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De Zeventiende Eeuw. Jaargang 12 (1996)

De Zeventiende Eeuw. Jaargang 12

(1996)– [tijdschrift] Zeventiende Eeuw, De–rechtenstatus Auteursrechtelijk beschermd

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Contrasting careers in astronomy: Huygens and Cassini
Albert van Helden

Much has been written about the times in which Christiaan Huygens lived and the research he did. I would like to examine his career in a slightly different light: the question of how one went about building a career in science in the middle decades of the 17th century. And also, I want to make some remarks about Huygens's activities in astronomy and how those compared to those of other astronomers. Specifically, I want to compare Huygens with Giandomenico Cassini (1625-1712), an almost exact contemporary.

First, how did one make a career in science in the 17th century and how did Huygens's background limit or expand the possibilities? The simple answer is that just as there was no science in our professional sense, there was no easy institutional career path for which one could opt. Huygens was taught mostly at home and for the most part he was an autodidact in science. His family had achieved a high social position in the service of the house of Orange, but just when Christiaan had reached the age at which he might have been expected to enter that service, his early twenties, William II died and the United Provinces decided to do without a stadholder. This situation lasted until 1672, by which time Huygens was in his early forties.1.

But the question was probably moot at any rate, for it appears that Christiaan only reluctantly performed secretarial and diplomatic tasks for which his father groomed him and would have refused a career in this kind of service: it bored him to tears.2. What were the options for a person in his position? First, one could obtain an appointment at a university or even a Latin school. To Huygens, the salary would have been almost negligible and the lecturing not to his liking. More importantly, the Huygens family was very wealthy and had entered the ranks of the nobility: a university career was socially beneath them. It appears that Christiaan never considered this option.

Outside the university, there were few choices. The church was not one of these for Huygens as it was for so many poor boys in Catholic lands. Scholars of modest means were basically at the mercy of patrons. Wealthy scholars had no need for preferment. Among Huygens's correspondents we find a number of examples: Ismael Boulliau was the secretary of a diplomat; Frans van Schooten was a professor of mathematics at Leiden; both were of modest means. Sir Robert Moray was independently wealthy, and so was René Descartes, who visited the Huygens home during Christiaan's youth.

High patronage or the life of an independent scholar were Huygens's only options. The model in this respect, as he was in so much of Huygens's research, was Galileo.3. In northern Europe, however, the high-patronage scene was dismal. Queen Christina of Sweden, who had given an appointment to Descartes, had abdicated and moved to Rome; in England, where the Crown had earlier offered op-

portunities to artists and engineers from the Low Countries (Van Dyck, Drebbel), there was no royal court during the Protectorate, and even with the Restoration the patronage of science was financially very limited. France was in turmoil until the ascendancy of Louis XIV, and in Holland the house of Orange sat on the sidelines from 1651 to 1672.

Looking farther afield, there was, of course, no hope for a Protestant to gain the favor of the Pope or a cardinal in Rome, but in Florence Grand Duke Ferdinand II and his brother Leopold had started the Accademia del Cimento. Huygens dedicated his Systema Saturnium of 1659 to Prince Leopold. This was entirely appropriate, for the book took up where Galileo left off in his celestial discoveries. A fruitful scientific correspondence between Huygens and Prince Leopold ensued, and it can be said that Huygens thus became a client of the Prince.4. That was, however, as far as it went.

For Huygens, the only real choice for status advancement was high patronage. And he achieved that goal; when, in his mid-thirties, he was offered a position at the French court, as the intellectual centerpiece of the newly founded Académie Royale des Sciences.5. For someone who specialized in natural philosophy/ medicine/mathematics, this was the richest prize in Europe, and there is little question that at that time Huygens was considered the finest scientific mind in Europe.

But getting there was one thing: making a success of it once there was perhaps something else, and here Huygens's personality came into play. In the currency of the patronage economy, novelty - understandable novelty - was highly prized. Huygens had done what Galileo had done: he had improved the telescope, discovered a satellite with it, and solved, by a breathtakingly elegant explanation, the puzzle of Saturn's appearances - first discovered by Galileo.6. He had also made a revolutionary new clock that improved the accuracy of timepieces by orders of magnitude.7. These were novelties that were spectacular and reached out to a wide audience. The inventor of these things belonged in the court of the Sun King. In that respect, Huygens was to the French Court what Galileo had been to the Tuscan Court.

But, as Galileo's career shows, the scientist had to continue producing novelties: it was a competitive environment. Galileo was rather good at it, although in the end he overreached himself. I would argue that Huygens was not so good at this and lost out in the competition. This loss came at the hands of Giovandomenico Cassini, whose career provides an interesting contrast to Huygens's. Let me therefore briefly sketch Cassini's career.

