GIORGIO STRANO GALILEO, RELIABLE OBSERVER: ASTRONOMICAL ACCURACY AND THE OPTICAL LIMITS OF THE TELESCOPE ESTRATTO da CELESTIAL NOVELTIES ON THE EVE OF THE SCIENTIFIC REVOLUTION 1540-1630 EDITED BY DARIO TESSICINI and PATRICK J. BONER Leo S. Olschki Editore Firenze BIBLIOTECA DI GALILÆANA .III. CELESTIAL NOVELTIES ON THE EVE OF THE SCIENTIFIC REVOLUTION 1540-1630 edited by DARIO TESSICINI and PATRICK BONER GALILÆANA Journal of Galilean Studies www.museogalileo.it BIBLIOTECA DI GALILÆANA III CELESTIAL NOVELTIES ON THE EVE OF THE SCIENTIFIC REVOLUTION 1540-1630 edited by DARIO TESSICINI and PATRICK J. BONER LEO S. OLSCHKI EDITORE MMXIII Tutti i diritti riservati CASA EDITRICE LEO S. OLSCHKI Viuzzo del Pozzetto, 8 50126 Firenze www.olschki.it Il volume è stato pubblicato grazie al contributo di ISBN 978 88 222 6254 7 CONTENTS PATRICK BONER – DARIO TESSICINI, Introduction . . . . . . . . . . . Pag. VII ADAM MOSLEY, Past portents predict: cometary historiae and catalogues in the sixteenth and seventeenth centuries . . . . . . . . . . » 1 TAYRA M.C. LANUZA NAVARRO – VÍCTOR NAVARRO BROTONS, Prophecy and politics in Spain: celestial novelties and the science of the stars, 1572-1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . » 33 DARIO TESSICINI, The comet of 1577 in Italy: astrological prognostications and cometary theory at the end of the sixteenth century » 57 ISABELLE PANTIN, Francesco Giuntini et les nouveautés célestes . . » 85 ELIDE CASALI, Astrologia ‘cristiana’ e nuova scienza. Pronostici astrologici sulle comete (1577-1618) . . . . . . . . . . . . . . . . . . » 105 JOHN HENRY, Jean Fernel on celestial influences and the reform of medical theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . » 133 NICK JARDINE, How to Present a Copernican Comet: The Form and Tactics of Christoph Rothmann’s Dialexis on the Comet of 1585 » 159 MIGUEL ANGEL GRANADA, Tycho Brahe’s anti-Copernican campaign: his criticism of Maestlin and Thomas Digges in the Astronomiae Instauratae Progymnasmata . . . . . . . . . . . . . . . . . » 185 FRANCESCO BARRECA – PATRICK J. BONER, A perfect similitude: science and politics in Kepler’s dedicatory letters to De stella nova and the Astronomia nova . . . . . . . . . . . . . . . . . . . . » 209 ÉDOUARD MEHL, Comètes et taches solaires en Allemagne (16101630): l’aile hétérodoxe (Faulhaber, Mayr, Mögling) et le point de départ de la ‘fable du monde’ cartésienne . . . . . . . . . . . . » 231 GIORGIO STRANO, Galileo, reliable observer: astronomical accuracy and the optical limits of the telescope . . . . . . . . . . . . . . . . . » 257 Index of names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . » 273 — V — GIORGIO STRANO * GALILEO, RELIABLE OBSERVER: ASTRONOMICAL ACCURACY AND THE OPTICAL LIMITS OF THE TELESCOPE INTRODUCTION Galileo Galilei (1564-1642), his telescope, and the celestial discoveries described in the Starry Messenger (Venice, 1610), in the Description and Demonstrations on Sunspots (Rome, 1613), and in The Assayer (Rome, 1623) form a fascinating topic. The context of the discoveries, their use as evidence in favour of the heliocentric cosmos – especially in the Dialogue on the Two Chief World Systems (Florence, 1632) – and their reception in Galileo’s time, are examined in countless books and articles.1 Attempts were also made to combine the fascination for the sky and the belief in the Copernican system with Galileo’s studies on the structure of matter and of the properties of motion.2 Notwithstanding such extraordinary attention, it is quite remarkable that only a few scholars further researched Galileo’s astronomical observations from a practical point of view. The ‘golden age’ of this activity dates back to the 1970s. In 1975, the astronomer Guglielmo Righini published the first article about the dating of Galileo’s lunar observations on the basis of the engravings in the Starry Messenger and the reconstruction of the moon’s aspect in the year 1609.3 Righini’s scientific approach was immediately rejected by * I would like to thank Paolo Del Santo and Giancarlo Truffa, with whom I had several conversations about the topics of this article. I hope that the article will serve as a starting point for a joint and comprehensive work about Galileo’s astronomical observations. I also would like to express my gratitude to Karen Giacobassi for her kind revision of the English text. 1 Among the recent publications, see MASSIMO BUCCIANTINI , Galileo e Keplero. Filosofia, cosmologia e teologia nell’età della Controriforma, Torino, Einaudi, 2003; MICHELE CAMEROTA, Galileo Galilei e la cultura scientifica nell’età della Controriforma, Roma, Salerno, 2004. 2 See PAOLO GALLUZZI , Tra atomi e indivisibili: la materia ambigua di Galileo, Firenze, Olschki, 2011. 