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. 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. Around the same time, the first European mention of sunspots is found, by Theophrastus. There were reports from Islamic and European astronomers of sunspots in the early ninth century; 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. 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. 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.
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), 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. With these amendments Galileo was authorised to take his book to print. 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. 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. 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.' 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.' 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 Apelles discusses the transit of Venus but is wrong about the size of the planet relative to the Sun; it is so much smaller than Apelles suggests that it may not even be possible for observers to see it making its transit, meaning that the lack of a definite sighting of the transit does not necessarily prove anything. (Scheiner had argued that since a transit of Venus was predicted but not seen, this must mean that Venus had passed behind the Sun, thereby lending support to Tycho Brahe's view that Venus, like all planets apart from the Moon, orbited around the Sun).
Responding to points in Apelles' third letter
Apelles reports that sunspots took around fifteen days to pass across the face of the Sun, and that he never saw the same spots re-emerge on the eastern limb of the Sun fifteen days after they disappeared on the western limb. He concludes that they could not therefore be features carried around the Sun on its surface by a regular rotation. Galileo responds that this would be the case if Apelles had shown that the spots were solid bodies, whereas it is obvious to observers that they are changing shape as they move around the Sun. He therefore says that Apelles has not proved that they could not be on the surface of the Sun. Apelles arguments are inconsistent. When considering his failure to observe a transit of Venus, he concludes that Venus must be behind the Sun (which was possible in Tycho Brahe's model of the universe but impossible in Ptolemy's); however when discussing parallax, in a later part of his argument, he claims that Venus only displays a small parallax (required in Ptolemy's system but impossible in Brahe's). Apelles argues that the spots are not in any of the 'orbs' of the Moon, Venus or Mercury; but according to Galileo these 'orbs', like deferents and epicycles, were only theoretical devices of 'pure astronomers' and not actual physical entities. 'Philosophical astronomers' have no interest in such concepts but are concerned with trying to understand how the universe actually works. Apelles does not even argue consistently on the basis of his assumptions that these orbs and other suppositional devices actually exist, for he says first that if the spots were phenomena in the 'orbs' of the Moon, Venus or Mercury (which only appear to us to be on the face of the Sun) then they would have to move with motion of those planets. However having concluded that the spots are in the 'orb' of the Sun, he maintains that they do not move with the motion of the Sun, but independently of it. Galileo then offers a different explanation to the one Apelles had suggested for the fact that as the sunspots approach the limb of the Sun in their rotation, they grow thinner. Apelles had included a diagram in his third letter to demonstrate how he believed this could be explained in terms of the spots being small moons, which went through phases. Galileo maintained that this was doubtful. As the dark area of sunspots approach the limb of the Sun, it appears from observation that the area of darkness reduces from the side facing away from the Sun – i.e. that the spots are actually getting thinner. If they were moons, the area of darkness would diminish from the side facing the centre of the Sun. Galileo points out inconsistencies in the arguments by Apelles which, in one place, would mean the spots had to be very close to the Sun, and, in another part, that they must be far away from it. The differences in speed between spots moving near the Sun's equator and those further away from it argue for their being on the surface, as the larger the notional 'orb' outside the Sun the spots might be carried on, the less visible this difference in speed would be. Galileo considers the possible 'essence' or substance of the sunspots, and says he does not believe there is yet any way of knowing it. He shows however that of all the things we observe on Earth, it is clouds that share the most characteristics with sunspots. Whatever they may be made of, they are certainly not 'stars' as Apelles suggests, since, as he himself shows, they cannot be observed making regular orbits of the Sun.
Apelles had tried to make the case that the sunspots were similar to two phenomena Galileo had discovered, the moons of Jupiter and the rings of Saturn. Galileo responds that there is no comparison in either case; the moons of Jupiter (Medicean Stars) move with an absolute regularity he has already described, while Saturn simply bears no comparison to the description Apelles provides of it. (Here Galileo provides two simple in-line sketches to show what he means). Galileo assures his reader that he can confirm, after long observation, that Saturn never changes its shape, as Apelles claims, and never will. Mercury, the planet closest to the Sun, completes its transit in about six hours; it makes no sense to propose that spots on some 'orb' which is much closer to the Sun than Mercury would take around fifteen days to complete theirs. Likewise planetary orbits appear constant in their speed, whereas Apelles has shown that sunspots move rapidly in the centre of the Sun but more slowly at its edges.
