10 - The impact of Galilean culture - From Bonaventura Cavalieri to Gian Domenico Cassini.

The successor of Magini was the Milanese Jesuit Bonaventura Cavalieri (1598-1647), one of Galileo’s friend. He was undoubtedly more mathematician than astronomer: his Geometria indivisibilibus continuorum nova quadam ratione promota (Bologna, 1635) marked a crucial step in the conquest of differential and integral calculus. He should also be credited, however, with making a decisive contribution to the spread of Galilean ideas to Bologna. (62) From the list of the lectures he gave in 1643 we know he taught the basics of the Copernican system - and this just a few years after Galileo had been condemned.Following the death of Cavalieri, the Bolognese Galilean school produced no more personalities of note. The eminent figures in this period came not so much from the University as from the Jesuits, including the great anticopernican from Ferrara Giovanni Battista Riccioli (1598-1671) and the Bolognese Francesco Maria Grimaldi (1618-1663). The in folio works of the former are veritable encyclopedias that were widespread and much consulted in their day: Almagestum novum..., Astronomia reformata..., Geographia et hydrographia reformata..., Chronologia reformata.... The second is famous above all for having discovered the effect that carries the name he gave it of diffractio luminis and for having explicitly set out in his book De lumine (published posthumously in 1665) that the speed of light is not infinite. He was also author of the lunar maps, which remained a part of planetary astronomy, with their reference to so-called laghi, mari and crateri (lakes, seas, and craters).

There were however in this period contrary signals to Jesuit anticopernicanism such as the publication, in Bologna, of the works of Galileo, albeit incomplete (the Dialogo dei massimi sistemi being obviously missing).

The Galilean line of thought was consonant with the work of the physician and biologist Marcello Malpighi (1628-1694) - the first to use the microscope for the systematic study of animal and vegetative structures - who, having lived two years in Tuscany, had close ties with the Accademia del Cimento and Geminiano Montanari (1633-1687) (about whom we shall say more later), founder of the Accademia della Traccia, "Bologna branch" of the Cimento. Both were undoubtedly figures of note and played a crucial role in training the next generation of natural philosophy scholars, in whose ranks was Luigi Ferdinando Marsili. It is worth recalling that neither of the two had an easy life in Bologna, Montanari probably paying for the strong stance he took against astrology.

In 1650 Bonaventura Cavalieri was succeeded by the Ligurian Gian Domenico Cassini (1625-1712) from Perinaldo, invited by the marquis Cornelio Malvasia (1603-1664) to visit his observatory at Panzano - near Modena, but today unfortunately destroyed - and introduced by him to the Bolognese scene. It is fair to say it was with him that Bolognese astronomy took the lead in Europe, at least until 1669 when Cassini, at the behest of Colbert, left Bologna for Paris.

Even though he could never say so openly, not even during his stay in France where the Church’s condemnation of Copernicus was even more rigorously observed than in the papal dominions themselves, there is no doubt that one, and perhaps the main, thrust of his work was an attempt to prove the validity of the Copernican system, an attempt which had moreover its fair share of luck.

A key criticism was over the question of the validity or otherwise, in the skies, of Aristotelian, or perhaps more accurately peripatetic, physics. It was a legitimate target given that the Church, in condemning heliocentrism, had abstained from officially adopting this school of physics as its own.

As soon as he arrived in Bologna Cassini had in fact immediately set about constructing a new meridian in San Petronio, the very long meridian line that can still today be admired in the floor of the church and which replaced the meridian built by Danti 80 years earlier.

Its size (almost 68m long), precision-made detail, and easy use meant its accuracy was equal to that of the new telescope-fitted instruments more than 50 years later. From observations made between 1655 and 1669 Cassini obtained three crucial results: a table of atmospheric refractions which remained the most accurate for over a century, a new value for the inclination of the ecliptic, and confirmation of the non-uniformity of the Earth’s orbital velocity, in line with Kepler’s laws of planetary motion.