3 GUGLIELMO RIGHINI , New Light on Galileo’s Lunar Observations, in MARIA LUISA RIGHINI 18 — 257 — GIORGIO STRANO the astronomer Owen Gingerich,4 and, in 1976, was instead approved by the historian Stillman Drake.5 Again, in 1978, Righini published (in Italian) his Contribution to the Scientific Interpretation of Galileo’s Astronomy. After a general introduction, he examined the observations of the 1604 nova, the moon, the satellites of Jupiter, the sunspots, the phases of Venus, the structure of Saturn, the other planets and comets.6 That same year, another astronomer, Ewan A. Whitaker, utilized Righini’s approach to date not only the lunar engravings of the Starry Messenger, but also Galileo’s preparatory watercolours.7 Since then, Whitaker seems to be the only scholar to have continued these lunar investigations in the past thirty years.8 After the 1970s, studies on the making of Galileo’s observational reports faded. They only revived sporadically with claims about Galileo’s alleged discovery of Neptune between December 1612 and January 1613.9 The celebrations for the fourth centennial of the invention of the telescope and of Galileo’s celestial discoveries, respectively in 2008 and 2009 (proclaimed the International Year of Astronomy), led many people – including me – to replicate old telescopes, perform new observations, and study the performances of those instruments. Therefore, it is not a surprise that during such a Galilean revival, in addition to other controversial historical discoveries,10 someone also noticed the peculiar optical properties BONELLI – WILLIAM R. SHEA (eds.), Reason, Experiment and Mysticism in the Scientific Revolution, New York, Science History Publications, 1975, pp. 59-76. 4 OWEN GINGERICH , Dissertation cum Professore Righini et Sidereo Nuncio, in M.L. RIGHINI BONELLI – W.R. SHEA (eds.), Reason, Experiment and Mysticism (cit. note 3), pp. 77-88. 5 STILLMAN DRAKE, Galileo’s First Telescopic Observations, «Journal of the History of Astronomy», 7 (1976), pp. 153-168. 6 RIGHINI , Contributo alla interpretazione scientifica dell’opera astronomica di Galileo, «Annali dell’Istituto e Museo di Storia della Scienza», suppl. fasc. 2 (1978), pp. 14-24 for the nova, pp. 24-44 for the moon, pp. 45-75 for Jupiter, pp. 75-92 for sunspots, pp. 92-95 for Venus, pp. 95-101 for Saturn and pp. 101-107 for other miscellaneous observations, except for the fixed stars. 7 EWAN A. WHITAKER, Galileo’s Lunar Observations and the Dating of the Composition of Sidereus nuncius, «Journal for the History of Astronomy», 9 (1978), pp. 155-169. 8 WHITAKER, Selenography in the Seventeenth Century, in MICHAEL HOSKIN (ed.), The General History of Astronomy, Cambridge, Cambridge University Press, 1984-, vol. 2.1, pp. 119-143; ID., Mapping and Naming the Moon: A History of Lunar Cartography and Nomenclature, Cambridge, University Press, 2000, pp. 19-25; ID., Dating of Galileo’s Sketches of the Moon, in PAOLO GALLUZZI (ed.), Galileo: Images of the Universe from Antiquity to the Telescope, Florence, Giunti, 2009, pp. 262-267. 9 DAVID N. JAMIESON, Galileo’s Miraculous Year 1609 and the Revolutionary Telescope, «Australian Physics», 46 (2009), pp. 72-76. 10 This is the case of the fake water-coloured copy of the Starry Messenger owned by Richard Lan and Seyla Martayan, about which see: HORST BREDEKAMP, Galilei der Künstler: der Mond, die Sonne, die Hand, Berlin, Akademie Verlag, 2007, pp. 149-174; GINGERICH, The Curious Case of — 258 — GALILEO, RELIABLE OBSERVER of Galileo’s telescope. In particular, in September 2010, Tom Pope and Jim Mosher described on the web a few of these properties,11 mentioned by Albert van Helden in his historical re-examination of the early telescopic observations.12 Returning once again to the internal and experimental studies of the telescope and the related observations reveals interesting aspects of Galileo’s early astronomical activity. Such aspects emerge only by merging the properties of the early telescopes with the actual appearance of the sky at the time of its observation, and by considering all of the Galilean observations as part of a single scientific program. Indeed, separating the observations of the moon from those of Venus, or of Saturn, or separating the observations of the Ptolemaic nebulae from those of the comets, will possibly mislead us from both Galileo’s intellectual aims and strategies. THE INSTRUMENT The correct understanding of Galileo’s use of the telescope requires a careful examination of the instrument itself. It is particularly essential to compare the Galilean and the Keplerian types of telescope. The differences between the two optical devices are such that they had momentous consequences for both the history of science and historical studies. Theoretically invented and described by Johann Kepler (1571-1630) in the Dioptrice (Augsburg, 1611),13 the Keplerian telescope became the astronomical refractor par excellence. The basic combination of two convex lenses underwent several modifications by Eustachio Divini (1610-1685), Giuseppe the M-L Sidereus Nuncius, «Galilaeana», 6 (2009), pp. 141-165; SHEA, Owen Gingerich’s Curious Case, «Galilaeana», 7 (2010), pp. 97-110. Such a ‘discovery’ led to the re-examination of Galileo’s observations from the point of view of the history of the book; see: PAUL NEEDHAM, Galileo Makes a Book: The First Edition of Sidereus Nuncius Venice 1610, in BREDEKAMP (ed.), Galileo’s O, Berlin, Akademie Verlag, 2010, 2 vols.: vol. 2. 