On Sunspots
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On Sunspots

Galileo Galilei, Christoph Scheiner, Eileen Reeves, Albert van Helden

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eBook - ePub

On Sunspots

Galileo Galilei, Christoph Scheiner, Eileen Reeves, Albert van Helden

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Galileo's telescopic discoveries, and especially his observation of sunspots, caused great debate in an age when the heavens were thought to be perfect and unchanging. Christoph Scheiner, a Jesuit mathematician, argued that sunspots were planets or moons crossing in front of the Sun. Galileo, on the other hand, countered that the spots were on or near the surface of the Sun itself, and he supported his position with a series of meticulous observations and mathematical demonstrations that eventually convinced even his rival.

On Sunspots collects the correspondence that constituted the public debate, including the first English translation of Scheiner's two tracts as well as Galileo's three letters, which have previously appeared only in abridged form. In addition, Albert Van Helden and Eileen Reeves have supplemented the correspondence with lengthy introductions, extensive notes, and a bibliography. The result will become the standard work on the subject, essential for students and historians of astronomy, the telescope, and early modern Catholicism.

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1

Introduction

The Sun too, both in rising and in withdrawing
Beneath the waves, will give you signs; the Sun
Commands most certain signs, both those he brings
At break of day and when the stars are rising.
If hiding in a cloud he wears his morning guise
Flecked, and the centre of his disk concave,
Beware of showers: the south wind, from the deep
Driving, bodes ill for trees and crops and herds . . .
This too still more it will pay you to remember:
When, having spanned Olympus, he’s departing
Often we see colours of varying hue
Wander across his face; purple means rain,
Flame-colour means east winds; but if the flecks
Begin to take a tinge of fiery red,
Then you will see a welter everywhere
Of winds and storm clouds both . . .
Who dares call the Sun a liar?
Virgil, Georgics1
The study of sunspots emerged in the second decade of the seventeenth century as an essential element of the debate about the nature of the heavens: if these evanescent phenomena were really located on the Sun, then the most splendid body in the heavens was imperfect, and with it the entire heavens. The instrument that made these and other celestial features visible, the telescope, was still a novelty, and the observations made with it were likewise subjects of controversy and negotiation. An entirely new dimension had been added to the study of the heavens; an instrument lately emerged from the mathematical or optical and mechanical traditions made available information that bore directly on philosophical and cosmological issues. Telescopic astronomy, as we might call it, was a subject that could not have been imagined in 1607—the year before the instrument came into use—and in 1612 and 1613, when the sunspot controversy raged, it had yet to find an institutional setting, an epistemological foundation, or even an adequate optical explanation. The initial arguments about the reality of the celestial phenomena originally observed through it, especially the uneven surface of the Moon and the satellites of Jupiter, were only just beginning to recede, giving way to debates about their interpretation, when the newly visible solar markings focused attention on the precision of telescopic observations, and added to the arguments concerning the nature of the heavens.
That Galileo’s interpretation of sunspots prevailed over those of his opponents is indisputable; what one could perhaps more reasonably debate is the question of priority in their discovery.2 This controversy, initiated by Galileo and his rival Christoph Scheiner themselves, was carried on with vigor by some nineteenth-century scholars, and as recently as 1974 John North weighed in with a defense of the priority of the English scientist Thomas Harriot.3 The very notion of “discovery” needs some qualification, for sunspots have been observed since Antiquity, and were documented for almost two millennia before the advent of the telescope. What little that can be added to the tired question of priority is that the first record of a telescopic observation of sunspots was made by Thomas Harriot on 18 December 1610 (Gregorian), and that the earliest printed reference to its telescopic appearance came in a publication by the Frisian Johannes Fabricius in the summer of 1611. We prefer to let the voluble claims of Galileo and Scheiner emerge within the context of the controversy over the nature of the spots.
At the heart of the debate lay the question of the location and nature of the spots: those issues bore directly on the ongoing cosmological quarrel that had preoccupied European astronomers and philosophers for over a century. The question of the nature of the heavens had been raised well before Copernicus; it had been addressed both by the mapping of comets in the fifteenth century and by astronomers’ novel claim that parallax measurements of those comets would show whether the original distinction between the perfect and unchanging heavens and the corrupt and ever-changing terrestrial region in Aristotelian cosmology could be maintained.4 While the pace and intensity of the debate increased with the publication of Nicolaus Copernicus’s De Revolutionibus in 1543, heliocentrism was by no means the only issue. As James Lattis has demonstrated, the prominent Jesuit Christoph Clavius exerted himself against Copernicus’s heliocentrism, Tycho Brahe’s geo-heliocentrism, Girolamo Fracastoro’s revival of homocentric spheres, and neo-Stoic notions of a fluid heaven in which celestial bodies flew like birds in the air or swam like fish in the sea. In each of these alternative cosmologies, the nature of the heavens was a central issue.
