, served as mayor of Bordeaux, and retired in 1571 at the age of thirty-seven to devote the remainder of his life to writing and revising his Essais, which are comparable to thought experiments. Montaigne’s views concerning the Copernican system appeared in a chapter entitled “An Apology for Raymond Sebond,” in which he described with satisfaction the decline of geocentrism:8

For three thousand years the skies and the stars were all in motion; everyone believed it; then Cleanthes of Samos, or according to Theophrastus, Nicetas of Syracuse decided to maintain that it was the Earth which did the moving, revolving on its axis through the oblique circle of the Zodiac; and in our own time Copernicus has given such a good basis to this doctrine that he can legitimately draw all the right astronomical inferences from it.9

Montaigne adopted a broadly skeptical position against certitude in scientific matters.10 The heliocentric model, he noted, should not be accepted simply because it was true, but rather because it dethroned man from his central place in the universe. He preferred the new theory for ethical reasons, noting that Epicurean and Stoic philosophers had favored the heliocentric model long before Copernicus, and that they had done so with an eye to questioning the importance that man had spontaneously attributed to himself.11

At the end of the sixteenth century, Copernicanism faced cultural and religious resistance from many thinkers. Montaigne was not among them. A fervent admirer of Lucretius’s De rerum natura (On the Nature of Things), Montaigne had contemplated the possibility of a plurality of worlds, a view following naturally from atomistic philosophy. He considered it more likely than the view held by Aristotle and, in particular, Thomas Aquinas, that the world was unique. The plurality of worlds would, in effect, make the earth disappear into the immensity of the universe, and reinforced for Montaigne the de-centering of the earth.

According to Marc Foglia, Montaigne endorsed the Copernican hypothesis

as a philosophical adversary of an institutionalized Aristotelianism that had become “religion and law,” and as a defender of the free exercise of thought. In contrast to a political order, science is not a set of truths that one must defend, but only a dominant tradition that one should be able to question critically.12

Montaigne may have read the anonymous preface to De revolutionibus, which attempted to neutralize the revolutionary implications of the book by depriving the heliocentric theory of any physical significance:

For these hypotheses need not be true nor even probable. On the contrary, if they provide a calculus consistent with the observations, that alone is enough. … Therefore alongside the ancient hypotheses, which are no more probable, let us permit these new hypotheses also to become known.13

The preface is now thought to have been written by Andreas Osiander, a Protestant theologian sympathetic to Martin Luther and Philip Melanchthon, both of whom had previously accused Copernicus of contradicting the Bible.

Book I of De revolutionibus begins with an author’s preface, written in the form of a dedication to Pope Paul III, in which Copernicus provides physical arguments in support of his model. These had been affirmed beforehand in the Narratio prima de libris revolutionum Copernici (The First Report on the Book of the Revolutions of Copernicus), a trial balloon of sorts for heliocentrism, published in 1540 by the sole pupil of Copernicus, Georg Joachim Rheticus. One can recognize in Osiander’s preface the interpretation known as instrumentalism, according to which astronomy only uses mathematical fictions in order to explain the planetary trajectories; its sole aim is to save the phenomena. This Platonically-inspired astronomy—as opposed to Aristotelian physics, which sought the causes of things—did not offer opinions on the nature of celestial phenomena.

Montaigne may have adopted this view in his Essais, but it was not to minimize the epistemological importance of Copernicus. On the contrary, Montaigne sought corroboration for his argument that human knowledge is illusory. The vision of the world suggested by the heliocentric theory certainly seemed better regulated and more harmonious than the geocentric theory of Ptolemy, with its complex system of epicycles. That did not establish its truth. In science, as in every discipline, Montaigne believed, one must reserve judgment, and it was thus natural to exercise caution regarding the Copernican system. One had to consider it simply as a recent, and therefore interesting, step toward a systematic description of the universe. It was thus necessarily provisional, inevitably to be replaced, sooner or later, by a better system: “What should we take from this, if not that we should not take either one? And who knows whether a third opinion, a thousand years from now, would not reverse the two preceding ones?”14 As it turned out, a thousand-year wait was unnecessary. New systems were proposed shortly thereafter by Tycho Brahe in 1583 and by Johannes Kepler in 1596.

