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JUST OVER 400 years ago, an Italian professor named Galileo Galilei aimed a telescope skyward and changed our view of the universe forever. It's hardly a surprise, then, that Italians venerate Galileo as a hero, in the way that England honours Newton and the United States celebrates the Wright brothers.

Yet discovering the Galileo trail can be a challenge. Many guidebooks pay little attention to the history of science and, often, one finds only a tiny plaque where a grand statue - or a museum - might have been expected. Galileo's birthplace in Pisa is a case in point. It wasn't mentioned in my Lonely Planet guide, and I had to track down its location, tucked away in the city's quiet San Francesco quarter, just north of the river Arno.

Galileo was born in a modest apartment on Via Giuseppe Giusti, at number 24. The simple, four-storey building hardly stands out; only an Italian flag and a small plaque indicate that the father of modern astronomy was born within the pinkish-brown walls. With a little imagination, however, we can picture the scene as it might have been in Galileo's time, when the neighbourhood was home to artists, craftsmen and shopkeepers. His father, Vincenzo Galilei, had settled here with his wife just a year before the birth of their son.

You may be wondering, as I was, why Galileo's parents gave him a name that was a virtual carbon copy of the family name. "'Galileo Galilei' - it sounds like an echo or a yodel," Dava Sobel, author of Galileo's Daughter, tells me. It was the custom at that time, she explains, for families in Tuscany to give their first son a name based on the family name. Over the centuries, the given name won out; history remembers the scientist simply as Galileo.

His father, Vincenzo, was a professional musician; many of his compositions have survived, and you can find modern recordings of them on CD. Perhaps the young Galileo learned to play the lute here at his family home, by his father's side; we know that in time he became an accomplished musician and poet in his own right. This musical knowledge would be of great use to him in later life, when his interests turned to the physical sciences.

In Galileo's day, there were no clocks or watches accurate enough to track time to less than a second - but someone with musical training could estimate time down to perhaps a sixty-fourth of a second, Sobel says.

GALILEO'S FATHER hoped his son would become a doctor; instead, the youngster developed an interest in mathematics. He studied at the University of Pisa and later took his first teaching job there. Many of the university buildings still look much as they did in Galileo's time.

The city's star attraction is, of course, the iconic Leaning Tower - actually one of three spectacular Romanesque structures that make up the Piazza dei Miracoli (the Field of Miracles), along with the enormous cathedral and adjacent baptistry. Visitors who pay the 15 euro admission fee, and climb the 293 steps to the top, are rewarded with a breathtaking view of the town.

As I peered out from the top of the tower, I kept thinking to myself; "Did he, or didn't he?" It must have been incredibly tempting: to drop objects of different weights from the top of the tower, watching them plunge 57 m to the ground below, to see if they really do land at the same time, as Aristotle had argued.

With its convenient tilt, the tower would have been the perfect place for Galileo to test his theories of motion, and of falling bodies in particular. (And, yes, the tower was already leaning in Galileo's time; built between 1173 and 1350, it had started to lean before it was even finished.)

The story of Galileo and the tower may be more legend than fact. There's only one written account, set down by a former student many years later. But I was lucky enough to witness a re-creation of the famous experiment by Steve Shore, an American physicist based at the University of Pisa, as part of the 400-year anniversary celebrations of Galileo first aiming a telescope at the night sky in 1609.

Shore was joined by an actor playing 'Galileo', along with musicians in period costume; other men, dressed in bright red and white robes, played the role of the sceptical Artistotelian professors - the naysayers who mocked Galileo for his foolish ideas. Hundreds of people had gathered to watch.

Shore, like most historians, doubts that Galileo actually carried out the experiment at the famous tower. "Something that dramatic, I think, people would have remembered," he says. At any rate, Galileo didn't need to; his work with inclined planes already told him that bodies must fall at the same rate, regardless of their weight. "Galileo already knew the result," Shore says.

