This is an interesting and lively book about an obscure professor of mathematics who ran a student boarding house in Padua and became a courtly star in Florence in the wake of his telescopic discoveries. Readers of this book will learn little about Galileo's physics, mathematics or astronomy, but they will have an enjoyable time discovering that he was human – all too human. According to Biagioli, if Galileo had a plan, it was not mainly about science but about Galileo. His first book, published when he was already forty-two years old, was a slim instruction manual for the use of the geometrical and military compass that he sold to well-heeled students to whom he gave private lessons for a hefty fee. Although he was paid to teach mathematics, elementary physics and astronomy at the University of Padua, Galileo never lectured more than thirty hours … per annum.

Biagioli argues that Galileo was in no hurry to publish because there was little money to be made that way. He eventually printed ‘to control, not to communicate’ (p. 4), namely when he found it useful to enlist the press as a witness to his claim that he had been the first to invent the compass, although he owed much to his predecessors. Galileo was a capitalist who wanted to make a profit from the products of his intellect. No sooner had he invented a horse-driven water pump than he applied for a patent. When he made a particularly powerful lodestone he found a broker to arrange for its sale to the Grand Duke of Tuscany. He also saw his telescope as a source of income. The Sidereus Nuncius, the booklet that disclosed to a spellbound audience that the Moon has mountains and valleys like the Earth, that the stars are vastly more numerous than anyone had surmised, and that Jupiter has four satellites, was aimed at prospective patrons rather than at the republic of letters. He provided only a bare diagram of his telescope and he did not disclose the specifications of the two lenses or the procedure to be followed to produce such an instrument. The first recipients of his telescope were princes and cardinals, not fellow scientists. Kepler, who had generously endorsed the instrument before seeing it, pleaded with Galileo to send him one, but Galileo told him that he was sorry because Kepler deserved only the best of telescopes which, unfortunately, he had already given the Grand Duke of Tuscany in whose honour he had named Jupiter's satellites ‘Medicean stars’.

Biagioli spends considerable time on the acrimonious debate over who discovered sunspots. Galileo and the Jesuit Christoph Scheiner observed them at roughly the same time but both claimed priority, although Thomas Harriot was probably the first to view them in December 1610. Galileo made better observations than Scheiner (with a projection apparatus developed by his pupil Benedetto Castelli), and he offered a better explanation of their nature, but his belligerent stance lost him the backing of the Society of Jesus from which he had until then received recognition and support.

It is against this psychological and sociological background that Biagioli re-examines Galileo's problems with the Church. He shows that Galileo was undoubtedly right in his distinction between the role of Scripture as ‘teaching how to go to heaven’ and the role of science as ‘teaching how the heavens go’, but that he made the fatal mistake of thinking that having moved to the courtly top of prestige he could impose his views on the ecclesiastical Establishment. His work was censured and he was placed under house arrest, but his condemnation ended up canonizing him as a hero of science and turning his punishment into posthumous credit.

What a guy! But can this be all? Of course not! If Galileo was proud of his image it was as a scientist and not merely as a courtier. He had not only discovered a new world in the sky with his telescope, he had also discovered, with the even more powerful instrument of his mind, the two laws of physics that Newton was to hail as the gateway to modern science: the law of freely falling bodies and the parabolic path of projectiles.

Not an easy man, Galileo, but a genius none the less.