11 TOM POPE – JIM MOSHER , http://pacifier.com/~tpope/About_Website.htm (site no longer active). 12 ALBERT VAN HELDEN , Galileo and the Telescope, in VAN HELDEN – SVEN DUPRÉ – ROB VAN GENT – HUIB ZUIDERVAART (eds.), The Origins of the Telescope, Amsterdam, KNAW Press, 2010, pp. 183-201. See in particular pp. 189-190. 13 JOHANNES KEPLER, Dioptrice, seu demonstratio eorum quae visui et visibilibus propter conspicilla non ita pridem inventa accidunt: praemissae epistolae Galilaei de ijs quae post editionem Nuncij siderij ope perspicilli, nova et admiranda in coelo deprehensa sunt: item examen praefationis Ioannis Penae Galli in Optica Euclidis de usu optices in philosophia, Augusburg, Typis Davidis Franci, 1611, pp. 42-44. — 259 — GIORGIO STRANO Campani (1635-1715), Christiaan Huygens (1629-1695), and many others, with the purpose of obtaining larger, sharper and erect images. The hegemony of the instrument reduced the popularity of the Galilean telescope which, within two decades from invention, returned to its original use as a spyglass. No further attempts to develop the combination of a convex objective lens and a concave ocular lens were made. The magnification decreased from the maximum of over 30 times, as announced in the Starry Messenger, down to 3 or 5 times typical for theatre spyglasses from between the 18th and 20th centuries. Finally, all but two of the original telescopes made by Galileo disappeared. Such a disappearance led historians to overlook several important questions. Why did the Dutch or Galilean telescope come first? And why did the Keplerian telescope have to wait about twenty years from its theoretical creation to gain supremacy? Moreover, what can one really see looking through a Galilean telescope? Rivers of ink have been poured over enquiries on the birth and the early evolution of the telescope, and about Galileo’s use – or disuse – of theoretical optics in perfecting the instrument. Such rivers usually avoid the above questions. On the one hand, historians are usually unable to study the optical properties of a telescope. Therefore, they miss the possibility to compare the content of ancient sources with the theoretical analysis and the practical use of the instrument. On the other hand, scientists (upon which historians have to rely to restore their gaps in scientific knowledge) have a background that prevents the correct understanding – and, therefore, the complete explanation – of the historical issues at stake. In order to understand the significance of this dilemma, let me first start by providing an example of the typical scientific optical background. In 1937, Francis A. Jenkins and Harvey E. White wrote Fundamentals of Physical Optics. They published an enlarged version of the book in 1950, Fundamentals of Optics, that became a basic manual for universities. The book underwent four editions, many reprints, and several translations. Under the paragraph ‘Astronomical Telescopes’, the authors write: «Historically the first telescope was probably constructed in Holland in 1608 by an obscure spectacle-lens grinder, Hans Lippershey. A few months later Galileo, upon hearing that objects at a distance could be made to appear close at hand by means of two lenses, designed and made with his own hands the first authentic telescope. The elements of this telescope are still in existence and may be seen on exhibit in Florence. The principle of the astronomical telescopes of today – Jenkins — 260 — GALILEO, RELIABLE OBSERVER and White conclude – is the same as that of these early devices», and is suitably illustrated (Fig. 1 – above).14 When the reader looks at the image, an obvious conclusion emerges: the Galilean and the Keplerian telescope are the same thing. If not, at least the study of the first type relies on the same principles as the second. With a step further, one can study the Galilean combination of lenses by applying the same methods he would apply to the Keplerian. Of course, this is the truth, but only half of it... Where is the Galilean telescope in Jenkins and White’s book? It appears a few pages before the astronomical telescopes, among the ‘‘Telephoto Lenses’’.15 The image included in the text is revealing. It shows a couple of lenses in which the concave lens increases the focal length of a convex lens (Fig. 1 – below). The image obtained with the lenses is enlarged and projected on a photographic film, or a screen. In the Keplerian telescope, as shown by Jenkins and White, the convex objective lens produces an upside-down image of the object being observed. This image is ‘real’: it can be projected on a screen placed in the spot where light rays concentrate. The convex