Book Reviews: Galileo: Watcher of the Skies | Galileo

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Galileo: Watcher of the Skies by David Wootton Yale UP, 354pp, £25 Galileo by JL Heilbron Oxford UP, 354pp, £20

It is 400 years since Galileo Galilei pointed his little home-made telescope at the sky and saw for the first time the mountains of the Moon, phases of Venus and satellites of Jupiter, finding evidence of Copernicus's theory that Earth was not after all the centre of the universe. The quatercentenary is marked by two excellent biographies, each a model of scholarship, though quite different in character. While both authors are historians of science, David Wootton's background is in history while JL Heilbron trained in physics. This is evident from first glance: Heilbron delves deeply into Galileo's theoretical methods, offering diagrams and equations that might intimidate the general reader but will appeal to mathematically literate ones, while Wootton's narrative is purely verbal, divided into bite-sized thematic chapters. But the contrast goes further: Heilbron's style is the more flamboyant, Wootton's the more readable. Heilbron's book is expansive, digressive, occasionally self-indulgent; Wootton's is pacey, keen to emphasise original findings, and reads in parts like a conference paper. Heilbron's is sure to be the definitive technical study for years to come; Wootton's is aimed at a wider audience. Galileophiles will naturally wish to have both, and will not be disappointed with either.

One thing everybody knows about Pisan-born Galileo is that he stood at the top of the Leaning Tower and dropped two objects of differing weight which landed at the same time, thus disproving Aristotle and establishing a basic principle of modern physics. Wootton devotes a chapter to this supposed event, for which the only testimony comes from Galileo's disciple Viviani, who also left us the story that by watching the swinging chandelier in the neighbouring cathedral Galileo realised that the rate of oscillation depended only on length, a fact that could be used to design a pendulum clock. Unfortunately, says Wootton, "Viviani's story has long been dismissed as a myth"; the great chandelier was installed after Galileo left Pisa, hence the Leaning Tower story appears equally fishy. Heilbron adopts a different tone: "Iconoclasts have thrown doubt on this vignette although the tower's tilt made it a perfect platform for the experiment." Some have even doubted whether Galileo was much of an experimenter at all, instead relying, as Einstein later would, on "thought experiments".

Both biographers reject this, with Wootton making the important point that experiments in real life are never as simple as ones done in schoolrooms once the "correct" answer has been decided. As a historian, he appreciates how messy the whole thing is.This messiness extends, of course, to life itself. Galileo never married but had a Venetian mistress, Marina Gamba. Heilbron speculates that she was a prostitute or, at best, an "honest courtesan" (the higher class who charged as much for conversation as for what Montaigne called "the entire business"). What is certain is that Galileo left Marina behind when he moved to Florence in 1610: his daughters were put in a convent. One of them, the saintly Maria Celeste, was to be his greatest emotional support in later years; her story has been told in Dava Sobel's Galileo's Daughter.

Pride, stubbornness and a degree of self-destructiveness are agreed as factors in Galileo's progression from modestly successful academic to celebrity when he announced his telescopic discoveries in a pamphlet called The Starry Messenger, then notoriety in his run-in with the Inquisition, leading to his final years under house arrest at a rented villa outside Florence.

Given that fame only came when Galileo was 45, both biographers are keen to explore the earlier part of his life. Heilbron emphasises the extent to which Galileo was a "Renaissance man": his father Vincenzo Galilei was a musician, and Galileo played the lute to professional standard. He was also a poet: Heilbron quotes from his verse and criticism, admitting that while modern commentators have found him excellent as a prose writer, his poetry largely stank. He painted too, and might have followed an artistic career if science had not beckoned.

Wootton highlights how Galileo's wanderings never took him further than Rome. He enjoyed gardening but had as little interest in animals as in foreign lands. His best friend, Sagredo, gave him a small caged bird from India. Sagredo was to appear posthumously as a character in Galileo's greatest work, the Dialogue On The Two World Systems, that was to get its author into so much trouble. In real life he was a sensitive soul, says Wootton, anxious that Galileo would take good care of the precious bird - but no word came back. "Eventually he extracted news of it from Galileo: the cat had caught it."

Heilbron's Sagredo would have taken this in his stride: his interests included "Jesuit-baiting, art collecting, and womanising"; he ran a private brothel and invented a combination wine glass and thermometer so his favourite tipple could always be in optimal condition. Galileo did his share of debauchery: Heilbron thinks it may have left him with syphilis, but Wootton insists his real interest was in the heavens.

Prior to 1610 he had probably made all his great discoveries concerning motion and mechanics, but would only expound his ideas in print towards the end of his life.It was when he heard of the recently invented telescope, and worked out how to make one of his own, that Galileo became a public figure. Quickly attacked for trying to pass it off as his own invention, he nevertheless brought it to greater refinement than anyone else had managed.

Was Galileo already a Copernican before looking through his telescope? Wootton cites a pamphlet by a writer calling himself Alimberto Mauri that came out in 1606, asserting that the Moon could be seen (with the naked eye) to have a rough surface. Wootton agrees with those who think Mauri was actually Galileo, already convinced the Moon was a world like ours. Heilbron is sceptical and considers the pamphlet unremarkable, though he quotes other early pseudonymous work by Galileo indicating radical thinking. Galileo maintained that he was a devout Catholic: it wasn't his fault if the Church thought Copernicanism was heretical, when really it did not contradict the Bible (the Vatican only acknowledged this in 1820). Wootton argues that Galileo was a materialist free-thinker from his early years, but that this did not necessarily contradict an unorthodox kind of Catholicism.

For the papal authorities the question hinged on whether one thought Copernicus's theory to be a convenient mathematical model or a statement of physical truth. But Galileo believed there was clinching evidence that showed the Earth moves: the tides. Knowing nothing about the Moon's gravity, he thought tides to be caused by the turning of the Earth as it orbits the Sun. Heilbron explains "this fateful and fallacious theory", which Galileo may have borrowed from Paolo Sarpi, while Wootton omits the minutiae but claims there to be a grain of truth in it. In any case, to the end of his life Galileo would continue to refine the idea, even though it predicted a different tidal cycle from what is observed. Such stubbornness was too much for the Inquisition. In 1632, the ailing Galileo, 68 years old, was summoned to Rome, threatened with torture, and recanted.

What of that other famous story, when at the trial he supposedly muttered, "But still it (the Earth] moves"? Heilbron dismisses the tale. Wootton tells us the story first appeared in 1757, "just at the time when the Catholic Church was beginning to acknowledge that further opposition to Copernicanism was futile". But like the one about the Leaning Tower, it will continue to be told.

Wootton reckons one of the best portrayals is Brecht's Life of Galileo, "a play which, for all its fictions, deserves to be taken seriously as historical interpretation". For Heilbron it is the science, not the life, that is ultimately of greatest importance: his book bears comparison with Pais's classic study of Einstein, Subtle Is The Lord. I'm grateful to both for shedding new light on one of the greatest episodes in intellectual history.