Letters on Sunspots
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'' Letters on Sunspots '' (''Istoria e Dimostrazioni intorno alle Macchie Solari'') was a pamphlet written by
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
in 1612 and published in
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...
by the
Accademia dei Lincei The Accademia dei Lincei (; literally the "Academy of the Lynx-Eyed", but anglicised as the Lincean Academy) is one of the oldest and most prestigious European scientific institutions, located at the Palazzo Corsini on the Via della Lungara in Rom ...
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 ''Sidereus Nuncius'' (usually ''Sidereal Messenger'', also ''Starry Messenger'' or ''Sidereal Message'') is a short astronomical treatise (or ''pamphlet'') published in New Latin by Galileo Galilei on March 13, 1610. It was the first published ...
,'' 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
sunspot Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s. The earliest apparent reference to them appears in the
I Ching The ''I Ching'' or ''Yi Jing'' (, ), usually translated ''Book of Changes'' or ''Classic of Changes'', is an ancient Chinese divination text that is among the oldest of the Chinese classics. Originally a divination manual in the Western Zho ...
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 Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek philosopher and the successor to Aristotle in the Peripatetic school. He was a native of Eresos in Lesbos.Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routledge ...
. 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; those occurring in 1129 were recorded by both
Averroes Ibn Rushd ( ar, ; full name in ; 14 April 112611 December 1198), often Latinized as Averroes ( ), was an Andalusian polymath and jurist who wrote about many subjects, including philosophy, theology, medicine, astronomy, physics, psycholog ...
and
John of Worcester John of Worcester (died c. 1140) was an English monk and chronicler who worked at Worcester Priory. He is usually held to be the author of the ''Chronicon ex chronicis''. ''Chronicon ex chronicis'' The ''Chronicon ex chronicis'' is a world wide ...
, whose drawings of the phenomenon are the earliest surviving today.
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
observed a sunspot in 1607 but, like some earlier observers, believed he was watching the transit of
Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
. The sunspot activity of December 1610 was the first to be observed using the newly invented
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
, by
Thomas Harriot Thomas Harriot (; – 2 July 1621), also spelled Harriott, Hariot or Heriot, was an English astronomer, mathematician, ethnographer and translator to whom the theory of refraction is attributed. Thomas Harriot was also recognized for his cont ...
, who sketched what he saw but did not publish it. In 1611
Johannes Fabricius Johann Goldsmid, better known by his Latinisation of names, Latinized name Johann(es) Fabricius (8 January 1587 – 19 March 1616), eldest son of David Fabricius (1564–1617), was a Frisians, Frisian/Germans, German astronomer and a discoverer of ...
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 Augsburg (; bar , Augschburg , links=https://en.wikipedia.org/wiki/Swabian_German , label=Swabian German, , ) is a city in Swabia, Bavaria, Germany, around west of Bavarian capital Munich. It is a university town and regional seat of the ' ...
banker and scholar
Mark Welser Mark Welser (1558–1614) was a German banker, politician, and astronomer, who engaged in learned correspondence with European intellectuals of his time. Of particular note is his exchange with Galileo Galilei, regarding sunspots. Biography We ...
. 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 The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme cr ...
, when in fact proper credit was due to others. They also implied that Galileo had copied Scheiner's
helioscope A helioscope is an instrument used in observing the sun and sunspots. The helioscope was first used by Benedetto Castelli (1578-1643) and refined by Galileo (1564–1642). The method involves projecting an image of the sun onto a white sheet of pa ...
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 Federico Angelo Cesi (; 26 February 1585 – 1 August 1630) was an Italian scientist, naturalist, and founder of the Accademia dei Lincei. On his father's death in 1630, he became briefly lord of Acquasparta. Biography Federico Cesi was ...
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 Angelo is an Italian masculine given name and surname meaning "angel", or "messenger". People People with the given name *Angelo Accattino (born 1966), Italian prelate of the Catholic Church *Angelo Acciaioli (bishop) (1298–1357), Italian Rom ...
which uncompromisingly asserted Galileo's primacy in discovering sunspots. The text was presented for censorship to the
Roman Inquisition The Roman Inquisition, formally the Supreme Sacred Congregation of the Roman and Universal Inquisition, was a system of partisan tribunals developed by the Holy See of the Roman Catholic Church, during the second half of the 16th century, respons ...
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 Giambattista della Porta (; 1535 – 4 February 1615), also known as Giovanni Battista Della Porta, was an Italian scholar, polymath and playwright who lived in Naples at the time of the Renaissance, Scientific Revolution and Reformation. Giamba ...
, 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 The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme cr ...
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 There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...
that he had himself discovered and described in ''
Siderius Nuncius ''Sidereus Nuncius'' (usually ''Sidereal Messenger'', also ''Starry Messenger'' or ''Sidereal Message'') is a short astronomical treatise (or ''pamphlet'') published in New Latin by Galileo Galilei on March 13, 1610. It was the first published ...
''. '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 frameless, upright=0.5 A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) the solar disk. During a trans ...
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 There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...
and the
rings of Saturn The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
. 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 or 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 Benedetto Castelli (1578 – 9 April 1643), born Antonio Castelli, was an Italian mathematician. Benedetto was his name in religion on entering the Benedictine Order in 1595. Life Born in Brescia, Castelli studied at the University of Padua and l ...
. 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 In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...
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 Lodovico Cardi (21 September 1559 – 8 June 1613), also known as Cigoli, was an Italian painter and architect of the late Mannerist and early Baroque period, trained and active in his early career in Florence, and spending the last nine years ...
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 1 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 14 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,
pelles The Fisher King is a figure in Arthurian legend, the last in a long line of British kings tasked with guarding the Holy Grail. The Fisher King is both the protector and physical embodiment of his lands, but a wound renders him incapable and hi ...
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... ndI say to your Lordship that this star too .e. Saturnand 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 The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme cr ...
and the
moons of Jupiter There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...
. In a letter to
Federico Cesi Federico Angelo Cesi (; 26 February 1585 – 1 August 1630) was an Italian scientist, naturalist, and founder of the Accademia dei Lincei. On his father's death in 1630, he became briefly lord of Acquasparta. Biography Federico Cesi was ...
, Galileo said: 'I have finally concluded, and I believe I can demonstrate necessarily, that they .e. the sunspotsare 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 n directionto 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.'John Michael Lewis, Galileo in France: French Reactions to the Theories and Trial of Galileo, Peter Lang, 2006 p.94


