GASSENDI, Pierre.

Mercurius in sole visus, et Venus invisa Parisiis, anno 1631. Pro voto, & Admonitione Keppleri. Cujus heic sunt ea de re epistola duae, Cum Observatis quibusdam aliis. [With:] SCHICKARD, Wilhelm. Pars Responsi ad Epistolas P. Gassendi de Mercurio sub Sole Viso.

Paris; Tubingen: Sébastien Cramoisy; Typis Theodorici Werlini; impensis Philiberti Brunni, 1632; 1632.

First edition, very rare, of Gassendi’s work containing the description of the first observation of the transit of Mercury across the Sun. It was addressed to Wilhelm Schickard, a professor of astronomy at Tübingen, and we include the first edition of Schickard’s reply. The importance of Gassendi’s observation was that it supplied for the first time an indisputable quantitative measure of the apparent diameter of a planetary disc. The fact that it was an order of magnitude smaller than predicted by Tycho Brahe and Johannes Kepler meant that Earth must be much farther from the Sun than previously thought. This was the first indication of the true scale of the solar system. “Kepler many years before had stated that Mercury would cross the sun's disc on May 20, 1607, and carefully but vainly watched for it. With the completion of the Rudolphine Tables in 1627, he again took up the problem of determining the times of transit, and in a small tract published in 1629, entitled Admonitio ad Astronomos rerumque celestium studiosos, de miris rarisque anni 1631 phaenomenis …, announced that Mercury would pass over the sun's disc on Nov. 7, 1631, and Venus on Dec. 6, 1631” (Stanford Encyclopedia of Philosophy). “Kepler had advised [astronomers] to project the image of the Sun on paper by means of a telescope, or by means of a simple camera obscura if they did not have a proper telescopic setup. For a number of reasons only a few men in Europe succeeded in observing the transit of Mercury. Most were foiled by the weather … Out of all this effort came only one published and usable observations of the transit of Mercury of 1631, that made by Pierre Gassendi. Gassendi was in Paris at the time, and when he read Kepler's admonition he began making preparations for the observations. The apparatus he used for observing sunspots from time to time would obviously suit his purpose best. Gassendi used a darkened room in which the image of the Sun was admitted through a simple Galilean telescope and projected onto a piece of paper” (van Helden, ‘The Importance of the Transit of Mercury of 1631,’ Journal for the History of Astronomy 7 (1976), pp. 3-4). “The great triumph of Gassendi's scanning of the skies was his observation of Mercury's transit before the Sun (1631), the first such recorded observation and a confirmation of Kepler's prediction of the planetary orbits in accordance with the Three Laws. This confirmation in turn enabled the subsequent calculations of the distance between the Earth, the Sun, and the other planets” (SEP). Schickard himself could not believe that Mercury could be as small as Gassendi had seen it, a third of a minute of arc in diameter, and tried to explain how the planet could be as large as everyone had thought it to be and could yet appear so small. In his Response to the Letter of P. Gassendi, he tried by various optical arguments to preserve for Mercury a diameter of at least one minute of arc. Mercury’s transit was observed at about the same time by Johannes Remus Quietanus and Johann Baptist Cysat, but neither published their observations. RBH lists only one other copy of Gassendi’s work (also in a modern binding), and none of Schickard’s since 1941.

“Of all the planets known in the seventeenth century, Mercury was by far the most difficult to observe. It was very small yet very bright, and it is never farther than about 23° from the Sun. While in 1610 Galileo had verified the phases of Venus, he was never able to do the same for the phases of Mercury, nor was he able to make a telescopic estimate of its angular diameter. In his Dialogue he wrote:

‘In Mercury no observations of importance can be made, since it does not allow itself to be seen except at its maximum angles with the sun, in which the inequalities of its distances from the earth are imperceptible. Hence such differences are unobservable, and so are its changes of shape, which must certainly take place as in Venus. But when we do see it, it would necessarily show itself to us in the shape of a semicircle, just as Venus does at its maximum angles, though its disc is so small and its brilliance so lively that the power of the telescope is not sufficient to strip off its hair so that it may appear completely shorn.’