ocular lens placed beyond this image works as a magnifying glass. It takes the image produced by the objective as a new object being observed and creates another image. Depending on the position of the ocular, this image is real or virtual. ‘Virtual’ means that the image cannot be projected on a screen placed on the spot where the protractions of the light rays concentrate. As Jenkins and White stress, the position of the ocular in astronomical telescopes is such that the final image is virtual and at infinity. The light rays exit the ocular as a parallel bundle. A third lens is indispensable in order to create a real image on the human retina, noticeably the eye crystalline. The final image is erect, and the brain perceives it as upside-down. The creation of a real image on the prime focal plane of the telescope has noteworthy consequences: 1) Since the ocular takes the image directly from the prime focal plane of the objective, its prime focal point must coincide as much as possible with the image. As the ocular is moved slightly back or forth, the image goes quickly out of focus. Moreover, if the objective lens is not well shaped, or if it is not achromatic, it produces secondary multiple images which are out 14 FRANCIS ARTHUR JENKINS – HARVEY ELLIOT WHITE , Fundamentals of Optics, New York, McGraw-Hill, 1957 (3rd ed.), p. 178. 15 Ivi, pp. 173-174. — 261 — GIORGIO STRANO Fig. 1. Above: The astronomical telescope; JENKINS & WHITE, Fundamental of Optics, New York, 1957, p. 179, fig. 10L; Below: Telephoto lens; ivi, p. 173, fig. 10D. of the prime focal plane of the telescope. The ocular amplifies whatever aberration that exists and gives rise to fuzzy or colourful telescopic images. 2) The ocular shows a magnified version of the image formed by the objective on the prime focal plane of the telescope. This image cannot vary in size or position if the eye is moved back and forth, or laterally behind the centre of the ocular. What one sees by looking through the telescope is therefore independent of the position of the eye. 3) The existence of a prime focal plane for the telescope means that the diameter of the tube of the instrument corresponding to the spot where the image is formed has direct influence on the amplitude of the field of view. The part of the tube farthest from the prime focal plane blurs the edge of the field of view. If a stop is placed on the main focal plane, the size of the field of view depends on the diameter of the hole at the centre of the stop. The ocular shows a magnified version of the hole. The edge of the hole reduces the original field of view and overlays the image of the far object. If — 262 — GALILEO, RELIABLE OBSERVER one inserts two cross-lines on the main focal plane, they are also magnified by the ocular and overlay the image of the far object. In brief, looking through any point of the ocular, the eye perceives the object, the stop hole’s edge, and the cross-lines forming a single image. The structure and the optical behaviour of the Galilean telescope are very different. The concave ocular stands before the objective prime focal point. No real image forms inside the telescope. Before the light rays converge on the prime focal point, the concave ocular makes them parallel, as required in an astronomical telescope. The third lens of the eye crystalline is indispensable for creating a real image on the human retina. This image is upsidedown, and the brain perceives it as erect. The lack of a real image in the prime focal plane of the Galilean telescope creates remarkable differences from the Keplerian: 1) Notwithstanding the lack of a real image, the optimal functioning requires that the prime focal point of the ocular lens and that of the objective lens coincide as much as possible. If the ocular, however, is moved slightly towards the objective, the image does not lose focus quickly. As in unaided vision, the crystalline – especially that of a young adult eye – adapts and compensates for the imperfect focusing of the instrument. In a reverse zooming effect, the image appears smaller and sharper. Moving the ocular towards the objective may also reduce the chromatic effect and other aberrations caused by imperfections of the objective. One of the exhibits at the Florentine version of the Galileo’s Telescope exhibition in 2008 included two parallel optical benches. Each bench carried a poor quality convex objective and an ocular lens. The two oculars had the same power, but one was convex, the other concave. The images perceived through the Keplerian set were very distorted, while those perceived through the equivalent Galilean set were legible.16 This experiment explains why the Galilean telescope materially appeared on the astronomical scene before the Keplerian. The Keplerian required higher quality lenses that only became available in the 1630s. 2) The ocular shows no magnified version of the image formed by the objective lens on the prime focal plane. The image