Cassini was born on 8 June 1625 into a family of relatively modest means in Perinaldo, a mountain commune overlooking the Ligurian Sea, halfway between Ventimiglia and San Remo. In sharp contrast to Huygens's education, Cassini's was entirely formal. After elementary school, he attended a Latin school in Vallebona for two years and then entered the Jesuit college in Genoa. Here he was educated following the ratio studiorum promulgated in 1599, for which the famous Christopher Clavius had laid down the mathematical requirements. Cassini's intellectual prowess quickly brought him to the attention of powerful churchmen: in 1646, he gave a lecture on theological and philosophical themes before Cardinal Durazzo, archbishop of Genoa and an alumnus of the college. Cassini learned to compose

verse in Latin and Italian, something he did for the rest of his life. But his talent lay in the mathematical subjects.8.

Cassini was not independently wealthy and had to make his living by his wits. He therefore assiduously acquired the manners of the courtier and was very successful in this role. He became close friends with Francesco Maria Imperiali-Lercaro, the scion of a powerful Genoese family, and it appears that he circulated among the Genoese elite and taught mathematics to a number of their children. It also appears that his initial reputation was in astrology. But astrology turned him to the serious study of astronomy, and he was encouraged in this endeavor by the Genoese patrician Giambattista Baliani, better known for his studies of the atmosphere and vacuum. It was Baliani who gave Cassini his first astronomical instrument. Cassini's prowess in astronomy and astrology brought him to the attention of Cornelio Malvasia, a powerful patron in Bologna, and the result was that in 1651, at the age of 26, Cassini was appointed to the chair of astronomy at the University of Bologna.9. How did a 26-year-old courtier who had never published anything become professor of astronomy at this famous university? The answer is clearly that Cassini was the client of powerful patrons and was a consummate player of the patronage game. Having obtained the position, he made the most of it.

In his application for the position, Cassini promised that, besides reading the usual traditional astronomical text to his students, he would make astronomical observations and take up again the long discontinued task of publishing annual almanacs.10. He made good on most of his promises in a very spectacular fashion. When a comet appeared at Christmas 1652, he made observations and published a little book on the subject within a few weeks.11. Here we see a marked contrast between Huygens and Cassini: whereas Huygens was usually slow to publish his discoveries and much of his scientific work remained unpublished at his death, Cassini always published his results quickly and left virtually no unpublished works at his death.

Having studied the Almagestum Novum (1651) of Father Giovanbattista Riccioli, S.J., a professor at the Jesuit college in Bologna, and having familiarized himself with the most important works of Johannes Kepler, Cassini decided to attack a very technical problem that lay at the heart of the struggle to make astronomical predictions more accurate: the relationship between solar parallax and atmospheric refraction. The former correction is due to the fact that the Earth cannot quite be considered a geometrical point when it comes to observing bodies within the solar system, and the latter correction is made necessary by the fact that light, when it enters the Earth's atmosphere, is bent toward the vertical, thus making objects appear higher in the sky than they in fact are.12. In terms of patronage economy this was a very recondite research subject with little hope of producing spectacular novelties. Cassini, however, managed to combine the routine with the spectacular.

With the help of his patrons, he convinced the appropriate authorities to allow him to rebuild an old meridian that had been laid down in the cathedral of San Petronio by Ignazio Danti eight decades earlier. Invitations were sent out for the laying of the first stone of the meridian line at the summer solstice of 1655 and for the observation of the autumn equinox that same year. Citizens could examine the meridian for themselves, and it quickly became the tourist attraction it still is today. When Queen Christina of Sweden visited Bologna, on her way to Rome, Cassini

published a broadsheet dedicating the instrument to her. A cathedral had been turned into an astronomical measuring instrument and measurement had become a public spectacle.13.

With the meridian, Cassini measured solar altitudes and compared them with his measurement of the altitude of the Pole. He found a consistent discrepancy of 2 arc-minutes in the resulting obliquity of the ecliptic if he used old-fashioned corrections for parallax and refraction. As a result he devised better corrections, which made the difference disappear. This was a fundamental contribution to astronomy and it lay at the heart of the improvement of predictive tables in the second half of the seventeenth century. Cassini published new solar tables and new tables of refraction and parallax in 1662.14. Important astronomical improvements resulted from the most public and spectacular observations.