The value obtained for the inclination of the ecliptic was 23°29’15" against the 23°31’30" of Tycho Brahe and the 23°50’ of Ptolemy (nearer therefore than the others to the actual value of 23°28’53"). Immediately after its completion, in 1656, comparing the variations in the diameter of the Sun with the speed of its apparent motion along the ecliptic - both measured with the new magnificent instrument - he was able to write that an actual variability in solar motion had been directly detected by observations for the first time (63). In 1736 Eustachio Manfredi would collect all the observations made with the San Petronio meridian in a volume entitled De Gnomone Meridiano Bononiensi.

A "physical" inequality in the motion, set against the "apparent" inequality that the ancients had interpreted as due to the eccentric position of the Earth with regard to the orbit which transported the Sun with uniform motion, destroyed Aristotelian physics in the skies. The motion of celestial bodies proved not to conform to the laws Aristotle had sought to impose on it.

Using one of the first excellent simple long focal lenses, which rightly made the lens-maker from Spoleto Giuseppe Campani (1636-1715) famous, Cassini discovered the rotation of Jupiter around its own axis in 1665 (64) and that of Mars in 1666 (65). It was thus proved that these planets spun on axes inclined somewhat on the ecliptic, in the same direction and with periods not too dissimilar to the Earth in the Copernican system. He also discovered the periods of rotation of the four "Medicean stars" - which enabled Römer to take the first measurement of the speed of light - and, taking advantage of a simultaneous observation of their eclipse behind Jupiter’s disk from two different points of the Earth, provided a practical way of measuring the geographic longitude.Among the various works of Cassini this last was of such topicality and interest that his fame reached the king of France Louis XIV, who called him to Paris to build the new Royal Observatory, the first stone of which was laid on June 21, 1667 the day of the summer solstice.

In Paris Cassini did some exacting investigative work on the Solar system with very long focal length telescopes, detecting the separation in the rings of Saturn (which still carries his name) and identifying four other satellites, again around Saturn. Bernard Le Bovier de Fontenelle (1657-1757), permanent secretary of the Académie des Sciences in Paris, insisted in his writings on the usefulness of the knowledge that had brought Cassini to Paris - especially the computation of the periods of rotation of Jupiter’s satellites - so as to be able to work out geographic longitudes exactly by means of astronomical observations. The availability of geographical maps that were much more accurate than the previous ones had been brought obvious advantages for shipping and trade.

To accomplish the actual measurement of the longitudes, Cassini made use of and enlarged on his vast network of correspondents. Even the restoration of the meridian line at San Petronio, carried out with Domenico Guglielmini (1655-1710), Superintendent of waters, was aimed at detecting possible variations in the meridians in order to "clear up this point, so essential to Geography and Navigation". A similar programme, as we shall see, was to meet with the approval of Marsili, author, with the help of Johann Christoph Müller, of astronomical observations "in open war and under military hardships", not for recreation purposes, but for carrying out reconnaissance of the territory that might be useful for his military duties: a direct and clear-cut relation this between science and power and one that underlay the protection (and financing) the kings of France and England lavished on the Observatories of Paris and Greenwich.

In Paris Cassini became a leading light in astronomy - and not just French astronomy - even if he was often criticized for his decidedly Cartesian approach that was negative from a theoretical point of view. From a practical point of view, however, the work of the Observatoire was the first instance of collaboration between intellectuals and government pursued by the Académie.

After Cassini left Italy in 1669, the Bolognese Senate formally retained his chair for him in the hope he would return. In the meantime the Astronomy course was entrusted to the Modenese Geminiano Montanari, first-rate practical astronomer, lens-maker, inventor (at the same time as others) of the ocular micrometer - an instrument he used to draw an accurate map of the Moon (66) - and the first person to systematically investigate the variations in light of Algol, a star he observed closely from 1668 to 1677 (67). The research into the light variability of Algol dealt a further blow to the ancient belief in the incorruptibility of the skies.

Montanari is also remembered for the memorable hoax he staged to ridicule astrology (68), a practice which was still officially around in Bologna in the XVIIIth century. It was in fact the Astronomy teacher’s task to compile the Taccuino (an astronomy-astrology almanac). This demonstrates to what extent the institutions lagged behind the ideas of the time: in the final rotulus of the history of the Studio (University), in 1799, we find assigned to the "cittadino dottore" (citizen doctor) Luigi Palcani Caccianemici (1748-1802) the job of compiling the astrological giudizio (judgement) and taccuino for use by the physicians of the city.