produced by the telescope is therefore directly dependent on the eye of the observer. Not only the correct focusing, but also the portion of the image perceived by the observer depends upon the position and the conditions of the eye. 3) The inexistence of a main focal plane inside the telescope leads to other consequences. There is no place inside the instrument to fix a field16 GIORGIO STRANO (ed.), Galileo’s Telescope: The Instrument that Changed the World, Florence, Giunti, 2008, p. 151, no. 5.1.3. — 263 — GIORGIO STRANO stop or cross-lines. Useful places can be found outside the tube of the telescope. In fact, one of these was found by Galileo in 1612, when he fixed his so-called ‘micrometer’ aside the tube and looked at it with the other eye.17 The other useful place is located inside the eye of the observer. In fact, the field-stop of the Galilean telescope is determined by the location and the aperture of the pupil. The closer the eye is to the ocular, the larger the portion of the object being observed. If the eye moves laterally behind the centre of the ocular, the observed portion of the object varies. This implies that the Galilean telescope has two distinct fields of view. The first is perceived by looking through the telescope from a fixed spot, while the second is fully perceived by moving the eye behind the ocular. The first field of view depends on the aperture of the pupil, which is smaller in daylight and larger by night.18 From a modern point of view, since these characteristics of the Galilean telescope appear to form an uncertain ground for observational activity, the astronomers consider them to be defects. Perhaps some of these are defects while others were practical advantages for an epoch in which no other telescopic device existed. It is a fact that combining two lenses by chance has a greater probability of producing legible images in the Dutch or Galilean combination than in the Keplerian. In any case, historians cannot overlook some information only because it is seen today as a defect by scientists. Not considering such information may lead to the creation of historiographic myths or to serious misunderstandings of the original process of discovery. GALILEO’S OBSERVATIONS In this section I will introduce a few provisional remarks about Galileo’s telescopic observations. Since they are provisional, these remarks are suitable to further investigation and revision. Study of the secondary literature reveals that the moon has stood at the very centre of scholarly attention. Galileo’s watercolours and the engravings 17 GALILEO GALILEI , Diaries of Observations, January 31, 1612, in ID ., Le opere di Galileo Galilei, edizione nazionale a cura di Antonio Favaro, 20 vols., Firenze, Barbera, 1890-1909 (hereafter cited as OG), III, p. 446. Regrettably, Galileo did not describe his «instr[ument]o ad intercapedines exactius accipiendas». A first account can be found in GIOVANNI ALFONSO BORELLI, Theoricae Mediceorum Planetarum ex causis physicis deductae, Florence, ex typographia S.M.D., 1666, pp. 143-144. See also STRANO (ed.), Galileo’s Telescope (cit. note 16), p. 149, no. 4.3.1. 18 See VAN HELDEN , Galileo and the Telescope (cit. note 12), pp. 189-190. — 264 — GALILEO, RELIABLE OBSERVER of the Starry Messenger have a strong visual impact. In comparison, other early 17th century representations of the lunar surface by Thomas Harriot or Teofilo Gallaccini appear to be much less reliable.19 In 1975, while opposing claims about Galileo’s inability as a cartographer,20 Guglielmo Righini started his analysis of the Starry Messenger. He looked for matching points between the lunar surface and the engravings. He carried on visibility and position calculations. Finally, he prepared a calendar of Galileo’s observations.21 Righini perfected this method in 1978.22 Of course, this is not the place to discuss the method. This is the place, however, to re-examine the procedure which Galileo followed in making his drawings, watercolours and engravings, and the meaning of the details which are visible in such representations. An alleged obstacle to early observations of the moon is the very small field of view of the Galilean telescope. In 1611, Kepler wrote that such an instrument revealed «barely half the diameter of the moon».23 Albert van Helden confirmed this statement in the case of the two surviving Galilean telescopes at the Museo Galileo. While the apparent diameter of the moon varies between 29 and 33 arc-minutes, both instruments inventory no. 2428, with a magnification of 21, and inventory no. 2427, with a magnification of 14, have a field of view of about 15 arc-minutes (Fig. 2).24 In 2010, van Helden pointed out that the telescope Galileo had at hand between the end of 19 THOMAS HARRIOT, Drawings of the Moon, July 26, and April 9-10, 1609, Coll. Lord Egremont, Petworth; see also: GALLUZZI (ed.), Galileo (cit. note 8), p. 361, no. VII.4.3-5. TEOFILO GALLACCINI, Monade celeste, o vero Trattato di Cosmografia, ms., post 1633-ante 1641; Biblioteca Comunale degli Intronati, Siena, Ms. L.VI.31, cc. 97r-97v. See also ALINA PAYNE (ed.), Teofilo Gallaccini, Selected Writings and Library, Florence, Olschki, 2012, pp. 385-388; ANNALISA PEZZO, Quasi in privata accademia: Il Palazzo delle Papesse ai tempi di Ascanio I e Ascanio II Piccolomini, in MARCO PERINI et al., Il Palazzo delle Libertà, Prato, Gli Ori, 2003, pp. 188-198: 191-193; STRANO (ed.), Galileo’s Telescope (cit. note 16), p. 139, no. 2.3.3. 