The Revolutionary Galileo

Over the six decades that followed the publication of De revolutionibus, only a handful of astronomers throughout Europe understood the importance of the Copernican theory. Among them were Thomas Digges and William Gilbert in England, Galileo Galilei in Catholic Italy, and Kepler, Michael Maestlin, Rheticus, and Christoph Rothmann in Lutheran countries. Adopting the argument provided by Osiander, most scholars of the time considered the Copernican system to be an ingenious mathematical fiction that facilitated and improved the calculation of celestial ephemerides. This could be seen in the Prutenic Tables, calculated by Erasmus Reinhold using the heliocentric model and published in 1551. These proved slightly superior to the Alphonsine Tables of 1483, which were calculated using Ptolemy’s geocentric model. Tycho, the most celebrated astronomer of his time and an experimentalist renowned for the quality of his observations, shared this sense of mistrust toward Copernicanism.15 While Tycho admired Copernicus’s work, he did not support heliocentrism, and in 1583 proposed a geo-heliocentric model in which the earth was immobile, orbited by the sun, moon, and fixed stars, while the five planets were in orbit around the sun. Tycho was thus able to remain faithful to both the principles of Aristotelian physics and a theological interpretation of the biblical account. This clever and comforting compromise quickly attracted the support of most astronomers, philosophers, and theologians.

In 1610, Galileo, who had previously not dared teach Copernican astronomy, published Siderius nuncius (Starry Messenger), in which he revealed the results of his observations made using a telescope. These findings contradicted the dogmas of Aristotelian physics: the moon had an uneven surface, just like earth; the sun, covered in spots, was imperfect; Venus had phases; Jupiter was at the center of a system with four moons; and there were many more fixed stars than could be seen by the naked eye. Galileo’s observations had implications that extended far beyond astronomy. The disruptive doctrine of Copernicus was elevated to center stage, especially when Galileo, in his public correspondence, began defending heliocentrism from theological attacks.

In February 1616, the Copernican theory was adjudged false by the ecclesiastical authorities, because it was contrary to scripture. Amendments and corrections were thus required to render De revolutionibus inoffensive in the eyes of the church. Robert Bellarmine, a powerful Cardinal, insisted that Galileo abandon heliocentrism, but did not forbid him to study the Copernican model as a scientific hypothesis, albeit one that had no basis in truth. Bellarmine argued that future developments could make Copernicanism credible, but the judgment would have to be made by the theologians of the Holy Office.

In the years that followed, Galileo appeared relatively docile. He avoided controversy until 1632 when he published Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), in which he mocked the Aristotelian conception of the universe, the geocentric theory that followed from it, and those who believed in it.

In adopting the heliocentric system, Galileo did not offer a hypothesis, he affirmed a reality.

Ruling that he had betrayed the terms of the agreement from earlier judicial proceedings, on June 22, 1633, the Congregation of the Holy Office required him to recant

of having believed and held the doctrine—which is false and contrary to the sacred and divine Scriptures—that the Sun is the center of the world and does not move from east to west and that the Earth moves and is not the center of the world.16

In a decree dated August 23, 1634, the Dialogo was placed on the Index Librorum Prohibitorum (List of Prohibited Books)—as had been the case with the Epitome Astronomiae Copernicanae (Abbreviation of the Copernican Astronomy) by Kepler, which was condemned in 1619.

The prospects for Copernicanism appeared uncertain. Galileo was now working under the shadow of the Inquisition. In Prague, the Holy Roman Empire having been ravaged by the Thirty Years War, Kepler lost his post as the imperial mathematician. In France, René Descartes, made cautious by the persecution of Galileo, decided not to publish Traité du monde et de la lumière (Treatise of the World and of Light) in his lifetime. Prudent. Descartes defended the heliocentric theory, going so far as to propose an infinite space in which each star is the center of a vortex similar to our solar system.

Amidst these difficulties, it was in the south of France that Copernican–Galilean astronomy found new adherents in the great Provençal humanists, Nicolas-Claude Fabri de Peiresc and Pierre Gassendi. The astronomer Jean-Dominique Cassini, also from Provence, would later elevate the field to new heights in Paris.

Prince of the Curious

Nicolas-Claude Fabri de Peiresc was born December 1, 1580 in Belgentier, a small town between Aix and Toulon. The outline of his life is known from a biography written by his close friend Gassendi. As a young man, Peiresc studied with the Jesuits in Avignon, and then Tournon. At the age of sixteen, he began the study of astronomy, which he found fascinating. At the time, astronomy involved making an inventory of the stars and tracking their movements by measuring the angles between them using a cross-staff or an astrolabe. Peiresc returned to Aix-en-Provence, before moving to Padua to study law, while also engaged in many other studies. Upon his arrival in Padua, he quickly befriended the Italian humanist Gian Vincenzo Pinelli, who became his teacher and role model. It was from Pinelli, whose library was the largest of the sixteenth century, that Peiresc acquired his appetite for books and his interest in various cabinets of curiosities.17 It was at Pinelli’s library that he met Galileo for the first time.