Galileo's Second letter – 14 August 1612
Galileo's second letter restates the key propositions from his first letter, and is otherwise mostly concerned with geometric proofs that the spots are on the surface of the Sun rather than above it. To accompany these proofs Galileo provides 38 detailed illustrations, which allow the reader to see how his observations relate to his calculations.
Further observations confirm what Galileo originally believed – that the spots were on or very close to the Sun, which carries them round as it rotates on its own axis. He notes that as sunspots approach the limit of their movement cross the visible field of the Sun, at the point where they are seen 'sideways on' from Earth, they sometimes appear as thin as a thread; if, as Apelles maintained, the spots were satellites, they would be clearly set apart from the surface of the Sun at this point. The apparent acceleration of the spots as they approach the centre of the Sun and their slower speeds towards the edges, are perfectly consistent with a circular rotation on the surface. The growth in apparent size of the gaps between spots as they approach the centre, and their apparent diminution towards the edges of the Sun, likewise confirm this. He uses a geometrical diagram to demonstrate the effects of foreshortening, showing how if the sunspots were removed from the surface of the Sun by even a twentieth part of its diameter, there would be a very observable difference in the visible foreshortening effect. The apparent distance to the observer from C to F is seven times smaller than the actual distance on the surface of the Sun from C to H; however, if the spots are just a small way above the surface of the Sun, the apparent distance from C to F corresponds to the actual distance from R to N, which is less than a third the length of C to H. Thus by measuring the differences in the apparent distances between spots as they move across the Sun, it is possible to know with certainty whether the foreshortening corresponds to the proportion CF:CH or to some other proportion. The changes in apparent distance observed leave no doubt on this question. He uses a second diagram to demonstrate the gaps between sunspots which can be seen right up to the point where they disappear at the limb of the Sun. This, he says, means they must be low against the sun and thin, rather than high above its surface and thick. Galileo then counters a number of arguments that might be put forward to show that sunspots are an effect in the Earth's atmosphere. These were not arguments Apelles had advanced; rather, he had also argued against them. Galileo's points were made for the sake of completeness, although, as he argues 'it is not necessary to waste time in re-examining every other conceivable position [for the sunspots], for anyone will immediately encounter manifest impossibilities and contradictions himself, so long as he has understood the phenomena I have recounted above.'. He says that because the spots change shape, it is difficult to be certain whether some complete a full revolution and reappear in changed form after disappearing round to the dark side of the Sun for fourteen or fifteen days. However he believes that this does in fact happen. 'I am inclined to this belief upon seeing a very large one appear and grow continuously while the visible hemisphere turns; since it is credible that it was generated long before its arrival, so it is reasonable to believe that it can last after its departure, such that its duration will be much longer than the time of half a revolution of the Sun. Therefore, some spots can doubtless, or rather necessarily, be seen twice by us. He considers arguments about the natural inclination of bodies for different kinds of motion in order to judge whether the spots are on the surface of the Sun or in its atmosphere, and concludes that the regularity of sunspot motion argues that they 'originate in a solid and firm body where the motion of the whole and of the parts is a single one.' (However, in his Third Letter he argued, against Scheiner, that 'there is no one so simple as to grant that the Sun is hard and immutable'). He describes his method of observing and recording sunspots, discovered by Benedetto Castelli. This is by way of explaining to the reader that the thirty-eight illustrations which follow are highly accurate (i.e. unlike Scheiner's). His last main point addresses those who say that his ideas and observations contradict Aristotle. 'If he argued for the immutability of the heavens because in times past no alteration whatsoever had been seen in them, it is entirely credible that if vision had demonstrated to him the things that it makes manifest to us, he would have arrived at the opposite conclusion. And I will further say that I think I contradict Aristotle's doctrine much less... with the supposition of mutable celestial material, than do those who would prefer to treat it as inalterable, because I am sure that he was never as certain of the conclusion of inalterability as he was of the notion that all human discourse must defer to evident experience.' He adds a postscript to say that while he was undertaking his observations, a sunspot appeared which was so large it could be seen with the naked eye between 19 and 21 August 1612. This is included in his series of illustrations.