20 RIGHINI , New Light on Galileo’s Lunar Observations (cit. note 3), pp. 65-66. This criticism was brought forth by JOHANNES CLASSEN, The First Maps of the Moon, «Sky and Telescope», 37 (1969), p. 82, and ZDENĚK KOPAL, The Moon, Dordrecht, D. Reidel, 1969, p. 225. 21 RIGHINI , New Light on Galileo’s Lunar Observations (cit. note 3), pp. 66-76. 22 ID., Contributo alla interpretazione scientifica dell’opera astronomica di Galileo (cit. note 6), pp. 24-35. 23 KEPLER, Narratio de Observatis a se quatuor Iovis Satellitibus Erronibus, Frankfurt, Sumptibus Zachariae Palthenii D., 1611, f. 3v: «Die 31 Augusti, vespere, Saturnum et Martem contemplati sumus, nullas in vicina vidimus amplitudine instrumenti, quae pene dimidiam Lunae diametrum capiebat»; see also OG, III, p. 185. 24 VAN HELDEN , Catalogue of Early Telescopes, Florence, Giunti, 1991, pp. 30 and 32 respectively. — 265 — GIORGIO STRANO Fig. 2. Galileo’s telescopes; Museo Galileo, Florence, inv. no. 2427 (above) and inv. no. 2428 (below). 1609 and the beginning of 1610, with a magnification of 20,25 also had a field of view between 10 and 20 arc-minutes.26 In order to establish the optical properties of the instrument, I used a good replica of the telescope inventory no. 2428. I placed a graduated target at 14.6 meters from the centre of the replica. Divisions on the target were spaced at 1 cm, and 25.5 cm subtended an angle of 1º. In artificial light, the field of view of the telescope was 7 divisions with the other eye closed (maximum aperture of the pupil) and 6 divisions with both eyes open (reduced aperture of the pupil). Therefore, the field of view of this Galilean telescope varied from 14 to 16 arc-minutes. This is in accordance with van Helden’s datum. By laterally moving the eye behind the ocular, however, it became possible to observe other portions of the target. The whole surface visible had a diameter of about 18 divisions, that is, about 42 arc-minutes. It is therefore important to distinguish between the impression of spying through a key hole, which one receives by taking a glance through the telescope, and what one can see when the eye is very close to the ocular and laterally moves behind it. This distinction weakens a historical myth: noticeably, that Galileo made his drawings of the moon by merging several partial (today 25 26 See GALILEI, Letter to Antonio de’ Medici, January 7, 1610; in OG, X, p. 273. VAN HELDEN, Galileo and the Telescope (cit. note 12), pp. 189-190 and p. 190, note 24. — 266 — GALILEO, RELIABLE OBSERVER lost) drawings. Once Galileo aimed the telescope at the sky by using some kind of mount,27 he could inspect the complete lunar surface by moving the eye behind the ocular of the telescope. A special problem concerns the reliability of Galileo’s depictions of the moon and, consequently, the possibility of dating his observations. In 1975, Owen Gingerich stated that any attempt to date the observations at the basis of the Starry Messenger is unreliable.28 Notwithstanding Ewan Whitaker’s insistence on such attempts, the engravings of the booklet represent the final stage of a process of intellectual elaboration. A first step of the engraving process may coincide with the watercolours in Galileo’s hand (Fig. 3 – left),29 and, perhaps, with other watercolours included in a letter to Antonio de’ Medici dated January 7, 1610.30 Both watercolour sets were obtained using a telescope with a magnification of 20. Unfortunately, the original letter is lost. Nevertheless, the images from the 17thcentury copy of the letter closely resemble those in Galileo’s manuscripts. Moreover, the text of the letter is similar to the text of the Starry Messenger. For example, both texts specify that «a place near the centre of the moon is occupied by a cavity much larger than the other ones, and perfectly circular in shape».31 On the one hand, because of such claims, Righini considered whether Galileo really observed the enormous crater included in the engravings of the Starry Messenger.32 In Righini’s opinion, the excessive size of the cavity was due to the coalescence of several smaller craters, which appeared as a unique structure to the not-so-powerful Galilean telescope.33 On the other hand, Gingerich guessed that Galileo might have observed the crater Albategnius. The size of the representation corresponds not to the material, but to the psychological impact that Galileo received by staring at the crater. Therefore, the GALILEI, Letter to Antonio de’ Medici, January 7, 1610, in OG, X, p. 278. GINGERICH, Dissertatio cum Professore Righini et Sidereo Nuncio (cit. note 4), p. 86. 