Following the death of Pinelli, Peiresc returned to France to continue his law studies. He completed his doctoral thesis in Montpellier and then, after trips to Paris, London and Flanders, he was appointed an advisor to the parliament of Provence.

Nonetheless, astronomy remained one of his major concerns. In the fall of 1604, Peiresc observed the meeting of the three superior planets—Mars, Jupiter and Saturn—an event known as the Great Conjunction that occurs only once every eight hundred years. At the same time, a star of the magnitude of Jupiter appeared in one of the legs of the constellation Ophiuchus, and was observed for more than a year. Lacking a celestial globe, Peiresc initially believed this particular star must be among those listed by the ancients. From letters he received a few months later, he learned that it was, in fact, a new star, and one that had also been observed by Galileo. Aristotle’s ancient doctrine about the immutability of the stars must have been in some sense defective.18

In 1608, astronomy was shaken by a new discovery. Dutch opticians realized that glass lenses, which had been used since the thirteenth century to correct vision, could be configured to magnify distant objects. Galileo immediately began constructing a telescope that could magnify distant objects by a power thirty times greater than the human eye. In contrast to his contemporaries, for whom the observation of distant terrestrial objects using the new device had become an increasingly popular pastime, Galileo pointed his telescope toward the heavens. In November 1609, he observed the surface of the moon, sunspots, and the phases of Venus. On January 7, 1610, he discovered that four new planets were in orbit around Jupiter; he named them the Medicis.19

Peiresc was informed of Galileo’s discoveries in a letter dated May 3, 1610. Quickly grasping the importance of the telescope, he began to work on an instrument of his own. Eight months later, Peiresc, equipped with his new device, started making observations from his terrace. He was joined by a group of amateur astronomers, including Joseph Gaultier de la Valette, vicar general of Aix. On November 24, Peiresc and Gaultier were the first in France to observe the four satellites of Jupiter, and on November 26 they discovered the Orion Nebula, which Peiresc described as follows: “At the center of Orion … a cloudiness composed between two stars and in some sense seen from the front and illuminated from behind, the sky not being perfectly clear.20

Peiresc dedicated himself to the observation of the Galilean satellites. He named them Cosmus Minor (Callisto), Cosmus Major (Ganymede), Maria (Europa), and Catharina (Io). His group of amateur astronomers followed their movements, measuring the lengths of their orbits around Jupiter and their disappearances behind it. They soon realized that these punctual and frequent eclipses could be helpful in determining terrestrial longitudes, and to this end they constructed tables listing predictions for the positions of the satellites at specific times. Upon learning that Galileo was working on the same problem, Peiresc elected not to publish his results, and generously gave up his project out of respect for the elder scholar. As it turned out, the predictions made by Peiresc’s group in Aix would prove more accurate than those of Galileo.

The orbital periods of the four Galilean satellites of Jupiter were estimated by Peiresc with great precision:21

Satellite Peiresc’s Value (Days) Current Value (Days)
Io (Catharina) 1.7 1.769
Europa (Maria) 3.5 3.551181
Ganymede (Cosmus Major) 7.14 7.15455296
Callisto (Cosmus Minor) 16.7 16.6890184

Peiresc’s greatest innovation was a graphical scheme in which he linked the successive positions of Jupiter, two of its satellites, and some bright stars by means of a sinusoidal curve. His diagrams reveal the first use of graph paper in modern terms, but more to the point, they reveal an astronomer prepared to track the motions of a heavenly body in four dimensions rather than three. This allowed Peiresc to determine the positions of the satellites for dates when poor weather had prevented observations, and to predict the positions they would occupy in the future. The curves described by Peiresc are still used today to prepare observations of Jupiter’s satellites, although they did not appear in the Annuaire du Bureau des longitudes until a century later.

Whatever Peiresc may have thought about Jupiter and its satellites, the Inquisition understood the implications of his work only too well. With moons in regular orbit around Jupiter, neither going astray nor disturbing the path of the planet, there was nothing, in theory, to prevent the motion of the earth around the sun.