Cassini was asked to lend his mathematical expertise in the search for solutions to two important non-astronomical problems: fortifications and water management. The first concerned a fort begun under Pope Urban VIII, called the Forte Urbano, and the second had to do with the changing courses of the Po and its tributary, the Reno, which had caused several sorts of disputes between Bologna and Ferrara. Through this work, Cassini became a consultant to the commune of Bologna and to the Papacy. As a result, he traveled widely and spent much time in Rome. The Pope also asked him to survey the water course of the Chiane, on the boundary between the Papal States and Tuscany, whose erratic flow affected both the Tiber and the Arno.15.

While in Rome, in 1664, Cassini met the world's most famous telescope maker, Eustachio Divini, who had recently been in a controversy with Huygens over the quality of their respective telescopes. He also met Giuseppe Campani, who was then challenging Divini's supremacy in a series of carefully staged confrontations - paragoni - in which Campani came out the clear winner. Campani gave Cassini access to his telescopes - a mutually beneficial move. Cassini had been observing the satellites of Jupiter in the hope of being able to make useful predictive tables: the Campani instruments allowed him to make better observations. The instruments, the best in the world, also allowed him to make celestial discoveries.16.

In telescopic astronomy, Huygens had a ten-year head-start on Cassini, but the Italian quickly made up the time. In 1665 and 1666 he published a series of tracts in which he described first seeing the shadows of Jupiter's satellites on the planet's disk, then a permanent spot on the planet itself which allowed him to deduce its rotation period, and finally the same for the planet Mars.17. Now Huygens's observations of Mars, dating from 1659, were actually better than those published by Cassini in 1666, but Huygens had not published his, except for a very vague and inferior figure in Systema Saturnium of 1659. Again, the publication habits of the two men were very different and to Huygens's disadvantage.

In 1668, Cassini published his tables of the motions of Jupiter's satellites.18. This was an important achievement, for it held out hope of solving the problem of determining longitude. Cassini's tables were so accurate that they eclipsed every earlier effort and were the foundation of the geodesic revolution that began over the next several decades.19. At the age of 43, then, Cassini had a number of achievements to his credit. He had turned astronomical observation into a public spectacle and produced major improvements in solar theory; he had made a number of spec-

tacular discoveries concerning the planets that made everyone compare him to Galileo; and he had produced astronomical tables that were fundamental for the improvement of astronomy, geodesy, and navigation. As the foundations of a royal observatory were being laid in Paris, there was little doubt as to who would be the most suitable person to take charge of it. Colbert wanted Cassini.

This was, however, a ticklish situation. Cassini was, after all an employee of the Pope and the commune of Bologna of whom the Pope was the overlord. Colbert made it known to the Pope that ‘His Most Christian Majesty need[ed] Cassini in France for some time,’20. and after delicate negotiations at the highest diplomatic levels, His Holiness acquiesced and also ordered the commune of Bologna to give Cassini leave from his teaching duties. Indeed, Bologna was ordered to allow Cassini to keep his chair and to continue paying him his salary in his absence. The commune rebelled at this last requirement because it flew in the face of centuries of well-founded regulations governing the absence of professors.21.

When it came to Cassini's pension at the French Academy, the situation was also a bit ticklish. Cassini was the highest paid professor at Bologna by far, earning 3,800 Bolognese lire, more than three times as much as his colleague Marcello Malpighi. It was, in fact, an astounding salary for the university. Moreover, with the emoluments he was receiving for his services for the commune and the Pope in fortifications and water control, his total income was 8,000 lire, not counting incidentals. As a result, Colbert set Cassini's stipend at 9,000 French livres, while Huygens's stipend was 6,000 livres.22.

Cassini's entry into the French Academy was by no means smooth. Since there was no room for him at the Bibliothèque du Roy, where the other academicians, including Huygens, were housed, he was put up in the Louvre. But there he could not make astronomical observations and he therefore rented a private house on the outskirts of the city. The ground floor of the observatory, designed by Claude Perrault, had been finished, and Cassini did not like what he saw: the building, he argued, was not well designed for astronomical observations. Over the protests of Perrault, some modifications were made to the design of the higher stories: Cassini was making his presence known. He moved into the building before it was finished, and he spent the next four decades there.23.

Cassini continued to make important contributions to astronomy in Paris. He took the lead in providing guidance in astronomical matters to expeditions to faroff regions, resulting in a definite verification of his modifications of certain astronomical parameters.24. He instituted a program of regular astronomical observations which quickly made the Paris Observatory the most important in the world. But he also paid attention to the more glamorous aspects of astronomy. With Campani telescopes he discovered four satellites of Saturn and the division in Saturn's ring that is still named after him. When the longer and longer telescopes could no longer be accommodated in and around the observatory building, he had the water tower of Marly moved (in one piece) to the observatory grounds to serve as a tower for attaching objective lenses of ‘aerial’ telescopes: once again, astronomical observation had become a spectator sport. All of Paris shared in the wonder of the new science. Further, Cassini was so successful as an organizer, that he was able to found a dynasty. His own heirs and the offspring of his sister ran the observatory until the French Revolution.