29 GALILEI , Watercolours of the Moon, 1609; Biblioteca Nazionale Centrale (BNCF), Florence, Ms. Gal. 48, ff. 28r and 29v. 30 ID., Letter to Antonio de’ Medici, January 7, 1610, in OG, X, pp. 274-276. 31 Ivi, p. 275: «[...] una ve ne ho io, non senza qualche meraviglia, osservata, che è posta quasi nel mezo della Luna, la quale apparisce perfettissimamente circolare, et è tra le altre assai grande»; and ID., Sidereus Nuncius, Venice, Apud Thomam Baglionum, 1610, p. 11r: «Unum quoque oblivioni minime tradam, quod non nisi aliqua cum admiratione adnotavi: medium quasi Lunae locum a cavitate quadam occupatum esse reliquis omnibus maiori, ac figura perfectae rotunditatis». 32 Ivi, ff. 9v, 10r and 10v. 33 RIGHINI , New Light on Galileo’s Lunar Observations (cit. note 3), pp. 72-74; ID ., Contributo alla interpretazione scientifica dell’opera astronomica di Galileo (cit. note 6), p. 31. 27 28 — 267 — GIORGIO STRANO representation is «a highly distorted and derivative version of the manuscript drawing» and a «symbolic illustration» (Fig. 3 – right).34 The second step in preparing the Starry Messenger, however, consisted in penetrating the structure of some parts of the lunar surface, as its peculiar relief. In another manuscript, Galileo depicts two parallel mountainous chains, a spare crater, and two lunar discs with something near their centres (Fig. 4).35 If we check the observational possibilities, the resolution of a Galilean telescope stopped at about 15 mm is about 10 arc-seconds. Thus, Galileo could really have observed crater Albategnius as a single entity. In the manuscript the size of the crater is realistic. The final engravings, however, were obtained by the sum of the general telescopic aspect of the moon, plus the feature of the particular structure of the lunar relief (craters), plus the intent to explain how a crater is made. In doing this, Galileo had no cartographic intent. Instead, he wanted to illustrate the specific terrestrial nature of the moon in the most expressive manner. The same intent can be found in Galileo’s description of the Sun – which I will not examine here – and two other planets. In 1610, Galileo wrote to Benedetto Castelli that he believed to have observed the phases that Mars should display in the Copernican system when the planet lies between conjunction with and opposition to the Sun.36 However, since the apparent dia- Fig. 3. Galileo’s moon at the last quarter; Left: Biblioteca Nazionale Centrale (BNCF), Florence, Ms. Gal. 48, f. 28r (detail on no. 4); Right: Sidereus nuncius, Venice, 1610, p. 10v (detail). GINGERICH, Dissertatio cum Professore Righini et Sidereo Nuncio (cit. note 4), pp. 85-87. GALILEI, Drawings of the Moon, 1609-1610; Biblioteca Nazionale Centrale (BNCF), Florence, Ms. Gal. 50, f. 68r; see also OG, III, p. 950. 36 ID ., Letter to Benedetto Castelli, December 30, 1610, in OG, X, p. 503. «Quanto a Marte [...] osservandolo da quattro mesi in qua, parmi che in questi ultimi giorni, sendo in mole a pena il terzo di quello che era in Settembre passato, si mostri da oriente alquanto scemo, se già l’effetto non m’inganna, il che non credo». 34 35 — 268 — GALILEO, RELIABLE OBSERVER meter of Mars varies between 4 and 26 arc-seconds, the phenomenon was far beyond the resolution limit of about 10 arc-seconds. After making this first claim, Galileo never mentioned Mars’ phases again. Venus was different. By repeating the observations or by theoretical extrapolation, it is possible to understand what Galileo could have observed. Venus’ phases are well visible, as Galileo wrote to his correspondents.37 The planet shows very smooth extremities of the crescent and fuzzy contours. Nevertheless, a comparison between Galileo’s later representation of Venus in The Assayer 38 and the real telescopic layout reveals three peculiar features (Fig. 5). In each of Venus’ phases as described by Galileo, the apparent diameter of the planet (from 10 to 66 arc-seconds) fits its distance from the Earth, as if the engraved 37 After sending the anagram «Haec immatura a me iam frustra leguntur o y» (i.e.: «Cynthiae figuras aemulatur mater amorum») to Giuliano de’ Medici (see GALILEI, Letter to Giuliano de’ Medici, December 11, 1610, in OG, X, p. 483), Galileo expounded his observations of Venus in two other letters to Christoph Clavius and Benedetto Castelli, both dated December 30, 1610, in OG, X, pp. 499-500 and p. 503 respectively. 