'Flaws' in the Sun

The
cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...
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 There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...
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 The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme cr ...
, 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 in 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 .e. the planetscomplete 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 Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...
and
mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objects r ...
. 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 Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
, which would later become
Newton's First Law of Motion Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows: # A body remains at rest, or in 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 Natural philosophy or philosophy of nature (from Latin ''philosophia naturalis'') is the philosophical study of physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior throu ...
to publish in Italian (for example
Lodovico delle Colombe Lodovico delle Colombe (1565(?) – after 1623) was an Italian Aristotelian scholar, famous for his battles with Galileo Galilei in a series of controversies in physics and astronomy. Early life Delle Colombe was born in Florence in the second ...
'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 The Accademia dei Lincei (; literally the "Academy of the Lynx-Eyed", but anglicised as the Lincean Academy) is one of the oldest and most prestigious European scientific institutions, located at the Palazzo Corsini on the Via della Lungara in Rom ...
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 .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.'J.L. Heilbron, Galileo, Oxford University Press, 2012 p.184 ''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 Strasbourg (, , ; german: Straßburg ; gsw, label=Bas Rhin Alsatian, Strossburi , gsw, label=Haut Rhin Alsatian, Strossburig ) is the prefecture and largest city of the Grand Est region of eastern France and the official seat of the Eu ...
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 A helioscope is an instrument used in observing the sun and sunspots. The helioscope was first used by Benedetto Castelli (1578-1643) and refined by Galileo (1564–1642). The method involves projecting an image of the sun onto a white sheet of pa ...
onto a large piece of white paper, on which he had already used a
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
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 ' is the "right" or "front" side and ''verso'' is the "left" or "back" side when text is written or printed on a leaf of paper () in a bound item such as a codex, book, broadsheet, or pamphlet. Etymology The terms are shortened from Latin ...
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 the sunspots. He reduced Mair's drawings further in size, and converted nine of the twelve from etchings or engravings into woodcuts, which lacked the subtlety of Mair's originals. Scheiner was evidently impressed by Greuter's work, as he commissioned him to create the illustrations for his own
magnum opus A masterpiece, ''magnum opus'' (), or ''chef-d’œuvre'' (; ; ) in modern use is a creation that has been given much critical praise, especially one that is considered the greatest work of a person's career or a work of outstanding creativity, ...
''Rosa Ursina'' in 1626. The 1619 work Galileo co-wrote with
Mario Guiducci Mario Guiducci (Florence 18 March 1583 - Florence 5 November 1646) was an Italian scholar and writer. A friend and colleague of Galileo, he collaborated with him on the '' Discourse on Comets'' in 1618. Early life Mario Guiducci was born in the ...
, ''Discourse on Comets'', mocked Scheiner for the 'badly colours and poorly-drawn images' in his work on Sunspots.