The Inquisition had by now condemned Galileo; he found himself under house arrest at his villa in Arcetri, near Florence. Peiresc was torn. He was not a cleric, but he was almost a man of the church; the pope and numerous cardinals knew him and respected his immense knowledge. Peiresc’s options were limited and, at best, he could only attempt to relieve Galileo’s misfortunes. In his correspondence with Francesco Barberini, a cardinal and nephew of the pope, Peiresc assumed the role of Galileo’s advocate (in a letter dated December 5, 1634): “My hope is that you will deign yourself to do something for the consolation of a good old septuagenarian, who is in ill health and whose memory will be difficult to erase in the future.”22 His pleas had no effect. Peiresc continued his efforts without success.

At the same time, he returned to the problem of determining longitudes. The best way to measure a difference in longitude was to observe a celestial event in two distant places; the time difference between observations is a measure of their longitude. No clocks were at the time both reliable and portable. Observing the satellites of Jupiter was not a task that could be easily carried out by sailors, for example, but lunar eclipses lent themselves more readily to this type of measurement.

With this in mind, Peiresc sought to arrange for the coordinated observation of a lunar eclipse by observers distributed along the Mediterranean. His correspondents were numerous; most were friars, since he had obtained the agreement of the leaders of the Jesuits and the Dominicans. After years of organizing, on August 28, 1635, the participants—in Aix, Aleppo, Cairo, Digne, Naples, Padua, Paris, and Rome—were ready to determine the local time at which the moon moved into the shadow of the earth.23 The results of the project were not insignificant; determining the longitude of Aleppo also yielded a new figure for the length of the Mediterranean, one that was almost a thousand kilometers shorter than previous estimates.

Yet Peiresc was not entirely satisfied. It was not when the moon entered or exited a shadow cone that one should make an observation, he noted. One had to fix on more precise points. For this purpose, a map of the moon’s surface would provide recognizable terrain. With the backing of Gassendi—whom he called the prince of the curious—Peiresc asked the engraver Claude Mellan to prepare the first detailed maps of the moon. These were based on telescope observations made from the personal observatory that Peiresc had built on the roof of his house. Two spectacular charts were engraved in 1636, but Peiresc’s death the following year prevented the task from being completed. The crater Peirescius, close to the Mare Australe in the moon’s southeastern hemisphere, is named in honor of Peiresc.24

Peiresc collected his own astronomical observations and those of his contemporaries in a series of manuscripts. He included numerous graphs and calculations, ephemerides, unpublished ideas, and a selection of letters he had received on various scientific subjects.25 A tireless correspondent, Peiresc wrote thousands of letters during his lifetime, most of which are now preserved at the Inguimbertine library in Carpentras and the Méjanes library in Aix-en-Provence. Peiresc’s most frequent correspondent was his dear friend Gassendi, but as a good humanist, interested in all the noble disciplines, he also corresponded with Galileo, the poet François Malherbe, Rubens, and the philosopher Tommaso Campanella, whom he attempted to defend from the attacks of the Inquisition.

Peiresc died in 1637.

Isaac Newton was born four days before the death of Galileo in 1643.

Astronomy to Atomism

Pierre Gassendi was born January 22, 1592, near Digne, in the Alps of the Haute-Provence. He began his studies in Digne, studied philosophy at the University of Aix, obtained a doctorate in theology at Avignon, and then, somewhat improbably, was named professor of rhetoric at Digne, and later a professor of philosophy at Aix. At his death on October 24, 1655, he was professor of mathematics at the Royal College in Paris. Gassendi was the very model of a polymath humanist, at once an astronomer, biographer, mathematician, philosopher, and theologian.26 But it was in astronomy and philosophy, in particular, that his legacy would prove most enduring.

In his youth, Gassendi discovered his passion for the wonders of the heavens while watching over his family’s flocks at night. He made regular observations throughout his life, using both telescopes and sighted instruments. During the first half-century following the invention of the telescope, the two methods were used in parallel. With telescopes one sought to make discoveries, while with traditional instruments, such as the quadrant or Jacob’s staff, one made measurements, something that could not yet be done with telescopes.27

Sunspots were one of the great novelties revealed by the telescope. Were the spots physically on the surface of the sun, or were they small satellites in orbit around the sun? Gassendi began a long series of observations in 1620, increasing in frequency around 1626, the year that Christoph Scheiner argued the spots were, in fact, satellites. Gassendi, for his part, was in agreement with Galileo, who suggested that the spots were marks on the surface of the sun itself, and thus evidence as they changed position for the rotation of our star. From his observations, Gassendi determined a speed of rotation for the sun, obtaining an estimate of 25 to 26 days. Most of Gassendi’s solar observations were lost because they were made before he began keeping notes in his journal systematically. On September 27, 1635, Gassendi wrote to Peiresc that:

[I]n order to prevent these scribbles and notes from going astray, I have begun for some while to write everything into a whole quire of paper, which I have sewn and covered with parchment for this purpose.

