Rome: Fabio di Falco, 1665.
First editions of these two exceptionally rare publications on the comet of 1664-5, which was observed my many astronomers, including Auzout, Borelli, Fabri, Hooke, Hevelius, Petit, and Newton as a studnt. The second work is also especially notable for containing the first published description of Jupiter’s Great Red Spot. Cassini observed the comet “in the presence of Queen Christina [to whom the first work is dedicated] and formulated on this occasion a new theory (in agreement with the Tychonian system) in which the orbit of the comet is a great circle whose center is situated in the direction of Sirius and whose perigee is beyond the orbit of Saturn” (DSB). The large engraved plate depicts the course of the comet in the southern celestial hemisphere from December 13, 1664 through the middle of January, 1665; it also shows the appearance and direction of the comet’s tail. Cassini’s detailed observations were made with a powerful new telescope which he describes in the preface to the first work. “Through his friendship with the famous Roman lens-makers 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…” (ibid). OCLC lists Brown (lacking plate) for the first work, and Brown, Cornell, Ohio State for the second. ABPC/RBH lists one copy of Theoriae, bound with Lettere (Bolaffi, 2014, EUR 21,250, modern binding), and the Macclesfield copy of Lettere alone (Sotheby’s, 2004, £4800, 18th century boards).
“During the early 1660s, Cassini often found himself in Rome overseeing engineering projects for the Papal court, where he took the opportunity in 1664 to make observations of the movements of the comet and publish those observations along with some of his astronomical beliefs. In a letter addressed to Leopoldo, Ottavio Falconeri wrote about the arguments Cassini was hoping to establish in this text. According to Falconieri, Cassini’s theory concerned with the motion of comets was aimed at demonstrating that they ‘did not move in a straight line perpendicular to the surface of the earth, but along the plane of the greatest circle [beyond the orbit of Saturn]’ around the sun, which is itself orbiting the stationary earth. More specifically, as he described in his published works on the topic [the offered works], Cassini believed that the 1664 comet travelled in epicycles around the distant bright star Sirius. While that star orbits the earth. In other words, he believed the comet to be moving around the earth, not the sun, as Falconieri had intimated in his letter to Leopoldo. Additionally, he clearly denied that the rapid movement of the comet when in opposition to the sun could be used by Copernicans as proof of the mobility of the earth, since, according to Cassini, such motion could also be explained within a Tychonic geocentric system.
“Therefore, Cassini was proposing a theory that dismissed Galileo’s claim about the rectilinear path of comets emanating from vapours in the earth’s atmosphere. Furthermore, by placing the comet amongst the sphere of stars, Cassini was proposing a radical departure from most theories since the late sixteenth century on the location and movements of comets. Nevertheless, he still maintained a finite geocentric and geostatic model with circular motion, consistent with Tychonic astronomy” (Boschiero, pp. 223-224).
The first observation of Jupiter’s Red Spot has sometimes been ascribed to Robert Hooke in 1664, but Falorni has shown that what Hooke observed was almost certainly the shadow of one of Jupiter’s moons. “As far as Cassini is concerned, it is beyond doubt that he repeatedly observed a spot quite like our modern Red Spot. It seems likely that his first observations were made at Cittá della Pieve, between the summer and the autumn of 1665; a full report was published the same year in the form of letters directed to the Abbot Falconieri [i.e., the present Lettere]. Cassini’s interest in the spot concerned its use in determining the planet’s rotation period, which at the time was unproven …
“First he took care to single out – by means of computing – which spots were caused by the transit of a satellite or a satellite’s shadow on the planet’s disc. Secondly, Cassini demonstrated that the remaining observable spots had to be located on the true surface of the planet. Among these latter, he finally recognized a spot that was exceptionally conspicuous and permanent, and proved ideal for determining a highly reliable rotation period.
““To that first light of distinction then followed the other of detecting amoing the number of the other spots a permanent one which was often seen to return in the same place with the same size and shape. It is the same spot that Yr. Ecc. Was able to see just touching the real northern edge of that belt of Jupiter which, among the three obscurer ones, lies more southerly. That one, which among the spots hitherto observed is the greatest, the most conspicuous and the more permanent … appeared to be different in colour, not so dark and black [as the shadows], but quite like that of the obscure belts … different in figure as being, when nearer to the centre, larger in accordance with the line of the belt which it grazes, or narrower when nearer to the circumference” (Lettere, p. 3). With these words Cassini described for the first time Jupiter’s Great Red Spot …
“Of all the Spot’s distinctive features, Cassini missed only its red colour, but it is out of the question that he would have been able to distinguish it because of the low light-grasp of telescopes of that time” (Falorni, p. 217).