Making predictions to test a hypothesis

In modern science
falsifiability Falsifiability is a standard of evaluation of scientific theories and hypotheses that was introduced by the philosopher of science Karl Popper in his book ''The Logic of Scientific Discovery'' (1934). He proposed it as the cornerstone of a sol ...
is generally considered important. In ''
De revolutionibus orbium coelestium ''De revolutionibus orbium coelestium'' (English translation: ''On the Revolutions of the Heavenly Spheres'') is the seminal work on the heliocentric theory of the astronomer Nicolaus Copernicus (1473–1543) of the Polish Renaissance. The book, ...
'' Copernicus had published both a theoretical description of the universe and a set of tables and calculating methods for working out the future positions of the planets. In ''Letters on Sunspots'' Galileo did as Copernicus had done – he elaborated his ideas on the form and substance of sunspots, and accompanied this with tables of predictions for the position of the moons of Jupiter. In part this was to demonstrate that Scheiner was wrong in comparing sunspots with the moons. More generally, Galileo was using his predictions to establish the validity of his ideas – if he could be demonstrably right about the complex movements of many small moons, his readers could take that as a token of his wider credibility. This approach was the opposite of the method of Aristotelian astronomers, who did not build theoretical models based on data, but looked for ways of explaining how the available data could be accommodated within existing theory.


Scholarly reception

Some astronomers and philosophers, such as Kepler, did not publish views on the ideas in Galileo's ''Letters on Sunspots''. Most scholars with an interest in the topic divided into those who supported Scheiner's view that sunspots were planets or other bodies above the surface of the Sun, or Galileo's that they were on or very near its surface. From the middle of the seventeenth century the debate about whether Scheiner or Galileo was right died down, partly because the number of sunspots was drastically reduced for several decades in the Maunder Minimum, making observation harder. After the
Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...
was built in 1667, Jean-Dominique Cassini instituted a programme of systematic observations, but he and his colleagues could find little pattern in the appearance of sunspots after many years of observation. However Cassini's observation did bear out Galileo's argument that sunspots indicated that the Sun was rotating, and Cassini did discover the rotation of Mars and Jupiter, which supported Galileo's contention that both the Earth and the Sun rotated.