“Galileo had finished writing the Dialogue early in 1630, but it did not, finally, appear in print until two years later. By that time, Mercury had, in fact, been observed shorn of its hair in a dramatic revelation … 

“Kepler himself had attempted to witness a transit of Mercury in 1607 by means of a camera obscura, and he had seen what turned out to be a sunspot. Five years later, when the telescope held out hope that transits could be observed, Galileo had criticized Scheiner for trying to observe Venus on the Sun during a superior conjunction, but he paid no further attention to the prospects of transits, perhaps leaving his reader with the idea that transits would be invisible events, even with the telescope. Kepler, however, never abandoned the hope of observing transits (at inferior conjunctions, of course), because he felt that the telescope would surely reveal Venus and Mercury when they crossed the Sun's disk.

“In preparing ephemerides for the years 1629 to 1636 from the Rudolphine Tables, Kepler and his assistant Jacob Bartsch found that in November 1631 a transit of Mercury would take place, followed a month later by a much rarer transit of Venus, although this latter event would probably not be visible in Europe. In the predictions for the year 1631, Kepler therefore included an ‘admonition’ to astronomers, urging them to observe these two transits. Bartsch made sure that Kepler’s admonition would come to the attention of the learned community by printing it separately in 1629, even before the ephemerides themselves had gone to press, and reprinting it in 1630 …

“[Kepler] saw the transits primarily as opportunities for determining the apparent diameters of Venus and Mercury – information that might have important consequences … According to Kepler's calculations, Venus should appear as a round dark spot with a diameter of 7'6" (almost a quarter of the Sun's apparent diameter) on the Sun's disk during its transit … Kepler did not predict Mercury's apparent diameter during its transit, but the same proportionality and Mercury's distance from the Earth at that time would produce an apparent diameter of about 2 1/2'. These large predicted apparent diameters led Kepler to a serious misjudgment. He advised astronomers that they would be able to observe the transits not only by projecting the Sun's image through a telescope, but also by projecting it through a simple pinhole. He probably had in mind a rather large camera obscura, such as the one of Philip, Landgraf of Hesse, but he also mentioned his own observation of 1607, when he thought he had observed Mercury on the Sun by simply holding a piece of paper in the beam of sunlight admitted into a dark attic through a small hole in the roof.

“Thanks to Bartsch's care, the astronomical community was alerted to the impending transits in good time, and a number of observers made preparations to witness them. Very few, however, were fortunate enough to observe anything. The transit of Venus was, in fact, not visible in Europe, and the transit of Mercury was missed by all but three observers because of cloudy weather or inappropriate instruments. The three astronomers who were fortunate enough to witness this important celestial event were Remus, now in Ruffach in Alsace, his correspondent Cysat in Ingolstadt, and Pierre Gassendi (1592-1655) in Paris. (Kepler himself had died on 15 November 1630). All three projected the Sun's image through a telescope, not a camera obscura.

“Gassendi was no stranger to the observation of sunspots. On a number of occasions he had traced the motion of spots across the Sun's disk by means of the telescopic projection method, and now, in anticipation of the two transits, he set up his apparatus in a room in Paris. The image of the Sun on the paper had a diameter of about 8 inches. He drew a circle of that size and divided its diameters (drawn at right angles to each other) into sixty equal parts, so that he would be able to plot the locations of the planet on the Sun’s disk in the way the locations of sunspots were usually plotted. Since the Sun's apparent diameter is about 30', each division represented about 30". Gassendi began his vigil on the fifth of November.

