Ephemerides Bononienses Mediceorum syderum ex hypothesibus, et tabulis …
Bologna: Emilio Maria Manolessi and brothers, 1668. First edition of this very rare and important work on the motions of Jupiter’s satellites, a work which led to the discovery that light has a finite speed, and whose tables were widely used for determining terrestrial longitude. In fact, Cassini himself was the first to deduce, as a consequence of his observations of the ‘Medicean stars’, that light travels at finite speed, although he later retracted this conclusion and the discovery is now attributed to the Danish astronomer Ole Rømer. “Through his friendship with the famous Roman lensmakers Giuseppe Campani and Eustachio Divini, Cassini, beginning in 1664, was able to obtain from them powerful celestial telescopes of great focal length. He used these instruments – very delicate and extremely accurate for the time – with great skill and made within several years a remarkable series of observations … Cassini likewise worked on establishing tables of movements of the satellites of Jupiter, a task that Galileo had undertaken primarily in order to obtain a solution to the problem of the determination of longitudes. While Galileo was unable fully to develop these tables owing to a lack of sufficiently precise and complete observations, … Cassini succeeded in this enterprise and published in 1668 his Ephemerides Bononienses mediceorem siderum. These ephemerides were employed for several decades by astronomers and navigators” (DSB). “Cassini found an inequality [irregularity] in the motion of Jupiter’s innermost moon (now called Io) which was strongly correlated with the distance between Jupiter and the Earth. Cassini initially hypothesized that this inequality was due to a finite speed of light, but then he rejected this notion. Rømer adopted Cassini’s stepchild, and predicted in September 1676 that the eclipse of Io that was to occur on 9 November of that year would take place 10 minutes later than the time that would be predicted if the ‘equation of light’ was ignored. His paper delivered to the Académie Royale des Sciences on 22 November 1676 was summarized in the issue of 7 December of the Journal des Sçavans” (Van Helden, p. 138). “The timing of the eclipses of the satellites [of Jupiter] varies systematically with the relative positions of the earth, sun, and Jupiter; and Rømer notices that these variations can be accounted for by the varying distance of travel of the light rays. His calculations both require and, for the remaining doubters, help substantiate the Copernican theory of the earth’s revolution around the sun” (Parkinson).Cassini’s tables contain woodcut illustrations showing the positions of the satellites at different times on different days. The introduction has a long history of telescopic observations (Mayr, Galileo, etc.), and also on the making of Cassini’s telescopes. RBH lists only two other copies: Macclesfield (Sotheby’s, June 10, 2004, lot 488, £9,600); and the dedication copy (Christie’s, Paris, June 25, 2004, lot 43, €23,500). OCLC lists Harvard, NYPL, Princeton, and Johns Hopkins in the US. Library Hub lists Science Museum, London, only in UK (not in BL). Provenance: Eighteenth-century ms. initials 'o i s’ on front free endpaper (slight show-through to title page); 19th-century shelfmark label on spine; Garisenda Antiquariato, Bologna (book label on front paste-down); Haskell F. Norman (1915-96), psychiatrist and bibliophile (bookplate on front paste-down), sold at; Sotheby’s, New York, June 16, 1998, lot 353, $8,050; W. P. Watson, Catalogue 10, no. 30, £9,500; Owen Gingerich (1930-2023), astronomer and bibliophile (bookplate on front paste-down). “Immediately after he discovered the four main satellites of Jupiter, Galileo proposed that their motion could be used as a natural clock. In 1692 Jean-Dominique Cassini wrote: ‘It is not by curiosity alone that the most famous astronomers of the present century have observed with so much care the planet Jupiter; they mainly did it in order to obtain an exact knowledge of longitudes, on which the perfection of geography and navigation depends. They estimated that one would have a fast and secure way to determine longitudes, if one could find in the sky some rapid phenomenon which could be observed at the same time from very distant points on the Earth. This being assumed, comparing with each other the times of observations done simultaneously in different locations distant from each other from the East to the West, it would be easy to know by how much one of these places is more to the East than the other; which indicates their difference in longitude.’ “The eclipses of the Jovian satellites thus allowed clocks in different locations to be synchronized. Measuring with clocks synchronized in this way the times of meridian transit of the Sun or of the same star at each location, one obtains by subtraction the difference of longitude of these places after small well-known corrections are made. Prior to this, lunar eclipses were used, but as Cassini noted, ‘... these eclipses are not frequent enough, and they are so difficult to observe that one has not found in this way the longitudes of many places.’ Improvements in instruments allowed easy observations of Jupiter’s satellites, at the very time when Cassini took over the leadership of the Paris Observatory (which was founded in 1667 by the French Academy of Sciences). Cassini continues: ‘This only became possible in 1668, when Mr. Cassini published ephemerides from these satellites, and the method to calculate their eclipses. Since that time, one has performed at the Observatory a large number of observations, together with astronomers of the Academy sent especially by order of the King in all parts of the world, and with other astronomers with whom mail was exchanged; and by the means of these observations one found in the longitudes indicated on all maps a large quantity of errors which have been corrected for.’ “This was obviously of prime importance, so that Bernard le Bouvier de Fontenelle (1657–1757) was able to write: ‘Were there no other use of astronomy than that drawn from Jupiter’s satellites, it would justify well enough these huge calculations, these diligent and scrupulous observations, this large ensemble of instruments built with so much care; [and] this superb building [the Paris Observatory] raised for our science’” (Bobis & Lequeux, pp. 97-98). “It was in 1650 that Cassini, then newly appointed professor of mathematics at the University of Bologna, took up the subject of the tables of the satellites of Jupiter. He examined Galileo's observations and those of Gassendi, which extended to 1645: he thus had at his disposal nearly three periods of the sidereal revolution of Jupiter. The compilation that he undertook convinced him that accurate prediction of the phenomena of the satellites could be achieved. “In 1652, using a copy of Torricelli’s telescope that the Marquis Cornelis Malvasia had had made for him, Cassini began a series of observations that would continue for fifteen years. On the basis of these observations he drew up tables of mean motions, and thence deduced ephemerides predicting the phenomena to be observed. In 1664 in Rome, with the aid of the new telescopes of Giuseppe Campani and his own ephemerides, he was able for the first time to detect the shadows of the satellites, projected by the Sun onto the illuminated disk of Jupiter. Timing of the transits of the shadows made possible a new level of accuracy of prediction. Some contemporaries doubted the existence of these shadows. Partly to convince them, and partly to make known the quality of his researches, Cassini addressed an open letter, dated 22 July 1665, to the Abbé Falconieri, foretelling therein the principal phenomena for the months of August and September of the same year. Observation confirmed his predictions. “In March 1668 were published the Ephemerides Bononienses Mediceorum syderum ex hypothesibus et tabulis Io. Dominici Cassini. Cassini here takes the motions of the satellites to be circular and uniform, but in his introduction he remarks that the second and third satellites are subject to inequalities [deviations from their predicted motions]. Eventually, he believes, these will be accounted for by such theories as Giovanni Alfonso Borelli has developed, using the lunar inequalities as an analogy; in the interim Cassini applies empirical equations that he does not divulge. “Among other subtleties not taken into account in the tables is the fact that, when Jupiter is in quadrature to the Sun [at 90° angular distance], about 1/50 of the face directed toward the Earth is unilluminated, so that a satellite can be occulted before it reaches the illuminated disk. But the tables are sufficiently accurate, Cassini asserts, to serve as a basis for their own further refinement and to make possible good determinations of differences in terrestrial longitudes. “Their accuracy depends crucially on Cassini's discovery of the principles for determining the latitudes of the satellites: the constant inclination of their orbits to the plane of Jupiter’s orbit, at an angle that Cassini takes to be double the inclination of Jupiter’s orbit to the ecliptic (it is actually half a degree larger), with the node moving about 6° per sidereal revolution of Jupiter. For on the latitudes depend the exact times of eclipses. Following the tables Cassini gives ephemerides of the principal phenomena for the year 1668. Throughout he uses for the satellites the names Pallas, Juno, Themis and Ceres. These names would be abandoned in the eighteenth century and replaced by Io, Europa, Ganymede and Callisto, the names proposed by [Simon] Mayr … “Cassini’s tables and ephemerides of Jupiter were to be of great importance for the future development of astronomy in France, for it was the quality of these tables that led to the invitation from Louis XIV to Cassini to come to France, to assist in the planning and operation of the new observatory, the first stone of which had been laid on 21 June 1667. Cassini arrived in Paris in 1669. and was installed in the Observatory some years later. He became the leading astronomer of France in his time, and the first of four Cassinis (the others all his direct