Christoph Scheiner

As Cesi had feared, the hostile tone of the ''Letters on Sunspots'' towards Scheiner helped turn the Jesuits against Galileo. In 1619,
Mario Guiducci Mario Guiducci (Florence 18 March 1583 - Florence 5 November 1646) was an Italian scholar and writer. A friend and colleague of Galileo, he collaborated with him on the '' Discourse on Comets'' in 1618. Early life Mario Guiducci was born in the ...
published ''A Discourse on Comets'', which was actually mostly written by Galileo, and which included an attack on Scheiner, although its focus was the work of another Jesuit,
Orazio Grassi Orazio Grassi, S.J. (b. Savona 1 May 1583 – d. Rome 23 July 1654), was an Italian Jesuit priest, who is best noted as a mathematician, astronomer and architect. He was one of the authors in controversy with Galileo Galilei on the nature of come ...
. In 1623, Galileo wrote ''Il Saggiatore'' (''The Assayer''), which accused Scheiner of trying to steal Galileo's ideas. In 1624, on a visit to Rome, Scheiner discovered that in ''The Assayer'', Galileo had accused him of plagiarism. Furious, he decided to stay in Rome and devote himself to proving his own expertise in sunspots. His major work on the topic was ''Rosa Ursina'' (1626–1630). It is widely believed, though there is no direct evidence, that the bitter dispute with Scheiner was a factor in bringing Galileo to trial in 1633, and indeed that Scheiner may have worked behind the scenes to bring the trial about. As a result of pursuing this dispute with Galileo and the years of research it entailed, Scheiner eventually became the world's leading expert on sunspots.


Raffaelo delle Colombe

Together with Niccolò Lorini and
Tommaso Caccini Tommaso Caccini (1574–1648) was an Italian Dominican friar and preacher. Born in Florence as Cosimo Caccini, he entered into the Dominican order of the Catholic Church as a teenager. Caccini began his career in the monastery of San Marco and gra ...
, delle Colombe was one of three Florentine Dominicans who opposed Galileo. Along with Raffaelo's brother
Lodovico delle Colombe Lodovico delle Colombe (1565(?) – after 1623) was an Italian Aristotelian scholar, famous for his battles with Galileo Galilei in a series of controversies in physics and astronomy. Early life Delle Colombe was born in Florence in the second ...
they formed what Galileo called the 'Pigeon League'. Caccini and delle Colombe both used the pulpit to preach against Galileo and the ideas of Copernicus, but only delle Colombe is known to have preached, on two separate occasions, against Galileo's ideas about sunspots. The first occasion was 26 February 1613, when his sermon concluded with these words:
'That ingenious Florentine mathematician of ours .e. Galileolaughs at the ancients who made the sun the most clear and clean of even the smallest spot, whence they formed the proverb 'to seek a spot on the sun.' But he, with the instrument called by him a telescope makes visible that it has regular spots, as by observation of days and months he had demonstrated. But this more truly God does, because 'the heavens are not of the world in His sight'. If spots are found in the suns of the just, do you think they will be found in the moons of the unjust?'
The second sermon against sunspots was on 8 December 1615, when the ''Letters on Sunspots'' had already been referred to the Inquisition for review. The sermon was delivered in
Florence cathedral Florence Cathedral, formally the (; in English Cathedral of Saint Mary of the Flower), is the cathedral of Florence, Italy ( it, Duomo di Firenze). It was begun in 1296 in the Gothic style to a design of Arnolfo di Cambio and was structurally c ...
on the
Feast of the Immaculate Conception The Solemnity of the Immaculate Conception, also called Immaculate Conception Day, celebrates the sinless lifespan and Immaculate Conception of the Blessed Virgin Mary on 8 December, nine months before the feast of the Nativity of Mary, celebrate ...
.
'an ingenious academic took for his device a mirror in the face of the sun with the motto 'it shows what is received'. That means he had carved in his spirit I do not know what kind of beloved sun. But what would be better for Mary? Who could fixedly look at the infinite light of the Divine Sun, were it not for this virginal mirror, that in itself conceives it he lightand renders it to the world? 'Born to us, given to us from an intact virgin?' This is 'Let what is received, be shown'. For one who seeks defects where there are none, is it not to be said to him 'he seeks a spot in the sun?' The sun is without spot, and the mother of the sun is without spot, from where Jesus is born.'