38 ID., Il saggiatore: nel quale con bilancia esquisita e giusta si ponderano le cose contenute nella Libra astronomica e filosofica di Lotario Sarsi..., Rome, appresso Giacomo Mascardi, 1623, p. 217. Fig. 4. Galileo’s drawings of the moon; Biblioteca Nazionale Centrale (BNCF), Florence, Ms. Gal. 50, f. 68r. — 269 — Fig. 5. Above: Venus as seen at a resolution of 10 arc-seconds; Below: Venus as seen by Galileo; Il Saggiatore, Rome, 1623, p. 217. GALILEO, RELIABLE OBSERVER relative sizes depended on calculations. Moreover, the extremities of the crescent are not smooth, but sharp. Finally, the external line of the crescent’s contour is smooth, while the internal line is rough. As for the moon in the Starry Messenger, Venus’ engravings are not the result of mere observations, but of an intellectual elaboration. Venus shows the same phases as the moon; therefore, besides proving the heliocentric structure of the world, it is also a terrestrial body like the Moon. Hence, Venus is isomorphic to the moon in every detail, including the pointed extremities of the crescent and the rough line of the terminator. In The Assayer, Galileo depicts Venus as it appears by applying his mind’s eye to the telescope. PROVISIONAL CONCLUSIONS I will not even begin to examine the other Galilean observations: Jupiter and its satellites, the ‘satellites’ of Saturn, sunspots, telescopic ‘star maps’ of the Milky Way and the Ptolemaic nebulae, and the comets. I will hopefully remit any further detailed discussion about such topics to an expanded version of this work. In any case, some concluding remarks about Galileo’s observational activity are indispensable. Since 1610, Galileo felt the urge to communicate his celestial discoveries in the most intuitive manner. Due to his Copernican predisposition, a few exigencies demanded priority: 1) Proving the heliocentric structure of the cosmos. The satellites of Jupiter and the phases of Venus supported this topic. Galileo revealed no scruples about extrapolating the content of the telescopic observations. For example, he replaced the observed apparent sizes of Venus with the calculated ones. He extrapolated the existence of the phases of Mercury from those of Venus. In addition, he welcomed the two ‘satellites’ around Saturn and the phases of Mars. As the latter two phenomena were deemed uncertain, he did not hesitate to remove their existence or their astronomical implications. 2) Proving the unity between the terrestrial and the celestial regions of the cosmos. The surface of the moon and of the Sun – the first displaying high mountains, the second dark clouds – were prominent topics. Again, Galileo did not hesitate to extrapolate the content of his telescopic observations. For example, he assumed that Venus’ surface was uneven like that of the moon. 3) Proving that the real cosmos was much larger than Ptolemy’s. The huge number of stars revealed by the telescope that form the Milky Way — 271 — GIORGIO STRANO and the Ptolemaic nebulae provided an introduction to the issue. Their very faint light suggested that these stars were placed at incredible distances from the Earth. This conclusion, indispensable to the Copernican structure of the cosmos, was however too disconcerting for Galileo’s contemporaries.39 Therefore, Galileo prepared the largest ‘star maps’ of the Starry Messenger starting from pre-existent and traditional material, both as a term of comparison and as an unquestionable ground. 4) Proving that the intimate structure of the universe was geometrical. Galileo systematically dissolved the Ptolemaic nebulae and the nebular appearance of the Milky Way and of comets. He did this by resolving the nebulae, but also by using a misinterpreted property of the telescope. The telescopic magnification affects in a different manner the luminosity of punctiform light sources (stars) and of diffused light sources (real nebulae or the coma and tail of comets). As the magnification increases, diffused light sources tend to disappear while punctiform light sources remain unaltered. As a result, the new universe emerging through the telescope was made of the same dots, circles and other geometrical figures in which the great Book of Nature was written. 39 On the question of cosmic proportion and the incredible distance of the stars in Tycho Brahe’s ‘anti-Copernican campaign’, see M.A. Granada’s contribution to this volume. — 272 — CITTÀ DI CASTELLO . PG FINITO DI STAMPARE NEL MESE DI MAGGIO 2013 ISSN 2038-3533 Comets, ‘new stars’ and other unexpected celestial phenomena up to Galileo’s telescopic discoveries have attracted the interest of historians of science, intellectual and cultural historians. These early modern ‘celestial novelties’ constitute the main subject of this volume, whose aim is to shed light on their reception and interpretation in science, natural philosophy, medicine, and their wider impact on European society. Comete, ‘nuove stelle’ e altri fenomeni imprevedibili, incluse le scoperte telescopiche di Galilei, hanno attirato l’attenzione di storici della scienza, delle idee e della cultura. Le novità celesti della prima modernità costituiscono il tema principale di questo volume, che si propone di far luce sulla loro ricezione e interpretazione nella scienza, nella filosofia naturale, nella medicina, e sul forte impatto che ebbero sulla società europea. ISBN 978 88 222 6254 7