The heavier cannonball fell faster.

This was the first of several corrections made to the legend of Galileo provided by Dava Sobel's engaging biography. Another assumption was that Galileo was an irreligious scientist, forced to recant his ideas by an adversarial Catholic Church. In fact, he was quite devout, and in old age and ill health would request a reprieve from house arrest to attend mass. He was aware from the start that his ideas were contrary to official doctrine, and carefully described his forceful published arguments for heliocentrism as mere theories, subservient to revealed wisdom, as if to demonstrate that good Catholics could understand heretical ideas as well as any Protestant. Meanwhile, he maintained private correspondence with non-Catholics who had embraced the Copernican revolution, and would ultimately turn to such friends outside Italy to publish his last work, Two New Sciences.

The arrival of Pope Urban VIII in 1623 seemed to bode well for Galileo, who was on good terms with the fellow Florentine and judged him a friend of the sciences. Galileo took no chances, however, and had his potentially incendiary Dialogue Concerning the Two Chief World Systems approved by Inquisition officials in both Rome and Florence. No matter, he was called down to Rome to face prosecution in 1633, basically on a technicality. The authorities had dug up the minutes of a 1616 meeting in which Galileo had been warned not to "in any way hold, teach, or defend [the Copernican theory] by word or in writing." For his part, Galileo had abided by the public edict of 1616, which censured Copernicus' book De revolutionibus orbium coelestium and named his ideas "false and contrary to Holy Scripture," but did not prohibit considering them hypothetically. The investigation dragged on, and Galileo was advised by a sympathetic official, Father Commissary Vincenzo Maculano da Firenzuola, to disown his ideas, avoid scandal, and let the matter drop. Thus, the country's most famous scientist testified:

A long time ago, that is, before the decision of the Holy Congregation of the Index, and before I was issued that injunction, I was undecided and regarded the two opinions, those of Ptolemy and Copernicus, as disputable, because either the one or the other could be true in Nature. But after the said decision, assured by the prudence of the authorities, all my uncertainty stopped, and I held, as I still hold, as most true and indisputable, Ptolemy's opinion, namely the stability of the Earth and the motion of the Sun.

The inquisitors were not convinced, and Galileo was pronounced "vehemently suspected of heresy." He was forced to sign, and read while kneeling before the tribunal, a humiliating statement that was publicized throughout the country: "...having held and believed that the Sun is the center of the world and immovable, and that the Earth is not the center and moves....I abjure with a sincere heart and unfeigned faith, I curse and detest the said errors and heresies...." According to legend, as Galileo stood he muttered "E pur si muove!" -- and yet it moves. The author points out that this would have been most imprudent for a man facing the death penalty.

Galileo was instead sentenced to indefinite imprisonment in the dungeons of the Holy Office. Continuous intercession by friends enabled him to serve his time first at the Tuscan embassy in Rome, and eventually in his home in Florence, though he was not allowed to publish or receive visitors. No less painfully, the Dialogue was added to the Index of Prohibited Books, and would remain banned until 1835.

In his long life of 77 years, during which he managed to avoid multiple outbreaks of bubonic plague, Galileo never married. However, he had three children with Marina Gamba of Venice, whose social station made her an unsuitable candidate for marriage. The two daughters, unmarriageable due to illegitimacy, became nuns, and the eldest would become Galileo's confidant and tireless supporter as he faced illness and trials. Quoting extensively from her letters, Sobel reveals the deep affection that united them, the despond that overcame the father at the death of his favorite daughter, and a surprise ending that is as touching as anything in a fictional love story.

Galileo's writings do not record the dropping of two balls from the Tower of Pisa; the story was first told, or invented, by one of his students after he died. The heavier ball, better able to overcome air resistance, would have dropped slightly faster. Galileo described the experiment in Two New Sciences:

Aristotle says that "an iron ball of one hundred pounds falling from a height of one hundred cubits reaches the ground before a one-pound ball has fallen a single cubit." I say that they arrive at the same time. You find, on making the experiment, that the larger outstrips the smaller by two finger-breadths, that is, when the larger has reached the ground, the other is short of it by two finger-breadths; now you would not hide behind these two fingers the ninety-nine cubits of Aristotle, nor would you mention my small error and at the same time pass over in silence his very large one.

Clearly Galileo is rightly recognized as the "father of physics," but he is also an early and inspiring example of scientific thought independent of conventional wisdom. As he wrote in his first book, The Assayer:

I say that the testimony of many has little more value than that of few, since the number of people who reason well in complicated matters is much smaller than that of those who reason badly. If reasoning were like hauling I should agree that several reasoners would be worth more than one, just as several horses can haul more sacks of grain than one can. But reasoning is like racing and not like hauling, and a single Barbary steed can outrun a hundred dray horses.