The Roman Inquisition

On 25 November 1615, the Inquisition decided to investigate the ''Letters on Sunspots'' because it had been mentioned by
Tommaso Caccini Tommaso Caccini (1574–1648) was an Italian Dominican friar and preacher. Born in Florence as Cosimo Caccini, he entered into the Dominican order of the Catholic Church as a teenager. Caccini began his career in the monastery of San Marco and gra ...
and Gianozzo Attavanti in their complaint about Galileo. Copies of the text were issued to the Inquisition's theological experts on 19 February 1616. On the morning of 23 February they met and agreed two propositions to be censured (that the Sun is the centre of the world, and that the Earth is not the centre of the world, but moves). Neither proposition is contained in ''Letters on Sunspots''. Shortly after the decision of the Inquisition, the
Congregation of the Index The ''Index Librorum Prohibitorum'' ("List of Prohibited Books") was a list of publications deemed heretical or contrary to morality by the Sacred Congregation of the Index (a former Dicastery of the Roman Curia), and Catholics were forbidde ...
placed Copernicus' ''De Revolutionibus'' on the Index. ''Letters on Sunspots'' was however not banned or required to undergo corrections. This meant that while Catholic scholars could no longer discuss heliocentrism, they could discuss the nature and origin of sunspots freely.


Francesco Sizzi

In 1611, before the ''Letters on Sunspots'' appeared,
Francesco Sizzi Francesco Sizzi, an Italian astronomer who lived during the 17th century, is credited with being the first to notice the annual movement of sunspots. He also argued against the existence of the Galilean satellites of Jupiter, discovered by Galileo ...
had published ''Dianoia Astronomica'', attacking the ideas of Galileo's earlier work, ''Siderius Nuncius''. In 1612 he went to Paris and devoted himself to the study of sunspots. In 1613 he wrote to Galileo's friend Orazio Morandi, confirming that his circle of colleagues in France agreed with Galileo that sunspots were not freshly generated with each revolution of the Sun, but could be observed passing round it several times. Furthermore, Sizzi drew to Galileo's attention something he had not yet noticed – that the inclination of the path travelled by sunspots varied with the seasons. Thus in one part of the year the sunspots appeared to be travelling upwards across the face of the Sun; in another part of the year they appeared to be travelling downward. Galileo was to adopt this observation and deploy it in his
Dialogue Concerning the Two Chief World Systems The ''Dialogue Concerning the Two Chief World Systems'' (''Dialogo sopra i due massimi sistemi del mondo'') is a 1632 Italian-language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was transl ...
in 1632 to demonstrate that the Earth tilted on its axis as it orbited the Sun.


Johannes Kepler

In his work ''Phaenomenon singulare'' (1609) Kepler had described what he took to be the transit of Mercury, observed on 29 May 1607. However, after Michael Maestlin pointed out Galileo's work to him, he corrected himself in 1617 in his ''Ephemerides'', recognising long after the event that what he had seen was sunspots. Welser sent Kepler a copy of Scheiner's first three Apelles letters, and Kepler replied before Galileo, arguing, like him, that Sunspots must be on the surface of the Sun and not satellites. Kepler reached this conclusion only by studying the evidence Scheiner's had provided, without making any direct observations of his own. Kepler did not however engage with the claims of Galileo in "Letters on Sunspots" or have further involvement in public discussion on the question.


Michael Maestlin

In his treatise on the comet of 1618, ''Astronomischer Discurs von dem Cometen, so in Anno 1618'',
Michael Maestlin Michael Maestlin (also Mästlin, Möstlin, or Moestlin) (30 September 1550 – 26 October 1631) was a German astronomer and mathematician, known for being the mentor of Johannes Kepler. He was a student of Philipp Apian and was known as the tea ...
made reference to the work of Fabricius and cited sunspots as evidence of the mutability of the heavens. He made no reference to the work of either Scheiner or Galileo, although he was aware of both. He concluded that sunspots are definitely on or near the Sun, and not a phenomenon of the earth's atmosphere; that it is only thanks to the telescope that they can be studied, but that they are not a new phenomenon; and that whether they are on the surface of the Sun or move around it is a question to which there is no reliable answer.