descendants) who took leading roles in the astronomy of France. The post of Director General of the Paris Observatory was created for the third Cassini by Louis XV in 1771. “On his arrival in Paris, Cassini was introduced to the king, and joined the Parisian group of astronomers, led by Jean Picard (1620-82) and Auzout, together with the Dutchman Christiaan Huygens (1629-95), who was in Paris from 1665 to 1681. One pressing need was to take advantage of the observations that Tycho Brahe had made at Uraniborg in the previous century, and for this it was necessary to reduce them to the meridian of Paris, by determining the difference in longitude between the two observatories. Cassini and Picard undertook to determine this difference by the method that Cassini had proposed in his Ephemerides of the Medicean stars. “Picard departed for Denmark in July 1671, while Cassini remained in Paris. Observations of the eclipses of the first satellite of Jupiter were made simultaneously in Paris and either at Uraniborg or in Copenhagen, the difference in longitude between the latter two places having already been accurately determined. Picard was assisted by a twenty-six-year-old Dane, Ole Christensen Rømer (1644-1710), who was studying the observations made by Tycho … “From their determination of the difference in longitude between Uraniborg and Paris, Cassini and Picard were able to reduce to the longitude of Paris not only Tycho's observations but also all the observations made during the course of the project of 1671-72. The latter brought to light some notable divergences between the observations and the predictions from Cassini's tables. These divergences could be as high as several minutes of time, and they were inexplicable, given the known precision of the measurements. Rømer, who had been tempted away from Denmark by Picard and was now installed at the Paris Observatory, joined with Cassini in undertaking a detailed comparison between observations spread over nearly ten years and the corresponding predictions derived from Cassini’s tables. Neither of them could admit the possibility of errors of such magnitude in either the tables or the observations” (Débarbat & Wilson, pp. 149-152). With rare exceptions, previous astronomers, both ancient and more recent – including Aristotle, Kepler, and Descartes – had held that light propagated itself instantaneously. Galileo, on the other hand, was not only convinced of its finite velocity, but also designed an experiment (although not an adequate one) by which the speed of light might be measured. These divergent views were discussed among the Paris academicians. A manuscript conserved in the Library of the Paris Observatory shows that, by August 1676, Cassini had ascribed the inequality that had been found to exist in the immersions and emersions of the first satellite of Jupiter to the fact that light travels at finite speed: “‘Inequality of Jupiter’s satellites, by M. Cassini. 22 August 1676 The selected observations of the satellites of Jupiter decided by the Academy five years ago yielded a new prostapheresis [irregularity of motion], the same for all the satellites, which is so important that it could give an error up to a quarter of an hour in the prediction of the eclipses; thus, for example, the next emersion of the first satellite on 16 November will occur about 10 minutes later than predicted by the calculation, which usually derives from the emersions which occurred immediately after the opposition of Jupiter and the Sun in the months of July or August. ‘This irregularity is related to a variation in the visible diameter of Jupiter, or to the distance of Jupiter from the Earth, and it seems to come from the fact that light arrives from the satellites with a delay such that it takes ten or eleven minutes [to cross] a distance equal to the half-diameter of the annual orbit. [our italics]. ‘But the difficulty with this element would make the calculation very intricate if one could not find at the same time a method to build tables in which the true times of the eclipses of any satellite are obtained only from its mean motion and from a single prostapheric table, without help from other tables. ‘This table will contain the inequality of the days or the true motion of the Sun [i.e. the inequality due to the eccentricity of the Earth’s orbit], the eccentric motion of Jupiter [i.e. the inequality due to the eccentricity of the orbit of Jupiter] and this new, not previously detected, inequality. This sort of table will surpass all those in use until now thanks to its shortness, to the ease of its use and to the extent of the data …’ “The first written account of the discovery [of the finite speed of light] is thus undeniably by Cassini” (Bobis & Lequeux, p. 99). However, Cassini then withdrew his support for this discovery on the grounds that the evidence in favour of the speed of light had been found only in the motion of a single satellite, while the other three showed complex irregularities the reasons for which had not yet been determined. “His carefulness explains why he proposed several hypotheses on the same footing: either the delays were due to the finite velocity of light, or they came from other causes, like a variation in the diameter of Jupiter. The possibility of such a variation looks absurd to us, but in Cassini’s time it was not, since nothing was known about the physical nature of the planets. Cassini himself discovered variable spots on Jupiter, and he thought that he saw dark zones on the satellites which made their apparent diameter variable. Cassini’s doubts about the hypothesis of the finite velocity of light are those of an experienced scientist: as claimed by Fontenelle, ‘an hypothesis must account for everything.’ Giacomo Filippo Maraldi I (1665–1729), Cassini’s nephew who also worked at the Paris Observatory, writes: ‘In order for an hypothesis to be accepted, it is not enough that it agrees with some observations, it must also be consistent with the other phenomena’” (ibid., pp. 100-101). “The wider reasons lying behind such a reversal are easy enough to identify. The only reputable astronomer who had previously admitted to thinking that the propagation of light might not be instantaneous was Galileo, and he had never succeeded in producing any evidence to verify his suspicion. In England, Robert Hooke included speculative comments about the speed of light in Micrographia (1665), pointing out that being propagated in the least imaginable time was not the same as being instantaneous and suggesting that eclipse observations hinted at a slower speed; but there is no evidence that anyone, in England or France, paid any serious attention to what would have seemed wild thoughts. For any of the Parisian astronomers to argue that light definitely moved at a finite speed would thus be a very radical departure from the prevailing orthodoxy. This situation helps explain why Cassini and others paid careful attention to the weakness of the evidence, and hence legitimate practical grounds for caution, and why once he had decided against the theory he was to become its most implacable opponent. “The pivotal role played by observations of Jupiter’s satellites on Rømer’s route to Paris may explain why he had the confidence to take up the idea that Cassini had dropped. As a mere assistant, and an imported one at that, he had little to lose by taking risks and perhaps something to gain by pursuing a line independent of Cassini’s” (Willmoth, p. 40). “Beginning from the point at which the earth and Jupiter were closest to each other, Rømer tried to predict the time of occurrence of an eclipse of Io at a later date, when the earth and Jupiter had drawn further apart. In September 1676 he announced to the members of the Academy that the eclipse predicted for 9 November of that year would be ten minutes later than the calculations made from previous eclipses would indicate. Observations confirmed his hypothesis, and Rømer correctly interpreted this phenomenon as being the result of the finite velocity of light. He was thereupon able to report to the Academy that the speed of light was such as to take twenty-two minutes for light to cross the full diameter of the annual orbit of the earth; in other terms, that the light from the sun would reach earth in eleven minutes (a time interval now measured to be about eight minutes and twenty seconds)” (DSB). “On 21 November Rømer read a memoir to the Academy in which he undertook to show that light is not transmitted instantaneously; the record of the meeting shows that Rømer was to confer with Cassini and Picard about publishing the memoir in the Journal des Sçavans. By this time Cassini had decided that the anomaly was due, not to the finite velocity of light, but to an actual inequality in the movement of the satellite. A week later “there was a discussion of the immersions and emersions of the first satellite of Jupiter … It was thought relevant that M. Cassini should give in writing the arguments that he proposed, and that M. Rømer should respond to them”. The following week Cassini duly expounded his arguments to the Academy. Two days later, on 7 December, the Journal des Sçavans carried Römer's memoir, ‘Démonstration touchant le mouvement de la lumière trouvé par M. Rømer’. There is no suggestion here that the explanation had been advanced earlier by Cassini” (Débarbat & Wilson, pp. 152-153). Parkinson p 106; Riccardi I 279; Macclesfield 488; Norman 411 (this copy). Bobis & Lequeux, ‘Cassini, Rømer and the velocity of light,’ Journal of Astronomical History and Heritage 11 (2008), pp. 97-105. Débarbat & Wilson, ‘The Galilean satellites of Jupiter from Galileo to Cassini, Rømer and Bradley,’ pp. 144-157 in: Taton & Wilson (eds.), Planetary Astronomy from the Renaissance to the Rise of Astrophysics, Part A: Tycho Brahe to Newton, 2008. Van Helden, ‘Rømer’s Speed of Light,’ Journal for the History of Astronomy 14 (1983), pp. 137-141. Willmoth, ‘Rømer, Flamsteed, Cassini and the speed of light,’ Centaurus 54 (2012), pp. 39-57.
Folio (278 x 196 mm), pp. [xxvi], 10, 48, with woodcut illustrations of Jupiter and the positions of its satellites in text (some occasional spotting). Contemporary limp vellum, gilt panels on sides. A very attractive copy.
Item #6519
Price: $28,000.00