His diary, or astronomical journal, was born at the same time as his recognition of the essentially historical nature of astronomy.

The project to create a lunar atlas, undertaken by Gassendi and Peiresc, was brought to an end by Peiresc’s death on 1637. Mellan remained in Paris, and a deeply affected Gassendi abandoned the project. As Gassendi explains in his biography of Peiresc, they had hoped to demonstrate that the lunar globe was similar to the terrestrial globe, and to offer support for Galileo’s intuition about the deep unity of terrestrial and celestial physics.28

Peiresc and Gassendi were not the first astronomers to chart the moon. They followed in the footsteps of Galileo. So did Scheiner, Giuseppe Biancani, and Thomas Harriot, who made the first known sketch of the moon in 1609. But Peiresc and Gassendi were the first to compile a complete lunar atlas, or selenography. In doing so they were following in the tradition of Galileo’s disciples, who were eager to prepare a detailed inventory of the sky.

From Gassendi’s diary, we can see that with respect to Mars, he was engaged in determining its angular distance to the stars; for Jupiter, he pursued the program undertaken by Peiresc. He was also interested in the strange shifting form of Saturn, not suspecting the existence of the planet’s rings.29

Gassendi’s most important observation was made when Mercury passed in front of the sun on November 7, 1631. Only Mercury and Venus can be observed from the earth during their solar transit. Similar passages took place in the months of May and November, around the 7th and the 9th of the month. For Mercury, one could expect this phenomenon to repeat every 7, 13, or 46 years. It was a rare observation.

The transit of Mercury on November 7, 1631 was the first to have been scientifically predicted and observed. It was difficult to know precisely when the event would take place. The tables astronomers possessed at the beginning of the seventeenth century were quite unreliable. In the ephemerides that Kepler had calculated for the years 1629 to 1631, on the basis of the Rudolphine Tables of 1627, he had added a note, titled Admonitio, indicating that Mercury would transit the sun on November 7, 1631. Following Kepler’s death in 1630, his son Jacob Bartsch had the note republished as an offprint. Gassendi had read Kepler’s note, but Peiresc had not. In a long letter written on July 9, 1631, Gassendi filled in the missing details. In Paris at the time, Gassendi did not expect to see much from a northern latitude. Trusting in Peiresc and his colleagues, he was counting on clearer skies in Provence.

November arrived, and with it the time when Mercury was to traverse the solar disk. Neither Peiresc nor Gaultier, nor any other member of the Provençal group saw anything. In fact, it was Gassendi in Paris who, alone in France, made the observation!

Gassendi took great pains with his observation. Since he could not look directly at the planet, he had the idea of projecting its image onto a sheet of paper. On November 5, he began his vigil, despite steady rain throughout the day. On the following day, he saw the sun briefly through a downpour. But on the 7th, the sun could be seen intermittently. Mercury was already visible on its surface, though Gassendi had difficulties in recognizing it due to its small size. Directly thereafter, he published an account of his observations in a pamphlet entitled Mercurius in sole visus (Mercury Seen in the Sun).30

Gassendi may have been shocked by the relatively tiny size of Mercury, but his observations confirmed Galileo’s predictions that the planets were much smaller than they seemed, and, indeed, smaller than astronomers had previously thought. Above all, Gassendi’s observations reinforced the authority of Kepler’s Rudolphine Tables, and in a more general sense, confirmed the validity of the new astronomy. It also obliged astronomers to reexamine the question of stellar and planetary diameters, and thus their distance with respect to the earth and the sun.

Throughout his career as an experimental astronomer, Gassendi rarely missed an eclipse of the sun or the moon, describing his pursuit “like a cat after mice.” The list of his observations, made over a period of thirty years, is lengthy. The care with which Gassendi worked marks an essential stage in the development of observational astronomy.

The course that Gassendi taught at the Royal College, edited and published under the title Institutio Astronomica, became a respected manual in England, France, Italy, and later in America. Gassendi was identified with the new astronomy due to his practical work, his biographies of Copernicus, Kepler, and Tycho, and his observation of the transit of Mercury. Re-edited many times, his manual appeared in public and private libraries alongside the other foundational texts of modern and revolutionary astronomy by Copernicus, Galileo, and Kepler.