Jean Tarde

The French churchman Jean Tarde visited Rome in 1615, and he also met Galileo in Florence and discussed sunspots with him, as well as Galileo's other work. He did not agree with Galileo's view that the sunspots were on or near the surface of the Sun, and held rather that they were small planets. On his return to France in 1615 he built an observatory at
La Roque-Gageac La Roque-Gageac (; oc, La Ròca de Gajac) is a commune in the Dordogne department in Nouvelle-Aquitaine, southwestern France. Perched above the river Dordogne, the village is a member of the ''Les Plus Beaux Villages de France'' ("The most beau ...
where he studied sunspots further. In 1620 he published ''Borbonia Sidera'', dedicated to
Louis XIII Louis XIII (; sometimes called the Just; 27 September 1601 – 14 May 1643) was King of France from 1610 until his death in 1643 and King of Navarre (as Louis II) from 1610 to 1620, when the crown of Navarre was merged with the French crown ...
, in which he declared the spots to be the 'Bourbon planets'.


Charles Malapert

The Belgian Jesuit
Charles Malapert Charles Malapert (1581–1630) was a Jesuit writer, astronomer and proponent of Aristotelian cosmology, from the Spanish Netherlands. He was considered one of the intellectual champions of the Roman Catholic Church. He used observations of come ...
agreed with Tarde that the apparent sunspots were in fact planets. His book, published in 1633, was dedicated to
Philip IV of Spain Philip IV ( es, Felipe, pt, Filipe; 8 April 160517 September 1665), also called the Planet King (Spanish: ''Rey Planeta''), was King of Spain from 1621 to his death and (as Philip III) King of Portugal from 1621 to 1640. Philip is remembered f ...
and christened them 'Austrian stars' in honour of the house of
Habsburg The House of Habsburg (), alternatively spelled Hapsburg in Englishgerman: Haus Habsburg, ; es, Casa de Habsburgo; hu, Habsburg család, it, Casa di Asburgo, nl, Huis van Habsburg, pl, dom Habsburgów, pt, Casa de Habsburgo, la, Domus Hab ...
.


Pierre Gassendi

Pierre Gassendi Pierre Gassendi (; also Pierre Gassend, Petrus Gassendi; 22 January 1592 – 24 October 1655) was a French philosopher, Catholic priest, astronomer, and mathematician. While he held a church position in south-east France, he also spent much tim ...
made his own observations of sunspots between 1618 and 1638. He agreed with Galileo that the spots were on the surface of the Sun, not satellites orbiting it. Like Galileo, he used observation of the spots to estimate the speed of the Sun's rotation, which he gave as 25–26 days. Most of his observations were not published however and his notes were not kept systematically. He did however discuss his findings with Descartes.


Rene Descartes

Rene Descartes was interested in sunspots and his correspondence shows that he was actively gathering information about them when he was working on ''Le Monde''. He was aware of Scheiner's ''Rosa Ursine'' published in 1630, which conceded Galileo's point that sunspots are actually on the face of the Sun. Whether he knew of Galileo's ideas primarily through Scheiner or whether he read ''Letters on Sunspots'' directly is not known, but in his ''
Principles of Philosophy ''Principles of Philosophy'' ( la, Principia Philosophiae) is a book by René Descartes. In essence, it is a synthesis of the ''Discourse on Method'' and ''Meditations on First Philosophy''.Guy Durandin, ''Les Principes de la Philosophie. Intro ...
'' (1644) he refers to "spots which appear on the sun's surface also revolve around it in planes inclined to that of the ecliptic", which appears to indicate at least a knowledge of Galileo's argument. Descartes used sunspots as an illustration of his Vortex Theory.


Giovanni Battista Riccioli

In his 1651 work ''Almagestum Novum'',
Giovanni Battista Riccioli Giovanni Battista Riccioli, SJ (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order. He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion ...
set out 126 arguments against the Copernican model of the universe. In his 43rd argument, Riccioli considered the points Galileo had made in his ''Letters on Sunspots'', and asserted that a heliocentric (Copernican) explanation of the phenomenon was more speculative, while a geocentric model allowed for a more parsimonious explanation and was thus more satisfactory (ref:
Occam's Razor Occam's razor, Ockham's razor, or Ocham's razor ( la, novacula Occami), also known as the principle of parsimony or the law of parsimony ( la, lex parsimoniae), is the problem-solving principle that "entities should not be multiplied beyond neces ...
). As Riccioli explained it, whether the Sun went round the Earth or the Earth round the Sun, three movements were necessary to explain the movement of sunspots. If the Earth moves around the Sun, the necessary movements were the annual motion of the Earth, the
diurnal motion Diurnal motion (, ) is an astronomical term referring to the apparent motion of celestial objects (e.g. the Sun and stars) around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's rotatio ...
of the Earth, and the rotation of the Sun. However, if the Sun moved around the Earth, this accounted for the same movement as both the annual and diurnal motions in the Copernican model. In addition, the annual gyration of the Sun at its poles, and the rotation of the Sun had to be added to completely account for the movement of sunspots. While both models required three movements, the heliocentric model required the Earth to make two movements (annual and diurnal) which could not be demonstrated, while the geocentric model was based on three observable celestial movements, and was accordingly preferable.


