Letters on Sunspots (Istoria e Dimostrazioni intorno alle Macchie Solari) was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it, Galileo outlined his recent observation of dark spots on the face of the Sun. His claims were significant in undermining the traditional Aristotelian view that the Sun was both unflawed and unmoving.A. Bowdoin Van Riper, Science in Popular Culture: A Reference Guide, Greenwood Publishing Group, 2002 p.111 The Letters on Sunspots was a continuation of Sidereus Nuncius, Galileo's first work where he publicly declared that he believed that the Copernican system was correct.
Previous observations of sunspots Galileo was not the first person to observe sunspots. The earliest apparent reference to them appears in the I Ching of ancient China, while the earliest recorded observation is also Chinese, dating to 364 BC.Stefan Hughes, Catchers of the Light: The Forgotten Lives of the Men and Women Who First Photographed the Heavens, ArtDeCiel Publishing, 2012 p.317 Around the same time, the first European mention of sunspots is found, by Theophrastus. There were reports from IslamicJ.M. Vaquero, M. Vázquez, The Sun Recorded Through History, Springer Science & Business Media, 2009 p.75 accessed 29 July 2017. and European astronomers of sunspots in the early ninth century;Arnab Rai Choudhuri, Nature's Third Cycle: A Story of Sunspots, OUP, 2015 p.7 those occurring in 1129 were recorded by both Averroes and John of Worcester, whose drawings of the phenomenon are the earliest surviving today. Johannes Kepler observed a sunspot in 1607 but, like some earlier observers, believed he was watching the transit of Mercury. The sunspot activity of December 1610 was the first to be observed using the newly invented telescope, by Thomas Harriot, who sketched what he saw but did not publish it. In 1611 Johannes Fabricius saw them, and published a pamphlet entitled De Maculis in Sole Observatis, which Galileo was not aware of before he wrote the Letters on Sunspots.
Critical dialogue with Scheiner
When Jesuit Christoph Scheiner first observed sunspots in March 1611, he ignored them until he saw them again in October. Then, under the pseudonym Apelles latens post tabulam (Apelles hiding behind the painting), he presented his description and conclusions about them in three letters to the Augsburg banker and scholar Mark Welser. Scheiner wanted to remain anonymous to avoid involving the Jesuit order and the church generally in an area of controversy. Welser published them on his own presses, sent copies to astronomers around Europe, and invited them to reply.Mario Biagioli, Galileo's Instruments of Credit: Telescopes, Images, Secrecy, University of Chicago Press, 2007 p.174Dennis Danielson, Paradise Lost and the Cosmological Revolution, Cambridge University Press, 2014 p.140 It was Welser's invitation which prompted Galileo to reply with two letters, arguing that the sunspots were not satellites, as Scheiner ('Apelles') maintained, but were features either on the Sun's surface or just above it.
In the meantime, Scheiner sent Welser two further letters on the subject, and after he had read Galileo's first letter, he responded with a sixth of his own. These later letters were different in tone from the first three, as they hinted that Galileo was claiming credit for having discovered the phases of Venus, when in fact proper credit was due to others. They also implied that Galileo had copied Scheiner's helioscope in order to do his research.John Michael Lewis, Galileo in France: French Reactions to the Theories and Trial of Galileo, Peter Lang, 2006 pp. 33–4
Having published Scheiner's first three letters under the title Tres Epistolae de Maculis Solaribus ("Three Letters on Solar Spots"), Welser now published his second three, also in 1612, as De Maculis Solaribus et Stellis circa Iovis Errantibus Accuratior Disquisition ("A More Accurate Disquisition Concerning Solar Spots and Stars Wandering around Jupiter"). Having read these second three letters, Galileo replied with a third of his own, much sharper and more polemical in tone than his earlier ones. Welser declined to publish Galileo's letters, perhaps because of the sarcastic tone they took towards Apelles, although the reason he gave Galileo was the exorbitant cost of producing all the illustrations Galileo wanted.J. L. Heilbron, Galileo, Oxford University Press, 2012 p. 191
Censorship by the Inquisition Publishing the Letters on Sunspots was a major financial and intellectual venture for the Accademia dei Lincei, and it was only the fourth title it had decided to issue. Federico Cesi paid for the publication himself, and wanted to strike a careful balance between introducing extraordinary new ideas and avoiding causing offence to people who might find those views problematic. This was consistent with the Accademia's project of acting as a centre for the dissemination of radical new scientific ideas, issued with the agreement of the Church authorities. Cesi tried to persuade Galileo to avoid an aggressive or polemical tone in his letters, to avoid antagonising the Jesuits (Scheiner's identity behind the pseudonym 'Apelles' was already suspected),John Michael Lewis, Galileo in France: French Reactions to the Theories and Trial of Galileo, Peter Lang, 2006 pp.34 but having read Scheiner's apparent accusations of bad faith in his later letters, Galileo did not heed his advice. Indeed, the published version of his Letters on Sunspots contained a preface by Angelo de Filiis which uncompromisingly asserted Galileo's primacy in discovering sunspots. The text was presented for censorship to the Roman Inquisition in order to obtain permission to print. The censors assigned were Cesare Fidelis, Luigi Ystella, Tommaso Pallavicini and Antonio Bucci.