Galileo's Third Letter – 1 December 1612 While Galileo's First and Second Letters had been written in response to Scheiner's Tres Epistolae, his Third Letter responded to Accuratior Disquisitio. Galileo was angry to see that once again Scheiner was making claims about the moons of Jupiter, since he regarded them as his own discovery. To demonstrate the falsehood of Scheiner's assertion that the moons of Jupiter were 'wandering stars', unpredictable in their movement, as well as to display his own clear superiority in observation and calculation of celestial movements, Galileo appended a complete set of Ephemerides for the Jovian moons to his third letter. Galileo shows the critical flaws in Scheiner's geometry, his understanding of the authorities he cites, his reasoning, his observations and indeed his own drawings.
Introduction Galileo says there is no point in speculating about the 'essence' of sunspots, or indeed of other things, but since writing his last letter he has spent time thinking about the uniform motion of the sunspots within a specific band around the Sun's surface. He asks, in passing, 'is there not still a controversy over whether the Earth itself remains immobile, or wanders?', which is an oblique reference to the idea, required by Copernicus' model of the universe, that the earth must rotate on its own axis every day. Lastly, he humorously compares scholars who insist that every detail of Aristotle's writing must be true, whether it corresponds with reality or not, with those artists who draw portraits of people in fruit and vegetables. 'As long as these oddities are offered as jokes, they are nice and pleasing... but if someone, perhaps because he had consumed all his studies in a similar style of painting, then wanted to draw the general conclusion that every other method of imitating was imperfect and blameworthy, surely Cigoli and other celebrated painters would laugh at him.'
Venus, sunspots and use of authorities Galileo takes up once again the question of whether there is any relation between the transit of Venus and sunspots. He criticises 'Apelles' for setting out a long and complex demonstration of the movement of Venus across the face of the Sun, when it was superfluous to his purpose. He criticises him further for giving an estimate of Venus's size as it crosses the Sun which is wrong, and for supporting this estimate with learned authorities from the past who did not have telescopes. Furthermore, Galileo argues, some of the ancient astronomers, including Ptolemy, made more cogent arguments than 'Apelles' suggests. Galileo notes that 'Apelles' has shifted his view on sunspots since his first letter. At first he insisted they were all spherical, like little moons; now he says they are irregular in shape, forming and dissolving. He previously said that the spots were at various distances from the Sun, wandering between it and Mercury, but he no longer maintains this view. 'Apelles' argues that the hardness and solidity of the Sun means that the fluid spots cannot be on its surface; but citing the authority of the ancients to confirm the Sun's solidity is pointless, since they had no idea of its structure; in any case the evidence of the spots themselves suggests the very opposite to the traditional view of the Sun's hardness. He agrees with 'Apelles' view that the spots are not chasms or pools on the Sun's surface, but nobody had ever argued that they were.