Athanasius Kircher

Athanasius Kircher Athanasius Kircher (2 May 1602 – 27 November 1680) was a German Jesuit scholar and polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans ...
succeeded Scheiner in the Chair of Mathematics at the
Collegio Romano The Roman College ( la, Collegium Romanum, it, Collegio Romano) was a school established by St. Ignatius of Loyola in 1551, just 11 years after he founded the Society of Jesus (Jesuits). It quickly grew to include classes from elementary school t ...
. In '' Mundus Subterraneus'' (1664), he rejected the views of both Scheiner and Galileo, reviving an earlier idea of Kepler's and arguing that sunspots were in fact smoke emanating from fires on the surface of the Sun, and that the surface of the Sun was therefore indeed perfect as the Aristotelians believed, although apparently disfigured by blemishes. Sunspots, he argued, just like the planets in astrology, had a profound influence on the Earth.


Sunspots in Galileo's later writings


''The Assayer''

In ''Il Saggiatore'' (''
The Assayer ''The Assayer'' ( it, Il Saggiatore) was a book published in Rome by Galileo Galilei in October 1623 and is generally considered to be one of the pioneering works of the scientific method, first broaching the idea that the book of nature is to be ...
'') (1623) Galileo was mostly concerned with faults in
Orazio Grassi Orazio Grassi, S.J. (b. Savona 1 May 1583 – d. Rome 23 July 1654), was an Italian Jesuit priest, who is best noted as a mathematician, astronomer and architect. He was one of the authors in controversy with Galileo Galilei on the nature of come ...
's arguments about comets, but in the introductory section he wrote :
'How many men attacked my Letters on Sunspots, and under what disguises! The material contained therein ought to have opened to the minds eye much room for admirable speculation; instead it met with scorn and derision. Many people disbelieved it or failed to appreciate it. Others, not wanting to agree with my ideas, advanced ridiculous and impossible opinions against me; and some, overwhelmed and convinced by my arguments, attempted to rob me of that glory which was mine, pretending not to have seen my writings and trying to represent themselves as the original discoverers of these impressive marvels.'
Christoph Scheiner took this to be an attack on him. He therefore used ''Rosa Ursina'' to mount a bitter riposte to Galileo, although he also conceded Galileo's main point, that sunspots exist on the Sun's surface or just above it, and thus that the Sun is not flawless.


''Dialogue Concerning the Two Chief World Systems''