Cassini was also interested in affairs of state, namely geodesy. Jean Picard had already begun this effort on the eve of Cassini's arrival, but Cassini organized the effort on a grand scale. First, Picard was sent on an expedition to Denmark in order to determine the position of Tycho Brahe's now derelict observatory with respect to Paris. This was fundamental, for Tycho's notebooks were still the largest body of accurate observational data. Then the latitudes and longitudes of every French town and village were determined. This resulted in a preliminary new map of France in the 1680s, on which the Atlantic coast was moved to the east by a full degree and the Mediterranean coast was moved by more than half a degree. It is reported that when Louis XIV saw this map, superimposed on the best previous map, he said that he was losing more territory to his astronomers than to his enemies. The completely new map of France was not finished until half a century after Cassini's death, but it was his project.25.

The effort had its parallel in the organization to collect longitudes and latitudes of faraway places. Here Cassini availed himself of, for example, Jesuit missionaries to the Far East. As a result, the longitudes of China and other places in East Asia were finally brought within reasonably accurate bounds. Often the longitude of a place in the Far East turned out to be in error by as much as 25o on existing maps. On the floor of the first story of the Observatoire, Cassini kept a large world map in polar projection that was not permanent. Its outlines were constantly changed as new information came in.26.

Cassini never particularly expressed himself on the merits of the Copernican theory. Moreover, he (and his descendants) were in error on several more theoretical issues (e.g., the shape of the Earth), and for that reason enlightenment historians such as Jean-Baptiste Delambre expressed disdain for Cassini's work.27. This disdain and the fact that he was half Italian and half French have meant that Cassini has, on the whole, been ignored by historians of science. That is now beginning to change. It can be said with confidence that Cassini was a major figure in the revolution in astronomy that occurred in the seventeenth century. But how do we compare him with Huygens? I will begin with an example.

Cassini's 1668 tables of the motions of Jupiter's satellites were most accurate for Io, the satellite closest to the planet, which circles the planet in 42 hours and whose eclipses therefore serve as a very useful celestial clock. The vast majority of observations of such satellite eclipses, as they enter and exit Jupiter's shadow cone, were for Io. In 1676 the Danish astronomer Ole Rømer discovered a systematic error of about 20 minutes in the predictions of the eclipses of this satellite. The period of this error was the same as the synodic period (opposition to opposition) of Jupiter. Rømer drew from this the conclusion that light travels with finite speed and that when Jupiter is farther away the light reflected by its satellites takes longer to reach us than when Jupiter is closer to us. This was a wonderful answer to a question which came directly out of the work of Descartes, who in one treatment considered light as an instantaneously transmitted pressure and in another as a stream of particles traveling with a finite speed. One now actually had an idea of what the speed of light was. Huygens supported Rømer in this explanation, and it was rapidly accepted by most astronomers and physicists, but not by Cassini.28.

The leap to the cosmic question is important here. Cassini stuck to the observations and argued that if this interpretation were true, then the effect ought to be

seen in the tables of the other three satellites as well. Since this was not the case, Rømer's explanation could not be correct. Now, we can sympathize with Cassini, for his tables for the other three satellites were just not sufficiently accurate to show the effect. But he went on to turn the time correction of the first satellite into its geometrical equivalent and then applied it to the other three satellites. In the case of Io it made no difference of course, but in the cases of Europa, Ganymede, and Callisto it led to ridiculous errors, and when Cassini published revised tables with (among other things) this new correction, in 1693, Edmond Halley pilloried him for this error in his review in the Philosophical Transactions.29.

Cassini was above all an observer and organizer. To us, who are used to well organized observatories, accurate tables, and accurate maps, his achievements have tended to become invisible. We remember him chiefly for his spectacular discoveries of satellites, the rotation periods of Jupiter and Mars, and the division in Saturn's ring. He found these not by accident but by constant application. As Professor Andriesse has so aptly put it, ‘Cassini was een jager, Christiaan ving by geluk.’30. A few years after Cassini's arrival in Paris, Huygens remarked to his brother Constantijn that Cassini was at the telescope every clear night and that he [Huygens] would never want to do that, being satisfied with his earlier astronomical discoveries which, at any rate, were much more important than those of Cassini's.31.