The movement of sunspots A large portion of the Third Letter is taken up with disproving Apelles' assertion that he had observed spots passing across the Sun at different speeds – one, on the diameter, taking sixteen days, and another, at a lower latitude, in just fourteen. (If sunspots moved at differential speeds, this tended to suggest they were moons moving independently of the Sun itself). Galileo says that in his own observations he has never seen this differential rate of movement, but that spots always move at a constant speed relative to each other. First Galileo demonstrates that points on two different sunspot trajectories at two different latitudes produce lines which maintain a constant proportion with each other at any point in the rotation. Next he shows that the larger the sphere on which sunspots appear, the less differential there is in their transit times at the same two latitudes. Finally, he shows that for a spot to move along the diameter of the Sun in a period 11⁄7 as long as another spot at a latitude 30° higher, the diameter of the Sun would need to be more than twice as great as observed. From this he concludes that Apelles is simply wrong, and it is not possible for one spot to traverse the Sun in sixteen days, while another takes only fourteen. Now Galileo turns to Apelles' illustrations of sunspots, and begins to use them to show how his arguments about sunspot motion are false. He recalls how Apelles depicts them coming into view, foreshortened, before appearing at their full width. He then demonstrates that for the spots Apelles had observed to change in apparent size as they did, they would need to be on the face of the Sun, because if they were even a short distance above its surface the foreshortening effect would be remarkably different. Galileo challenges Apelles' assertion that he had seen different spots moving at different speeds; particularly that he had seen spots on the Sun's diameter rotate more rapidly than those at higher latitudes. This, he says, is contradicted not only by observation but by Apelles' own statement in another place in his work that spots in the middle of the Sun remain longer than those passing nearer its limb. Finally, Apelles' own illustrations clearly show spots transiting the Sun in around 141⁄2 days, and nothing in his illustrations supports his contention that some take 16, and others 9.
Observations on other planets Having disproved Apelles' arguments on sunspots, Galileo addresses a number of his other errors. He briefly responds to Apelles' views on extraterrestrial life; then disposes of the idea that the Moon is translucent. He then returns to Apelles' analogy between sunspots and the moons of Jupiter, where he notes that Apelles has subtly moved from arguing that sunspots are like planets, to arguing that planets are like sunspots. 'Carried away by the desire to maintain what he had originally said, and unable to accommodate the spots exactly to the properties once associated with the other stars, [Apelles] has accommodated the stars to the properties that we know belong to the spots.' To dispense once and for all with Apelles' claim that the moons of Jupiter 'appear and disappear', Galileo provides predictions for their positions for the next two months to prove the regularity of their motions. To demonstrate that natural philosophy must always be led by observation and not try to fit new facts into preconceived frameworks, Galileo comments that the planet Saturn had recently and surprisingly changed its appearance. In his First Letter, he had argued that Saturn never changes its shape, and never will. Now, he agrees, it has changed shape. He does not try to prove his earlier views right in spite of new facts, but makes cautious predictions about how its appearance may change in future. Galileo concludes his remarks by criticising those who doggedly adhere to Aristotle's views, and then, drawing together all he has said about sunspots, the moons of Jupiter, and Saturn, ends with the first explicit endorsement of Copernicus in his writings:
I think it is not the act of a true philosopher to persist – if I may say so – with such obstinacy in maintaining Peripatetic conclusions that have been found to be manifestly false, believing perhaps that if Aristotle were here today he would do likewise, as if defending what is false, rather than being persuaded by the truth, were the better index of perfect judgement... [and] I say to your Lordship that this star too [i.e. Saturn] and perhaps no less than the emergence of the horned Venus, agrees in a wondrous manner with the harmony of the great Copernican system, to whose universal relations we see such favourable breezes and bright escorts directing us.
Significance of Letters on Sunspots
Ideas The common belief until Galileo's time was that the heavens beyond the Moon were both perfect and unchanging. Many of the arguments between Scheiner and Galileo were about things observed in the skies that appeared to be changing, and what the nature and significance of that change was. Although the behaviour of sunspots was the main topic of their debate, they also touched on other disputes, such as the phases of Venus and the moons of Jupiter. In a letter to Federico Cesi, Galileo said: 'I have finally concluded, and I believe I can demonstrate necessarily, that they [i.e. the sunspots] are contiguous to the surface of the solar body, where they are continually generated and dissolved, just like clouds around the earth, and are carried around by the sun itself, which turns on itself in a lunar month with a revolution similar [in direction] to those other of the planets... which news will be I think the funeral, or rather the extremity and Last Judgement of pseudophilosophy.... I wait to hear the spoutings of great things from the Peripatetics to maintain the immutability of the skies.'