In philosophical terms, Gassendi can be categorized among the atomists, and in particular among the Epicurians, whom he helped rehabilitate—a rather surprising position for an ecclesiastic. He was long opposed to the official theories of Aristotelianism and supported Galileo in his denunciation of geocentrism. He was equally opposed to astrology.

The works of Gassendi had a particular importance for Italian scientists. Scientists intimidated by the condemnation of Galileo, were, like Gassendi, in search of a philosophical system that could coherently explain the experiments of the time. Gassendi’s system offered the Italian intelligentsia an alternative to neo-Aristotelianism, without invoking the deterministic mechanism of Descartes. Gassendi and Descartes were, needless to say, engaged in an interminable epistolary dispute.

Unlike Giordano Bruno, Gassendi did not defend the idea of a plurality of worlds. This was a theory with unacceptable theological implications, and the inflammatory Bruno was tried for heresy by the Inquisition and burned at the stake in 1600. Gassendi owned a copy of Bruno’s De innumerabilibus, and in his own writings noted his agreement with some aspects of Bruno’s theories. Both Bruno and Gassendi concluded that the stars are, in fact, themselves suns, each with their own orbiting planetary systems. Gassendi was a follower of Digges in this respect, rejecting the notion that the universe is enclosed within a spherical shell of fixed stars, an idea that had persisted since antiquity and was not contested by Copernicus. Digges had proposed that the stars were scattered throughout the universe.31

In England, John Locke and Newton were profoundly influenced by Gassendi. Gottfried Leibniz noted the impact of the French humanists on Locke’s Essay on Human Understanding:

This author is pretty much in agreement with M. Gassendi’s System, which is fundamentally that of Democritus: he supports vacuum and atoms, he believes that matter could think, that there are no innate ideas, that our mind is a tabula rasa, and that we do not think all the time; and he seems inclined to agree with most of M. Gassendi’s objections against M. Descartes.32

Cassini the First

Educated in the Jesuit school of Genoa, Gian-Domenico Cassini’s talents brought him to the attention of a wealthy amateur from Bologna, the marquis Cornelio Malvasia. In 1644, the marquis engaged Cassini to work at the Observatory of Panzano, which was still under construction at the time. Cassini worked with two noted Jesuit astronomers, Giovanni Riccioli and Francesco Grimaldi, who completed his education. Cassini was named professor of astronomy and mathematics at the Jesuit University of Bologna in 1650. He was just 25 years old.

The Roman Catholic Church impelled Cassini to teach Ptolemaic astronomy. Nevertheless, after observing the comet of 1652–1653, he adopted Tycho’s geo-heliocentric system, already favored by the Jesuits; he would not accept the Copernican model until later.

An expert in both hydraulics and engineering, Cassini was charged by the senate of Bologna and the pope himself with a number of scientific and political missions. But it was astronomy to which he devoted most of his energies. In 1665, Cassini discovered Jupiter’s giant red spot, and determined the rotational speed of Jupiter, Mars, and Venus. His reputation spread beyond the borders of Italy, and in 1668, Jean-Baptiste Colbert, who sought out foreign scientists for the new Académie des Sciences in Paris, asked him to become a corresponding member. Cassini accepted.

Colbert then invited Cassini to France in order to construct a new observatory. Cassini arrived in Paris in August 1669, and began collaborating at the Académie, modifying Claude Perrault’s plans to better adapt the building for astronomical observations. Even before the adaptations were complete, Cassini discovered two of Saturn’s satellites: Iapetus in 1671 and Rhea in 1672. He was named director of the Observatory of Paris at the request of Louis XIV, and charged with making it the most important astronomical and scientific center of the time.

Despite requests from the pope that he return to Italy, Cassini preferred to remain in France, where he obtained citizenship in 1673. He then Gallicized his first name to Jean-Dominique. The same year, Cassini made the first precise measurement of the distance from the earth to the sun, using a measurement of the parallax of Mars deduced from the observations of Jean Richer in Cayenne. Two years later, he discovered the division of the rings of Saturn, now known as the Cassini Division, and, in 1684, two new satellites of Saturn: Tethys and Dione. In 1679, Cassini presented a map of the moon to the Académie des Sciences that would be unequaled in precision until the invention of photography. Oddly enough, Cassini refused to recognize Ole Rømer’s demonstration that the speed of light was finite, despite the fact that Rømer used Cassini’s own ephemerides of Jupiter’s satellites in his calculations.33 Around 1690, he was the first to observe that Jupiter’s atmosphere is in differential rotation.