Several phenomena had already brought the solidity of the heavens into question.5 By the first decade of the seventeenth century, an increasing number of astronomers believed that the new stars of 1572, 1600, and 1604,6 as well as the comet of 1577, had been shown by the best measuring instruments to be located above the Moon. Philosophers, however, were slower to change their minds. In the wake of the Council of Trent (1546–1563) the connections between cosmology and biblical interpretation came increasingly under scrutiny, and in the religious orders the pressure for conformity became greater.
The resulting tension was especially apparent within the Society of Jesus, which had become prominent in education and had produced a number of talented investigators of nature. On the one hand, in the 1570s Father Robert Bellarmine, basing himself solely on biblical sources, had depicted an entire cosmology in his Louvain lectures; on the other, Father Clavius used Scriptural authority in arguments against alternative cosmologies and in support of the Aristotelian-Ptolemaic system. Jesuit teachers of the mathematical subjects in the provinces looked to their colleagues at the prestigious Collegio Romano for guidance, but it appears that they were often left to decide for themselves what they could and could not teach about the world system. A letter of 1607 from Father Johannes Lanz of the Jesuit University of Ingolstadt in South Germany to Father Clavius in Rome suggests that recent discussions of comets and of the New Star of 1604 were particularly perplexing. And the frequent queries of the erudite and well-connected Augsburg Humanist Marc Welser, closely associated with the Society, were doubtless flattering, but also served to increase the pressure on Jesuit astronomers both in Rome and beyond. A letter that Welser wrote in the fall of 1607 to his friend Joseph Scaliger suggests that he had rather high expectations of the sky watchers at Ingolstadt: “You may read in the attached passage what was written to me from Prague about the appearance of the heavens; our rather sleepy astronomers here have as yet seen nothing of this, but they will do so, once alerted by me. Let me know, please, what your astronomers think about it.”7
The telescope greatly exacerbated these issues. The older verbal explanation of the spots visible to the naked eye on the surface of the Moon—that they were caused by “rarer and denser” matter distributed within a smooth sphere—did not survive the advent of the telescope by more than two or three years. The satellites of Jupiter were an entirely unexpected revelation, and they proved, regardless of one’s preference for a particular world system, that there was more than one center of motion in the universe. But the phenomena themselves, and the instrument that produced them, remained controversial for a year or more after the publication of Sidereus Nuncius, even in Italy, where the best telescopes were to be found.
The role played by the Jesuit mathematicians of the Collegio Romano in the reception of the new instrument, the verification of the new phenomena, and the exegetical problems these produced was therefore crucial, for mathematicians from that order from all over Europe, as well as those in the Far East and the New World, took their cue from their counterparts in Rome.8 Within that institution, the cautious self-censorship of its senior mathematician, Christoph Clavius, was balanced by a certain readiness to refine their telescopes, to undertake celestial observations, and to consider new cosmological models among his younger colleagues, particularly Christoph Grienberger, Giovanni Paolo Lembo, Odo van Maelcote, and Paul Guldin.9
In South Germany, another group of Jesuit scholars soon became interested in the new celestial phenomena, and in a relatively short period began to produce research results. Christoph Scheiner emerged as the leading figure among these astronomers. Intelligent and ambitious, he did not want his publication of his observations of the novel phenomena of sunspots to be delayed by the bureaucratic machinery of the order; through the agency of Marc Welser in nearby Augsburg, in January and September 1612 he was able to publish his results and arguments, the Tres Epistolae and the Accuratior Disquisitio, under a pseudonym, and without submitting these works to censors. Scheiner was unable, however, either to maintain his secret identity or to impress his superiors with his publications, for he had proposed what was in essence a new cosmology, one in which innumerable planetary objects moved through fluid heavens and around an immaculate Sun. Thus while Galileo attacked Scheiner from a position of covert Copernicanism,10 the rather unorthodox nature of the Jesuit astronomer’s views proved troubling to conservative thinkers within his order and to the Catholic Church at large. It is therefore significant that the order’s general, Claudio Aquaviva, circulated letters in 1611 and 1613 reminding all Jesuits to adhere to Aristotelian tradition in matters of cosmology, and that when Scheiner sought to publish later works under his own name, his texts were scrutinized closely by his superiors.
Scheiner was in some regards a tragic figure. Depicted as a dull and conservative thinker by Galileo, he appeared ungovernable to many within his own order. He was convinced by Galileo’s arguments and went on to do an extended research project on sunspots, the Rosa Ursina of 1630, which remained the standard work on the subject for a century.11 He was not, however, successful in Rome, where he went in the 1620s, and was condemned to spend the last two decades of his life in relative obscurity in the German provinces.12 His Prodromus pro Sole Mobili, a refutation of Galileo’s argument for the motion of the Earth on the basis of the motions of sunspots, completed shortly after the appearance of Galileo’s Dialogo (1632), was published only posthumously in 1651 and read by few. Over the past two decades, interest in Scheiner and in his fellow investigators of nature in the Jesuit order has steadily increased.13 Until recently, Scheiner was a figure of little moment to historians, while Galileo, whose subsequent career likewise took a tragic turn, has been a celebrated cultural icon for four centuries.

2

Sunspots before the telescope

In the Western tradition, the scientific study of...

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