Galileo’s Daughter: A Drama of Science, Faith and Love. Dava Sobel

heavens, as Aristotle described them, consisted entirely of a fifth element – the quintessence, or aether – that remained incorruptible. It was thus impossible for a new star suddenly to materialise. The nova, the Aristotelians argued, must inhabit the sublunar sphere between the Earth and the Moon, where change was permissible. But Galileo could see by comparing his nightly observations with those of other stargazers in distant lands that the new star lay far out, beyond the Moon, beyond the planets, among the domain of the old stars.

      In his playful, provocative way, Galileo presented the nova controversy to the public in a dialogue between a pair of peasants speaking Paduan dialect, which he published under the pen name Alimberto Mauri. Call the new star ‘quintessence’, his gruff hero concluded, or call it ‘polenta’! Careful observers could measure its distance just the same.

      Having thus impugned the immutability of the heavens, Galileo further attacked the defensive Aristotelian philosophers by turning the telescope on their territory in 1609. His telescopic discoveries transformed the nature of the Copernican question from an intellectual engagement into a debate that might be decided on the basis of evidence. The roughness of the Moon, for example, showed that some of the features of Earth repeated themselves in the heavens. The motions of the Medicean stars demonstrated that satellites could orbit bodies other than the Earth. The phases of Venus argued that at least one planet must travel around the Sun. And the dark spots discovered on the Sun sullied the perfection of yet another heavenly sphere. ‘In that part of the sky which deserves to be considered the most pure and serene of all – I mean in the very face of the sun,’ Galileo reported, ‘these innumerable multitudes of dense, obscure and foggy materials are discovered to be produced and dissolved continually in brief periods.’

      Galileo rued the stubbornness of philosophers who clung to Aristotle’s views despite the new perspective provided by the telescope. He swore that if Aristotle himself were brought back to life and shown the sights now seen, the great philosopher would quickly alter his opinion, as he had always honoured the evidence of his senses. Galileo chided the followers of Aristotle for being too timid to stray from their master’s texts: ‘They wish never to raise their eyes from those pages – as if this great book of the universe had been written to be read by nobody but Aristotle, and his eyes had been destined to see for all posterity.’

      Several of Galileo’s Aristotelian opponents sputtered that the sunspots must be a new fleet of ‘stars’ circling the Sun the way the Medicean stars orbited Jupiter. Even professors who had vociferously rejected the moons of Jupiter, damning them as demonic visions spawned by the distorting lenses of Galileo’s telescope, now turned to embrace them as the Sun’s last hope for maintaining its steady stateliness.

      One of the first scientists to see sunspots, Galileo gathered important correspondents among foreign astronomers seeking to compare observations and interpretations with him. In January of 1612, while still convalescing at the Villa delle Selve outside Florence, Galileo heard much about sunspots from a German gentleman and amateur scientist named Marcus Welser. ‘Most Illustrious and Excellent Sir,’ Welser hailed Galileo,

      Already the minds of men are assailing the heavens, and gain strength with every acquisition. You have led in scaling the walls, and have brought back the awarded crown. Now others follow your lead with the greater courage, knowing that once you have broken the ice for them it would indeed be base not to press so happy and honourable an undertaking. See, then, what has arrived from a friend of mine; and if it does not come to you as anything really new, as I suppose, nevertheless I hope you will be pleased to see that on this side of the mountains also men are not lacking who travel in your footsteps. With respect to these solar spots, please do me the favour of telling me frankly your opinion – whether you judge them to be made of starry matter or not; where you believe them to be situated, and what their motion is.

      Enclosed Galileo found several essays by Welser’s ‘friend’, an anonymous astronomer (later revealed as Father Christopher Scheiner, Jesuit professor at the University of Ingolstadt), who tried to explain the new phenomenon according to the old philosophy, protecting his identity behind the pseudonym ‘Apelles’.

      Galileo took nearly four months to formulate his reply, constrained at first by his illness (‘a long indisposition,’ he called it, ‘or I should say a series of long indispositions preventing all exercises and occupations on my part’), and even further by the calumny of his enemies, not to mention the mysterious nature of the spots themselves.

      ‘The difficulty of this matter,’ Galileo finally conceded to Welser, ‘combined with my inability to make many continued observations, has kept (and still keeps) my judgment in suspense. And I, indeed, must be more cautious and circumspect than most other people in pronouncing upon anything new. As Your Excellency well knows, certain recent discoveries that depart from common and popular opinions have been noisily denied and impugned, obliging me to hide in silence every new idea of mine until I have more than proved it.’ Nevertheless, Galileo expounded on the essence and substance of sunspots for many pages, initiating an ongoing correspondence with Welser – and through him ‘the masked Apelles’ – that sounded the full thunder of the new debate. Indeed, Galileo’s letters on sunspots speak almost as much about the system of the world as they do about the solar spots.

      ‘With absolute necessity we shall conclude,’ Galileo wrote early in the first of his three letters to Welser, ‘in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the Sun just as do all the other planets…No longer need we employ arguments that allow any answer, however feeble, from persons whose philosophy is badly upset by this new arrangement of the universe.’

      Apelles upheld the idea that the dark spots must be many small stars circling the Sun. Galileo saw nothing starlike about them. To his mind, they more closely resembled clouds: ‘Sunspots are generated and decay in longer and shorter periods; some condense and others greatly expand from day to day; they change their shapes, and some of these are most irregular; here their obscurity is greater and there less. They must be simply enormous in bulk, being either on the Sun or very close to it. By their uneven opacity they are capable of impeding the sunlight in differing degrees; and sometimes many spots are produced, sometimes few, sometimes none at all.’

      But he quickly added: ‘I do not assert on this account that the spots are clouds of the same material as ours, or aqueous vapours raised from the Earth and attracted by the Sun. I merely say that we have no knowledge of anything that more closely resembles them. Let them be vapours or exhalations then, or clouds, or fumes sent out from the Sun’s globe or attracted there from other places; I do not decide on this – and they may be any of a thousand other things not perceived by us.’ (He could never have imagined, despite his long-standing interest in magnets, that the spots marked the sites of the Sun’s most potent magnetic fields.)

      ‘If I may give my own opinion to a friend and patron,’ Galileo continued, ‘I shall say that the solar spots are produced and dissolve upon the surface of the Sun and are contiguous to it, while the Sun, rotating upon its axis in about one lunar month, carries them along, perhaps bringing back some of those that are of longer duration than a month, but so changed in shape and pattern that it is not easy for us to recognise them.’

      In closing this first letter, Galileo begged Welser’s indulgence:

      And forgive me my indecision, because of the novelty and difficulty of the subject, in which various thoughts have passed through my mind and met now with assent and again with rejection, leaving me abashed and perplexed, for I do not like to open my mouth without declaring anything whatever. Nevertheless, I shall not abandon the task in despair. Indeed, I hope that this new thing will turn out to be of admirable service in tuning for me some reed in this great discordant organ of our philosophy – an instrument on which I think I see many organists wearing themselves out trying vainly to get the whole thing into perfect harmony. Vainly, because they leave (or rather preserve) three or four of the principal reeds in discord, making it quite impossible for the others to respond in perfect tune.

      These off-key reeds that Galileo decried sounded several flat notes, including the immutability of the heavens, the farrago of the celestial spheres and the immobility of the Earth.

      Welser