In 1632 Galileo published ''Dialogo sopra i due Massimi Sistemi del Mondo'' (''
Dialogue Concerning the Two Chief World Systems The ''Dialogue Concerning the Two Chief World Systems'' (''Dialogo sopra i due massimi sistemi del mondo'') is a 1632 Italian-language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was transl ...
''), a fictitious four day-long discussion about natural philosophy between the characters Salviati (who argued for Copernican ideas and was effectively a mouthpiece of Galileo), Sagredo, who represented the interested but less well-informed reader, and Simplicio, who argued for Aristotle, and whose arguments were possibly a parody of those made by
Pope Urban VIII Pope Urban VIII ( la, Urbanus VIII; it, Urbano VIII; baptised 5 April 1568 – 29 July 1644), born Maffeo Vincenzo Barberini, was head of the Catholic Church and ruler of the Papal States from 6 August 1623 to his death in July 1644. As po ...
. The book was reviewed by the Roman Inquisition and in 1633 Galileo was interrogated and found 'vehemently suspect of heresy' because of it. He was forced to renounce his belief in heliocentrism, sentenced to house arrest and banned from publishing anything further. The ''Dialogue'' was placed on the
Index Index (or its plural form indices) may refer to: Arts, entertainment, and media Fictional entities * Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, an item on a Halo megastru ...
. The ''Dialogue'' is a broad synthesis of Galileo's thinking about physics, planetary movement, how far we can rely on our senses in making judgements about the world, and how we make intelligent use of evidence. It drew together all his findings and recapitulated arguments made in earlier years on specific topics. For this reason, there is no 'section on sunspots' in the ''Dialogue''. Rather, they are referred to at various points in arguments about other topics. In the ''Dialogue'', that sunspots are on the surface of the Sun and not planets was taken as established fact. The discussion concerned what inferences could be drawn about the universe from their rotation. Galileo did not argue that the existence of sunspots conclusively proved that the Copernican model was correct and the Aristotelian model wrong; he explained how the rotation of sunspots could be explained in both models, but that the Aristotelian explanation was much more complicated and suppositional. Day 1 The discussion opens with Salviati arguing that two key Aristotelian arguments are incompatible; either the heavens are perfect and unchanging, or that the evidence of the senses is preferable to argument and reasoning; either we should rely on the evidence of our senses when they tell us changes (such as sunspots) take place, or we should not. Holding both positions is not tenable. Day 2: Salviati argues that sunspots prove the rotation of the Sun on its axis. Aristotelians had previously held that it was impossible for a heavenly body to have more than one natural motion. Aristotelians must therefore choose between their determination that only one natural movement is possible (in which case the Sun is static, as Copernicus argued); or they must explain how a second natural motion occurs if they wish to maintain that the Sun makes a daily orbit of the Earth. This argument is resumed on Day 3 of the Dialogue.Maurice A Finocchiaro, ''Galileo's Dialogue'', Routledge 2014 pp.158.182


See also

*
Copernican Revolution The Copernican Revolution was the paradigm shift from the Ptolemaic model of the heavens, which described the cosmos as having Earth stationary at the center of the universe, to the heliocentric model with the Sun at the center of the Solar Sys ...
*
List of Catholic clergy scientists This is a list of Catholic clergy throughout history who have made contributions to science. These churchmen-scientists include Nicolaus Copernicus, Gregor Mendel, Georges Lemaître, Albertus Magnus, Roger Bacon, Pierre Gassendi, Roger Joseph Bo ...
*
Solar observation Solar observation is the scientific endeavor of studying the Sun and its behavior and relation to the Earth and the remainder of the Solar System. Deliberate solar observation began thousands of years ago. That initial era of direct observation g ...


References


External links


(video) Lecture by Paulo Galluzzi, Director of the Museu Galileo, on the involvement of the Accademia dei Lincei in publishing ''Letter on Sunspots''Galileo's ''Letters on Sunspots'' (Rome,1613)Video clip showing how Galileo deduced the circular motion of the Sun from the movement of sunspotsanimation of Galileo's sunspot drawingsSchreiner's ''Tres epistolae de maculis solaribus'' (Augsburg 1612)Scheiner's ''De Maculis solaribus et stellis circa Iovis errantibus accuratior Disquisitio'' (Augsburg, 1612)Sizzi's ''Dianoia Astronomica'' (Venice 1611)Scheiner's ''Sol ellipticus'' (Augsburg, 1615)Scheiner's ''Rosa Ursina sive Sol'' (Bracciano, 1626–30)Tarde's ''Borbonia Sidera'' (Paris, 1620)Malapert's ''Austriaca Sidera Heliocyclia'', (Douai, 1633)Riccioli's ''Almagestum Novum'' (Bologna 1651)Scheiner's ''Prodromus pro sole mobili et terra stabili contra ... Galilaeum a Galileis'' (Prague, 1651)Kircher's ''Mundus Subterraneus'' (Amsterdam 1665)
{{Authority control 1613 books 1613 in science Galileo Galilei Astronomical controversies History of astronomy