“It rained that entire day and almost all of the following day. The seventh began little better. Shortly before 9 a.m., however, the Sun appeared through a gap in the clouds and produced a clear image on the paper. Gassendi noticed a small spot on the Sun's disk, but it did not occur to him that this might be Mercury. Kepler's figure for Venus's expected size, as well as the traditional notion as to how large Mercury ought to appear, led Gassendi to expect a spot several minutes in diameter, or about a fifteenth part of the Sun's diameter. He wrote:

‘… I was far from suspecting that Mercury would project such a small shadow. For such was its smallness that its diameter hardly appeared to exceed half of one of the divisions marked. I thought rather that it was a spot which I had not noticed on the Sun on a previous day.’

“Whereas earlier observers had believed that they were observing Mercury when, in fact, they were looking at sunspots, Gassendi now thought that he was seeing sunspots when, in fact, he was observing Mercury on the Sun!

“As the clouds parted intermittently, Gassendi was able to observe the Sun from time to time. He measured the position of the small spot in the hope of using it as a reference point. Such a spot, sharing the Sun's 26-day axial rotation, would show no sensible change of position over a period of hours. Mercury, on the other hand, would move across the entire disk of the Sun in a matter of a few hours, should it appear. After two measurements, however, Gassendi found to his surprise that this dot, this supposed sunspot too minute to be Mercury, was moving much too rapidly to be a sunspot:

‘Thereupon, thrown into confusion, I began to think that an ordinary spot would hardly pass over that full distance [of four divisions] in an entire day. And I was undecided indeed. I could hardly be persuaded, however, that it was Mercury, so much was I preoccupied by the expectation of a greater size. Hence I wondered if perhaps I could have been wrong in some way about the distance measured earlier. And when the Sun shone again, and I ascertained the apparent distance to be greater [again] by two divisions …, then at last I thought that there was good evidence that it was Mercury’ …

“Convinced, now, that he was actually observing Mercury on the Sun, Gassendi compared the diameter of this little dark spot with the diameter of the Sun and found that it ‘could scarcely exceed two-thirds of one division, that is, the third part of a minute, or 20 seconds.’ This was only about a sixth of the apparent diameter which Tycho’s figures and Kepler's admonition would lead one to expect.

“In his report, Mercurius in Sole Visus et Venus Invisa, published as an open letter to his colleague Wilhelm Schickard (1592-1635), Maestlin's successor in the chair of astronomy at Tübingen, Gassendi shared his astonishment with the astronomical community. Although others, especially Galileo, had pointed out that, when observed with the naked eye, the stars and planets are clothed in adventitious rays and appear smaller when stripped of these rays by the telescope, no one had yet seen this very bright and very small planet well defined. Now, Gassendi had seen Mercury stripped naked and starkly outlined against the Sun's disk …

“For the time being Gassendi's observation of Mercury was the only one to find its way into print. His surprise at the planet's smallness was shared by his colleagues. Only Galileo received the news without surprise … The Landgraf of Hesse and his staff had also observed all day without seeing anything. Led astray by Kepler, they had used the Landgraf’s camera obscura.

“Wilhelm Schickard, to whom Gassendi's tract was addressed, had also been led astray. Following Kepler's example of the pre-telescopic observation of what turned out to be a sunspot in 1607, he had made a hole in one of the tiles of his roof, and with some of his friends he had awaited the glorious event in his attic. They had, however, been frustrated by the weather: it was cloudy all day, and the Sun hardly poked through the clouds once or twice. Had the Sun shone all day, they would have been equally frustrated!

“In his printed reply to Gassendi, Schickard expressed his surprise at Mercury's ‘entirely paradoxical smallness’ and gave his opinion that Gassendi would ‘hardly escape the censures of the critics, who will doubt from this whether you have really seen Mercury himself.’ Since, however, the motion of the spot proved that it could not have been a sunspot, Schickard agreed that it had to be Mercury passing across the Sun's disk. Tycho and Kepler had, however, predicted a much larger apparent diameter, and Schickard therefore set himself the task of explaining away Mercury's observed paradoxical smallness.