'Flaws' in the Sun The cosmology of Galileo's time, based on Aristotle's Physics, held that the Sun was 'perfect' and unflawed. Only with the invention of the telescope was it possible for sunspots to be systematically observed. Many who had never seen them found the idea of them morally and philosophically repugnant. Those who could see them, like Scheiner, wanted to find an explanation for them within the Aristotelian system. Galileo's arguments in Letters on Sunspots were intended to demonstrate these claims as false; and if they were false, Aristotelian assumptions about the universe could not be true.
Moons of Jupiter Galileo had discovered the moons of Jupiter in 1609. Scheiner argued that what appeared to be spots on the Sun were in fact clusters of small moons, thereby trying to deploy one of Galileo's own discoveries as an argument for the Aristotelian model. In his Letters on Sunspots Galileo showed how sunspots were nothing like the moons of Jupiter, and the comparison was false. Scheiner claimed that the sunspots, with their irregular movements, were like the moons of Jupiter whose positions were similarly hard to predict. To counter this argument, Galileo published tables of predictions for the future position of the moons of Jupiter, so that astronomers could easily distinguish between the regular, predictable movements they followed with the ephemeral and irregular sunspots.
Rotation of the Sun Showing that the Sun rotated had two effects. Firstly, it showed that the traditional Aristotelian model of the universe must be wrong, because that model assumed that the Sun had only a diurnal (daily) motion around the earth, and not a rotation on its own axis. Secondly, it showed that there was nothing necessarily unusual about rotation of a body in space. In the Aristotelian system, night and day were explained by the Sun moving round a static Earth. For Copernicus' system to work, there had to be an explanation for why half the Earth was not in permanent daylight, and the other in permanent darkness, as it completed its annual motion around the Sun. This explanation was that the Earth rotated on its own axis once every day. However it was very difficult to prove that the Earth was rotating, so to show that the Sun rotated made the Copernican model at least more plausible. While the rotation of the Sun did not prove Copernicus right, it proved his opponents wrong and made his ideas more likely to be true.
Phases of Venus In the Letters on Sunspots Galileo responded to claims by Scheiner about the phases of Venus, which were an important question in the astronomy of the time. There were different schools of thought about whether Venus had phases at all – to the naked eye, none were visible. In 1610, using his telescope, Galileo had discovered that Venus, like the Moon, had a full set of phases, but only in Letters on Sunspots did he commit this finding to publication. The fact that there was a full phase of Venus, (similar to a full moon) when Venus was in the same direction in the sky as the Sun meant that at a certain point in its orbit, Venus was on the other side of the Sun to the Earth. This indicated that Venus went around the Sun, and not around the Earth. This provided important evidence in support of the Copernican model of the universe.
Copernicus At least as early as 1597, Galileo had concluded that the Copernican model of the universe was correct but had not publicly advocated this position. In Siderius Nuncius Galileo included in his dedication to the Grand Duke of Tuscany the words ' while all the while with one accord they [i.e. the planets] complete all together mighty revolutions every ten years round the centre of the universe, that is, round the Sun.' In the body of the text itself, he stated briefly that in a forthcoming work, 'I will prove that the Earth has motion', which is an indirect allusion to the Copernican system, but that is all. Copernicus is not mentioned by name. It is at the end of the Third Letter that Galileo explicitly declares his belief in the Copernican system.
Movement of the Sun Galileo remarks in one passage that the Sun might not be revolving, but in another he states more definitely that the Sun does have a motion, and wonders what causes it. Here he establishes a connection between cosmology and mechanics. Galileo wrote, "I seem to have observed that physical bodies have physical inclination to some motion." Letters in Sunspots is also the first of his works to mention the concept of inertia, which would later become Newton's first law of motion.