Montanari did in fact publish an almanac, Frugnolo degli Influssi del Gran Cacciatore di Lagoscuro ("Hunting Lantern of the Influences of the Great Hunter from Dark Lake") which, produced purposely by chance, appeared to be more accurate in its forecasts than "official" astrology.

In this context Geminiano Montanari is of more interest to us than the afore-mentioned Marcello Malpighi because of his overriding concern with mathematics, astronomy and experimental physics. He took an active part in international networks of scientific information, was an eager propagator of new ideas, and helped organize and promote (as we have seen) the Accademia della Traccia, important not just for the level of scientific debate but for the actual training it gave Montanari’s best students.

Montanari also drew a clear line of metaphysical neutrality, based on a sharp distinction between metaphysics and "natural philosophy", from the experience of the Accademia del Cimento. It may be that a certain degree of prudence informed this attitude, given that Bologna was part of the Church State, though such prudence, exercised by a majority of Bolognese scholars over a long period, was so long-lasting as to suggest a certain degree of personal conviction. We find it again, for example, in the archdeacon Anton Felice Marsili, promoter of another academy concerned with the study of natural philosophy.

The Galilean model was not the only one known of and valued and was often integrated with Baconian philosophy; the ideas that circulated most persistently, in fact, were those of Descartes (1596-1650), with a tendency to focus at one and the same time on reason, mathematics and experiment.

When Geminiano Montanari was summoned to Padua in 1679, the lecturing went on more or less, albeit at a slightly lower level, but astronomical research, in the last two decades of the XVIIth century, was practically nonexistent in Bologna, surviving in only a few private observatories including that of the afore-mentioned marquis Malvasia.

Very few traces remain of this period on account of the destruction of the private observatories and the loss of the astronomers’ personal instruments. One of these traces, in memory of the work of Montanari in Bologna, is the meridian line he designed in the building in via Farini which belonged to the Vassé Pietramellara (today Sassoli de’ Bianchi). In a corridor, with baroque frescoes of celestial divinities, one can see up high a hole in the centre of a shield (or lens), supported by two puttoes. It was this the eye of the meridian line drawn on the floor by Gemininano Montanari twenty years or so after that of San Petronio.

Under the title "Observationes Astronomicae habitae ad Gnomonem Vassaeorum et Cassina~", the Archives of the Department of Astronomy contain (busta III) the records of the observations made with this meridian, between 1674 and 1690, and their comparison with those performed with the San Petronio meridian line.

  1. - L. Lombardo Radice: 1966, Bonaventura Cavalieri. La geometria degli indivisibili, UTET, Torino. G. Vernazza: op. cit.
  2. - G.D. Cassini: 1656, Specimen Observationum Bononiensium ..., Bononiae. Nel 1736 Eustachio Manfredi riunirà tutte le osservazioni fatte con la meridiana di San Petronio in un volume dal titolo De Gnomone Meridiano Bononiensi. A. Cassini: 1994, Gio: Domenico Cassini. Uno scienziato del Seicento, Comune di Perinaldo Ed.
  3. - G.D. Cassini: 1665, Lettere astronomiche all'abate Ottaviano Falconieri sopra la verità delle macchie osservate in Giove, Roma.
  4. - G.D. Cassini: 1666, Martis circa axem proprium revolubilis, Bononiae.
  5. - C. Malvasia: 1662, Ephemerides novissimae motuum coelesium, Mutinae.
  6. - G. Montanari: 1671, Sopra la sparizione d'alcune stelle ed altre novità celesti, Bologna. Vedi anche: F. Bianchini: 1737, Astronomicae ac geographicae observationes selectae..., Verona, p.268.
  7. - M. Cavazza: 1983, La cometa del 1680-81: astrologi ed astronomi a confronto, Studi e Mem. Storia Un. Bologna, Nuova Serie, III, p.409.