“After the telescope had stripped away the extraneous rays that appear to surround Mercury, Schickard argued, the remaining disk should have a diameter of about 1'. It appeared smaller, however, for three reasons. A stick held closely in front of a flame appears thinner from a distance than it is in reality, because it is the nature of light to spread itself in all directions. Likewise, Mercury's disk, surrounded by the brillance of the Sun, would appear smaller than it really is. Furthermore, as Cysat also argued, the Sun, being so much larger, illuminates more than half of Mercury's globe, leaving a smaller unilluminated side visible from the Earth. And, last, perhaps Mercury was not entirely opaque but had a solid core surrounded by a translucent coat. This last farfetched argument to preserve Mercury's expected, canonical size had already been used in 1610 by Lodovico delle Colombe (1565-ca. 1615), leader of the opposition against Galileo in Florence, in a vain effort to preserve the perfection of the Moon in the face of telescopic evidence about its mountainous nature.

“The Landgraf's observation, however, would tend to support Gassendi's observation. Schickard pointed out that the Landgraf's celebrated camera obscura could not reveal sunspots with diameters of less than 2'. He concluded that although Gassendi's observation itself was correct, Mercury's smallness was only apparent. In reality its apparent diameter was considerably larger, that is, about 1'. For various optical reasons, however, Mercury showed itself as a spot of only about 20" on the disk of the Sun …

Kepler's admonition had served to alert observers to a phenomenon never before observed in the heavens. In their attempts to witness these transits astronomers were aware, of course, that their observations could provide excellent opportunities to check the times and positions predicted in the tables and to improve the parameters of the planetary models of Mercury and Venus. This was especially important in the case of Mercury, the most difficult planet to observe. They were also not unaware that these transits could perhaps serve to determine the parallaxes of Mercury and Venus, provided that suitable observations could be made at different locations and compared. For both reasons Gassendi made preparations to have an assistant measure the Sun's altitude with a quadrant (in order to determine the time) in a room directly below him each time he gave the signal by stamping his foot. But the young man had gone out, convinced that the clouds would not allow the observation to be made, and by the time he had been found there was time for only one measurement before Mercury had left the Sun's disk.

“Accustomed as we are to thinking about transits primarily in connection with parallax measurements, we must be careful not to make the mistake of thinking that this measurement or even the correction of planetary elements, was considered by Gassendi and his colleagues to be the most important aspect of Mercury's transit of 1631 … the main issue was Mercury's apparent size. For Kepler this measurement was of crucial importance for his scheme of sizes and distances. Although corrections were made in Mercury's elements as a result of Gassendi's observation, no parallax of Mercury resulted from it. The planet's ‘entirely paradoxical smallness’ was by far the most important result of the observations of the transit of 1631.

In principle this smallness was implicit in Galileo's various statements about planetary sizes, from Sidereus Nuncius of 1610 to his Dialogue of 1632. In the absence of a table of measurements to take the place of the traditional measures, however, the full meaning of Galileo's general utterings on this subject had been lost on scientists in general and even astronomers in particular … even those like Gassendi who regularly scrutinized the heavens through a telescope, had not rid themselves yet of the traditional notion of how large planets and fixed stars ought to appear. Gassendi's announcement that Mercury's apparent diameter was an order of magnitude smaller than almost anyone had supposed it to be provided an essential and dramatic illustration of Galileo's general point, so vigorously reiterated that same year in the Dialogo” (Van Helden, Measuring the Universe (1985), pp. 95-104).

Lalande, p. 199. Macclesfield 256 (a sammelband of 5 works including this copy of Gassendi).

[Gassendi:] 4to (169 x 120 mm), pp. [2], 3-47, [1, blank], woodcut illustrations in text (trimmed a little too close as often happened with Macclesfield sammelbands but without any loss of text). Modern calf with gilt tooling. [Schickard:] 4to (176 x 132 mm), pp. 40, woodcut illustrations in text. Nineteenth century red morocco tooled in gilt, marbled paper pastedowns.

Item #6619

Price: $18,500.00