Queen Christina’s “interest in comets is an instance of the change of ideas from the Renaissance to the Enlightenment. Millenial and messianic predictions associated with comets that were then current in Protestant circles may have influenced her decision to abdicate from the throne of Sweden and her choice of 1654 to do so. The later comet of 1664 carried similar associations and Cassini thought it might be a return of Tycho's new star of 1572” (Cook, p. 122). Cassini had been a member of Christina’s circle since her arrival in Rome in 1655, and had dedicated his Specimen observationum Bononiensium (1656) to her. Falconieri was also a member of her circle in Rome.
Cassini (1625-1712) was born in Perinaldo, Republic of Genoa. “Cassini’s early studies were principally observations of the Sun, but after he obtained more powerful telescopes, he turned his attention to the planets. He was the first to observe the shadows of Jupiter’s satellites as they passed between that planet and the Sun. His observation of spots on the surface of the planet allowed him to measure Jupiter’s rotational period. In 1666, after similar observations of Mars, he found the value of 24 hours 40 minutes for Mars’s rotational period; it is now given as 24 hours 37 minutes 22.66 seconds. Two years later he compiled a table of the positions of Jupiter’s satellites that was used in 1675 by the Danish astronomer Ole Romer to establish that the speed of light is finite. In addition, he wrote several memoirs on flood control, and he experimented extensively in applied hydraulics.
“Hearing of Cassini’s discoveries and work, King Louis XIV of France invited him to Paris in 1669 to join the recently formed Academie des Sciences. Cassini assumed the directorship of the Observatoire de Paris after it was completed in 1671, and two years later he became a French citizen.
“Continuing the studies begun in Italy, Cassini discovered the Saturnian satellites Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684). He also discovered the flattening of Jupiter at its poles (a consequence of its rotation on its axis). In 1672, as part of a concerted effort to determine the size of the solar system more accurately, Cassini sent his colleague, Jean Richer, to South America so that roughly simultaneous measurements of the position of Mars could be made at Paris and Cayenne, French Guyana, leading to a better value for the Martian parallax and, indirectly, for the distance of the Sun. Between 1671 and 1679 Cassini made observations of the Moon, compiling a large map, which he presented to the Académie. In 1675 he discovered the Cassini Division and expressed the opinion that Saturn’s rings were swarms of tiny moonlets too small to be seen individually, an opinion that has been substantiated. In 1683, after a careful study of the zodiacal light, he concluded that it was of cosmic origin and not a meteorological phenomenon, as some proposed.
“In 1683 Cassini began the measurement of the arc of the meridian (longitude line) through Paris. From the results, he concluded that Earth is somewhat elongated (it is actually somewhat flattened at the poles). A traditionalist, he accepted the solar theory of Nicolaus Copernicus within limits, but he rejected the theory of Johannes Kepler that planets travel in ellipses and proposed that their paths were certain curved ovals, which came to be known as Cassinians, or ovals of Cassini. Although Cassini resisted new theories and ideas, his discoveries and observations unquestionably place him among the most important astronomers of the 17th and 18th centuries” (Britannica).[Theoriae:] Lalande, p. 261; Riccardi, I 276. J.J. Le F. de. Bib. astronomique, p. 261. [Lettere:] Riccardi I, 277. Boschiero, Experiment and Natural Philosophy in Seventeenth-Century Tuscany, 2007; Cook, Edmond Halley, 1998; Falorni, ‘The discovery of the Great Red Spot of Jupiter,’ Journal of the British Astronomical Association, vol. 97 (1987), pp. 215-219.
Two vols in one, folio (286 x 210 mm), pp [iv] 60 [recte 62]  22 , with woodcut diagrams and 1 large folding enrgaved plate in first work; contemporary carta rustica, end papers renewed, some leaves with a little damp staining, a few small worm holes mostly to margin, in all a fine copy of this highly rare work.