Cassini went blind in 1710, and died two years later in Paris at the age of 87.34

The Adventurers

Peiresc and, to a lesser extent, Gassendi, however much they may figure in scholarly accounts, are not well known to the public. They have remained in the shadows. Yet it is these intellectual adventurers who continued the revolution begun in the previous century by Copernicus. The Provençal humanists of the seventeenth century were fundamental in shaping the subsequent development of Western society. Newton stood directly in their future. Seeking God in nature, as in the scriptures, Newton left behind a world in which religion was markedly diminished. The eighteenth century saw the triumph of a perfectly mathematical form of celestial mechanics.

“Thus liberated, science has taken humanity on a journey to the edge of the universe, describing for us the touching fragility of our tiny planet.”35

Translated by the editors.


  1. Nicolaus Copernicus, On the Revolutions, trans. Edward Rosen (Baltimore: Johns Hopkins University Press, 1992), 5. See also Nicolaus Copernicus, On the Revolutions of the Heavenly Spheres, trans. Alistair Duncan (New York: Barnes and Noble, 1976). 
  2. See Michel-Pierre Lerner et al., Nicolas Copernic (1473–1543) De revolutionibus orbium coelestium / Des révolutions des orbes célestes (Paris: Les Belles Lettres, 2015); Nicolaus Copernicus, On the Revolutions of the Heavenly Spheres, trans. Alistair Duncan (New York: Barnes and Noble, 1976). 
  3. The complete work, De Hypothesibus Motuum Coelestium a se Constitutis Commentariolus, was not published until the nineteenth century. See, for example: Introductions à l’astronomie de Copernic: le Commentariolus de Copernic, la Narratio prima de Rheticus, ed. Henri Hugonnard-Roche, Edward Rosen, and Jean-Pierre Verdet (Paris: A. Blanchard, 1975). 
  4. See Stanford Encyclopedia of Philosophy Archive, “Petrus Ramus.” 
  5. See, for example, Jean Bodin, Universae naturae theatrum, in quo rerum omnium effectrices causae et fines quinque libris discutiuntur (Lugduni: Jacobum Roussin, 1596). 
  6. The original French is as follows:
    Il se trouve entre nous des esprits frénétiques
    Qui se perdent toujours par des sentiers obliques
    Et, de monstres forgeurs, ne peuvent point ramer
    Sur les paisibles flots d'une commune mer.
    Tels sont comme je crois ces écrivains qui pensent
    Que ce ne sont pas les cieux ou les astres qui dansent
    A l'entour de la terre, mais que la terre fait
    Chaque jour naturel un tour vraiment parfait.
    Guillaume de Salluste du Bartas, La sepmaine ou création du monde, ed. Victor Paul Bol (Arles: Actes Sud, 1988), 106–107. For an English translation see Guillaume de Saluste du Bartas, The Divine Weeks and Works of Guillaume de Saluste, Sieur du Bartas, trans. Josuah Sylvester, ed. Susan Snyder (Oxford: Oxford University Press, 1979). 
  7. Thomas Kuhn, The Copernican Revolution (Cambridge, MA: Harvard University Press, 1957). 
  8. For the original text, see Wikisource, “Essais/Livre II/Chapitre 12.” For an English translation, see Michel de Montaigne, An Apology for Raymond Sebond, trans. M. Screech (London: Penguin, 1987). 
  9. Michel de Montaigne, An Apology for Raymond Sebond, trans. M. Screech (London: Penguin, 1987), 149. 
  10. Marc Foglia, Montaigne, pédagogue du jugement (Paris: Classiques Garnier, Paris 2011). See also Stanford Encyclopedia of Philosophy, “Michel de Montaigne.” 
  11. There is also the lost treatise by Aristarchus of Samos, an Alexandrian astronomer during the third century BCE, who even then argued for a heliocentric system. 
  12. Marc Foglia, Montaigne et la révolution copernicienne, 16. 
  13. Nicholas Copernicus, Complete Works, vol. 2, ed. and trans. Edward Rosen (Baltimore: Johns Hopkins University Press, 1992). 
  14. Paul Mathias, Montaigne: ou l’usage du monde (Paris: Librairie Philosophique J. Vrin, 2006), 39. Translation by the editors. 
  15. For a detailed comparison see Frank Tipler and Wesley Bollinger, “Ptolemy versus Copernicus,” Inference: International Review of Science 1, no. 3 (2015). 