Language While Scheiner wrote his letters in Latin, Galileo's reply was in Italian. Scheiner did not speak Italian, so Welser had to have Galileo's letters translated into Latin so he could read them. This was not the first time Galileo had published in Italian, and Galileo was not the first natural philosopher to publish in Italian (for example Lodovico delle Colombe's account of the 1604 supernova was in Italian, as was Galileo's reply). However Letters on Sunspots was the first book the Accademia dei Lincei published in Italian. Galileo later said of his preference for Italian over Latin:
'I wrote in Italian because I wished everyone to be able to read what I wrote.... I see young men.... who, although furnished.... with a decent set of brains, yet not being able to understand things written in gibberish [i.e. Latin], take it into their heads that in these crabbed folios there must be some grand hocus-pocus of logic and philosophy much too high up for them to think of jumping at. I want them to know, that as nature has given eyes to them, just as well as to philosophers, for the purpose of seeing her works, she has also given them brains for examining and understanding them.' While Scheiner's lack of Italian hindered his response to Galileo in 1612 while they corresponded through Welser, it also meant that when Galileo published Il Saggiatore in 1623, which accused Scheiner of plagiarism, Scheiner was unaware of this until he happened to visit Rome the following year.
Use of diagrams and illustrations
Most readers of the time did not have a telescope, so could not see sunspots for themselves – they relied on descriptions and illustrations to make clear what they looked like. For this reason the quality and number of illustrations was essential in building public understanding. Scheiner's book of letters had contained illustrations of sunspots which were mostly 2.5 cm in diameter, leaving little space for detail and portraying sunspots as solid, dark entities. Scheiner himself had described them as 'not terribly exact' and 'drawn without precise measurement'. He also indicated that his drawings were not to scale, and the spots in his illustration had been drawn disproportionately large 'so that they would be more conspicuous.' A reader looking at these illustrations might be inclined to agree with Scheiner's view that sunspots were probably planets. Although the sunspots were constantly changing position, Scheiner presented his observations over a period of six weeks in a single fold out plate. All of his figures are small except for the observations in the top left corner. He admitted to his readers that his drawings were not made to scale, and that other factors such as variations in the weather, lack of time, or other impediments may have reduced their accuracy. Scheiner also showed the formation of spots in different orientations. Sometimes the configurations of the spots were linear following consecutive days, but the orientations became more complex over time that there was a lack of an obvious pattern. For Galileo to persuade his readers that sunspots were not planets but a much more transient and nebulous phenomenon, he needed illustrations which were larger, more detailed, more nuanced, and more 'natural.' Letters on Sunspots carried 38 engravings of sunspots, providing a visual narrative of the sun's appearance from 2 June – 8 July 1612, with some additional illustrations from August. This extensive visual representation, with its large scale and high-quality reproduction, allowed readers to see for themselves how sunspots waxed and waned as the sun rotated. The impact of this series of illustrations was to create a near-photographic sense of reality. This sense undermined the claims made by Scheiner before any argument was mounted to refute them.
Galileo and Prince Cesi selected Matthaeus Greuter to create the sunspot illustrations. Originally from Strasbourg and a convert from Protestantism, Greuter moved to Rome and set up as a printer specialising in work for the Jesuit order. His work ranged from devotional images of saints through to mathematical diagrams. This relationship may have recommended him as one whose involvement in a publication would perhaps ease its path through censorship; in addition his craftsmanship was outstanding, and he devised a novel etching technique specially in order to make the sunspot illustrations as realistic as possible. Galileo drew sunspots by projecting an image of the Sun through his helioscope onto a large piece of white paper, on which he had already used a compass to draw a circle. He then sketched the sunspots in as they appeared projected onto his sheet. To make his illustrations as realistic as possible, Greuter reproduced them at full size, even with the mark of the compass point from Galileo's original. Greuter worked from Galileo's original drawings, with the verso on the copperplate and the image traced through and etched. The cost of the thirty-eight copperplates was significant, amounting to fully half of the production costs of the edition. Because half the copies of the Letters also contained the Apelles Letters, Greuter reproduced the illustrations that Alexander Mair had done for Scheiner's book, allowing Galileo's readers to compare two distinct views of