The petulance of the last part of this statement is understandable for someone who sees himself overshadowed by a competitor. The first part is more important for it gives us an insight in Huygens's character. It was not astronomy per se that interested him. In this respect, to use an anachronism, Cassini was a professional and Huygens a dilettante. Huygens turned to Saturn because the planet's appearances presented a celebrated puzzle that had defeated Galileo and everyone else: he solved it in a year. And he solved it by thinking big, by invoking the Cartesian vortices and mathematical notions of symmetry, turning a two-dimensional gestalt into a three-dimensional one.32. And Systema Saturnium (1659), his book about the subject, was more than an explanation of his ring-theory: it was a book about the solar system, about harmony and cosmology. For Huygens, the particulars of Saturn's ring confirmed certain analogies between the Earth and the other planets.

This was a cosmic question that is usually ignored. But let us consider it for a moment. In the old cosmology of Aristotle, the Earth had been absolutely different from heavenly bodies in just about every respect, and when the followers of Copernicus thought of the Earth as a planet, a fundamental question arose: to what extent is the Earth like the other planets? If the Earth turns on an axis inclined to its orbital planet, then is that also true for other planets and at the same inclination? If the Earth has inhabitants, do the other planets? Is there a harmonic relationship that governs the sizes of the planets? If their periods are linked to their heliocentric distances by a mathematical function, is this true for their sizes as well? Huygens was thus reaching here into the very depths of Copernican ontology: what does it mean to say that the Earth is a planet? These are the sorts of questions that Cassini never raised in his publications, and as far as I can tell never asked himself.

When we come to the telescope, Huygens and his brother Constantijn continued their efforts to build more and more powerful instruments. In this they were at a disadvantage because they were dilettantes: Eustachio Divini and Giuseppe Cam-

pani made their living doing this, and it is clear that by the middle 1660s Huygens knew that he was beaten at this game. But for Huygens, ever the mathematician, what was really interesting about the telescope was the optics, and here no one could touch him. Both he and Campani invented compound eyepieces that resulted in a clearer image and a flatter field, but Huygens was the only one in Europe who could begin to explain by geometrical analysis why those particular configurations improved the image.33. To him, this was what was exciting about telescopes, and we can say the same about clocks. No one in the seventeenth century was oblivious to the fame and perhaps fortune that a better telescope or clock, or any fashionable device, could bring, but that is quite different from the device's intrinsic interest, and it was always there that Huygens went in his studies.

His mathematical constructions had to be perfect and elegant, as measured against the highest standards of the ancients (modified only slightly by Descartes). If his results are what strikes us, to him the method was equally important: he had to please himself. And pleasing himself meant holding back what was not in every respect a perfect and complete mathematical construction or proof. Hence Huygens published little. Cassini, in sharp distinction, was never bothered by such considerations: to him a result was a result and he had no sooner made the discovery or worked out a result than he published it. In the long run, Huygens's approach did not fit well into the patronage scheme, and as time went on his position at the French Academy, compared to Cassini's, became weaker and weaker as a result.

Huygens was no great organizer, a supervisor of assistants, a teacher of what we might call graduate students. Intellectually he was a loner, a scientist who was happiest in his study with pen, paper, and a few instruments. There has, of course, always been a need for such scientists. To make a public career in science in the seventeenth century, however, one had to sustain a public posture, and I think that this was uncomfortable for Huygens all his life. The public life took away too much energy from his beloved private study. And to be a drudge at the eyepiece of a telescope every night for one's entire life was anathema. When he was on the trail of something important, he observed assiduously, but after he had satisfied himself he stopped and turned back to mathematical studies, always the center of his interest.

It is, in the end, difficult to compare Huygens's contributions to astronomy to those of Cassini. Was Cassini's systematic work to increase the precision of solar theory matched by Huygens's development of the micrometer? Was Cassini's organizational genius that established the procedures of modern observatories matched by Huygens's invention of the pendulum clock which eventually led to a revolution in the productivity of those observatories? We ultimately come back to the point at which we started. Making a career for oneself as a scientist in the seventeenth century was a difficult proposition. And it was especially difficult for a ‘solitary genius’ such as Huygens. In a way, then, Huygens was both lucky and successful: lucky because he was born to wealth and status, successful because he was recognized by 1660 as the finest scientific mind in Europe and chosen to be the central figure in the newly founded French Academy. We must be careful not to use too much retrospect in evaluating him, knowing that just as he was being installed in the Bibliothèque du Roy a young Englishman was having an annus mirabilis under an apple tree in Lincolnshire. Nor must we judge him by the standards of

professionalized astronomy of the 19th and 20th century. To Huygens, the heavens and the instruments used to study them posed interesting problems of which he stayed abreast and which from time to time occupied him totally. But they were always part of a larger quest after a deeper mathematical understanding of nature.


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