  16. Famous Trials: Trial of Galileo Galilei 1633, “Papal Condemnation of Galileo (June 22, 1633).” Cited in Michel-Pierre Lerner et al., Nicolas Copernic (1473–1543) De revolutionibus orbium coelestium / Des révolutions des orbes célestes, vol. 1 (Paris: Les Belles Lettres, 2015), 16. 
  17. Wikipedia, “Cabinet of Curiosities.” 
  18. Known today as supernovae, the new star observed by Peiresc is named after Kepler, who spent nearly a year observing and studying the supernova in Ophiuchus. See Johannes Kepler, De Stella nova in pede Serpentarii (On the New Star in Ophiuchus’s foot) (Prague: Typis Pauli Sessii, 1606). 
  19. The first use of the term satellite is attributed to Kepler. The German astronomer Simon Marius (1573–1625) gave Jupiter’s moons their modern names: Callisto, Europa, Ganymede, and Io. 
  20. In Orione media ... Ex duabus stellis composita nubecula quamdam illuminata prima fronte referabat coelo non oie sereno
  21. See, for example: Amis du Planétarium d’Aix-en-Provence, Les Cahiers Peiresc: Numéro spécial Octobre 2009, ed. Philippe Malburet (Aix-en-Provence, France: Planétarium Peiresc, 2009). 
  22. The Trial of Galileo: Essential Documents, ed. Maurice A. Finocchiaro (Indianapolis, IN: Hackett, 2014), 146. 
  23. The instructions and the instruments did not arrive in Tunis in time for the eclipse. Mt. Sainte-Victoire, which towers over Aix-en-Provence, was climbed by a group of observers who missed the event because they were asleep! 
  24. The scientific world’s tributes to Peiresc have, rightly, not been limited to naming a lunar crater in his honor. The asteroid 19226, discovered in October 1993, was also named after him, a rather belated recognition of his contributions. 
  25. This manuscript, titled Astronomica, is kept at the Inguimbertine Library in Carpentras, France. 
  26. In addition to his biography of Peiresc, Gassendi also left us invaluable biographies of the astronomers Copernicus, Kepler, Regiomontanus, and Tycho. See, for example, Pierre Gassendi, The Life of Copernicus (1473–1543), with notes by Olivier Thill (Fairfax, VA: Xulon Press, 2002). 
  27. The micrometer, developed by William Gascoigne in 1639, was not widely known at the time. 
  28. Pierre Gassendi, Viri illustris Nicolai Claudii fabricii de Peiresc, senatoris aquisextiensis vita (Paris: Sebastiani Cramoisy, 1641). 
  29. In 1655, the year of Gassendi’s death, the Dutch astronomer Christiaan Huygens (1629–1695) was the first to describe the rings of Saturn as a disc. 
  30. Pierre Gassendi, Mercurius in Sole visus, et Venus invisa Parisiis, anno 1631, 1632 (Paris: Sebastiani Cramoisy, 1632). A scan of the original Latin text is available online at the Bibliothèques Virtuelles Humanistes
  31. Thomas Digges, A Perfit Description of the Cælestiall Orbes according to the most aunciente doctrine of the Phythagoreans, latelye reuiued by Copernicus and by Geometricall Demonstrations approued. (London: Thomas Marsh, 1576). 
  32. Gottfried Leibniz, New Essays on Human Understanding, trans. and ed. Peter Remnant and Jonathan Bennett (Cambridge, UK: Cambridge University Press, 1982), 70. 
  33. In 1676, at the Paris Observatory, Ole Rømer (1644–1710) demonstrated for the first time that the speed of light was not infinite. Rømer estimated that, “light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit,” a result that, strangely, Cassini refused to accept. See Wikipedia, “Rømer’s Determination of the Speed of Light.” 
  34. Succeeded by his son, Jacques Cassini, three generations of Cassinis reigned at the Observatory of Paris over 122 years. Jean-Dominique Cassini would be designated by the regal title Cassini I. 
  35. Jean-Pierre Luminet, Les Bâtisseurs du ciel (Copernic, Kepler, Galilée, Newton) (Paris: J. C. Lattès, 2010). 

Jean-Pierre Luminet is Director of Research at the CNRS Astrophysics Laboratory in Marseille and the Paris Observatory.

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