Ensuring the book was ready to print was a collaborative process involving the censors, Galileo, Cesi and others in working on the text until it was acceptable to the Inquisition, and the censors were well acquainted with the leading figures of the Accademia. Antonio Bucci, for example, was a physician who had previously been involved in reviewing work by Giambattista della Porta, also published by Cesi. In the case of Letters on Sunspots his critical support appears to have been helpful in ensuring that publication was not prevented by influential Dominicans of the Sacred Palace. Indeed, in his comments Bucci praised Galileo's work, with which he was already familiar, as he had been invited to take part in the Accademia's discussions about it before the manuscript was presented for censorship.
The censors insisted that Galileo remove from his text any reference to scripture or claims for divine guidance. Thus the pamphlet was to have opened with a quotation from Matthew 11:12 'The kingdom of heaven suffers violence, and men of violence take it by force.' The censors objected that this could be understood to mean that astronomers wanted to overpower theology. It was therefore amended to 'Already the minds of men assail the heavens, and the more valiant conquer them.' Further on in the text Galileo's claim that 'divine goodness' had led him to advocate the system of Copernicus was struck out, and replaced with 'favourable winds'. Galileo's text referred to the idea that the heavens were immutable as 'erroneous and repugnant to the indubitable truth of Scripture.' Like all other mentions of Scripture, the censors insisted that this too was removed. Galileo wanted to claim divine inspiration for his findings and show how they accorded with Holy Writ; the censors wanted to keep unusual new ideas at a safe distance from core tenets of the faith.William R. Shea & Mariano Artigas, Galileo in Rome, OUP 2003 pp.49–50 With these amendments Galileo was authorised to take his book to print.Thomas F.Mayer, The Roman Inquisition: Trying Galileo, University of Pennsylvania Press, 2015 p.7
Half of the printed edition of 1400 copies of Letters on Sunspots contained both the Apelles Letters and Scheiner's illustrations as well as Galileo's replies. The other half contained Galileo's work only. The total cost of the book was 258.70 scudi, of which 44 scudi was the cost of the illustrations and tables and 6 scudi was the cost of engraving the frontespiece.
Galileo's First Letter - 4 May 1612 Galileo describes how he has observed sunspots for eighteen months. His key conclusions are that sunspots were real and not merely optical illusions; and that they were not static, but moved.William R. Shea & Mark Davie, Selected Writings, Galileo, OUP 2012 p.32 The sunspots had a single motion, moving across the Sun in a uniform fashion. Galileo argued that the Sun was a perfect sphere and that it moves by itself on its own center. The Sun carries these spots until they disappear from view at its rim in about one lunar month.
Scheiner's view that the spots were satellites prompts Galileo to comment on the phases of Venus and how they supported a heliocentric view.William R. Shea & Mark Davie, Selected Writings, Galileo, OUP 2012 p.36 He develops his argument to show that sunspots were not permanent and did not have a regular pattern of movement as they would if they were heavenly bodies – they were nothing like the moons of Jupiter that he had himself discovered and described in Siderius Nuncius. 'The sun, turning on its axis, carries them around without necessarily showing us the same spots, or in the same order, or having the same shape.'William R. Shea & Mark Davie, Selected Writings, Galileo, OUP 2012 p.38 He noted the parallels between sunspots and clouds over the Earth, but did not assert that they were made of the same material. His comment on 'Apelles' (the pseudonym of Scheiner) was:'It seems to me therefore that Apelles has a free, and not a servile mind; he is well able to grasp true teaching; and now, prompted by the strength of so many new ideas, he is beginning to listen and to assent to true and sound philosophy, especially as regards the arrangement of the universe. But he is not yet able to detach himself completely from the fantasies he absorbed in the past, to which his intellect sometimes returns and lends assent by force of long-established habit.'William R. Shea & Mark Davie, Selected Writings, Galileo, OUP 2012 p.39
Much of Galileo's first letter is devoted to demonstrating weaknesses in Scheiner's arguments – inconsistencies, false analogies, and unlikely conclusions from the observations he had made.
Responding to points in Apelles' first letter Apelles says the sunspots move from east to west, when he should have said they moved from west to east. This is not in fact a disagreement about the direction of the spots, but a reminder of the conventions used by astronomers to describe them. From the point of view of the Earth, the sunspots move to move from east to west, but astronomers describe celestial movement from the 'highest' (i.e. furthest away from the Earth) point of their cycles. Apelles has not conclusively demonstrated that the spots cannot be on the surface of the Sun, simply by asserting that because it is bright, it cannot have dark parts. Apelles is wrong to say that sunspots are much darker than dark spots on the Moon; the spots are in fact not as dark as the area immediately around the Sun which is most strongly illuminated by it, and this area is itself so bright that the Moon would be invisible if we tried to observe it in that position.
Responding to points in Apelles' second letter