I watched from below as Shore dropped water-filled bottles of various sizes from the top of the tower. The verdict - as you might guess - came down in Galileo's favour: the bottles crashed to the ground at the same time. (I recorded the event. It can be viewed here).

THE GALILEO TRAIL continues in the northeastern city of Padua, where the scientist accepted a teaching position in 1592. He lived there for 18 years, calling it the happiest and most productive period of his life. And it was here that he first learned of a curious optical device from Holland, said to make distant objects appear nearby.

Galileo first saw its value as a military tool, and showed off an improved version of the telescope to a delegation of Venetian statesmen. Observing from the top of the campanile (bell tower) at the Piazza San Marco in nearby Venice, they were able to identify ships more than two hours before they arrived in port. A few months later, he aimed the device at the night sky - and was stunned by what the telescope revealed.

Galileo observed that the planet Venus goes through phases like our own Moon; that Jupiter has four moons; that the Sun is peppered with dark spots; and that the Moon is covered with mountains and craters - all of which contradicted the established wisdom of the ancient Greeks. Moreover, the observations of Venus and Jupiter seemed to support the Sun-centred (heliocentric) model of the Solar System described by Copernicus, rather than the ancient model of Aristotle and Ptolemy.

Galileo was probably leaning toward the Copernican view even before he began his astronomical observations; what he saw through the telescope, however, clinched it. He described the findings in a book called Sidereus Nuncius (The Starry Messenger), published in 1610. It was an instant bestseller.

In Padua, the street where Galileo lived has been renamed in his honour, and the medieval university buildings where he taught - in constant use since the 13th century - are remarkably well preserved. Visitors who tour the university's central building, known as the Palazzo del Bo, can see the old wooden podium from which he lectured.

With the fame that accompanied his telescopic discoveries, Galileo was able to negotiate a better-paying job in his native Tuscany. Having spent his teenage years in Florence, he now returned with the lofty title of chief mathematician and philosopher to the Grand Duke of Tuscany. (The Grand Duke in question was the young Cosimo II - but look for a stately equestrian statue of his grandfather, Cosimo I, looming over the Piazza della Signoria.)

WORDS HARDLY DO justice to the city of Florence. The phrase 'Renaissance theme park' crossed my mind, but even that doesn't quite convey the sheer density of spectacularly well-preserved art and architecture in this city, home to the greatest cultural flowering the world has ever seen.

Art enthusiasts can expect long queues for admission to the dazzling Uffizi gallery - but science buffs can treat themselves to an equally fascinating (and far less crowded) attraction, the Museum of the History of Science.

Among its thousands of artefacts is a small, wooden telescope about a metre long, with a lens just four or five cm across - one of only two surviving original telescopes used by Galileo. Also on display are his notebooks, in which he recorded his observations of Jupiter and Saturn. An adjacent display case houses a more peculiar relic - one of Galileo's fingers (or what's left of it), removed from his body nearly a century after his death.

Another striking tribute can be seen on Via Sant'Antonio, not far from the train station, where one of Galileo's students, a man named Vincenzo Viviani, lived at number 11. Viviani assisted Galileo in the final years of the scientist's life, and was clearly in awe of his master. He later had a bust of Galileo erected above his door, along with elaborately engraved scrolls of Latin text, describing the great man's scientific discoveries.

The Galileo story continues, for better and for worse, in Rome. On a visit in 1611, he showed off his telescope to the Jesuit astronomers at the Collegio Romano, the country's leading scientific research centre of the time. The Jesuits confirmed Galileo's telescopic observations, and lavished him with praise.

But trouble was brewing. Conservative professors, who earned their living teaching Aristotle's ideas, resented Galileo's growing fame. A few jealous churchmen denounced Galileo from the pulpit. Eventually, they convinced the Vatican that the Copernican system was contrary to Scripture, and in 1616 Galileo was warned that he could neither teach nor defend the 'new astronomy'.

The problem had little to do with Copernicanism as such. As long as the heliocentric model was used as a mathematical tool, the Church wasn't particularly concerned. But the notion of a moving Earth posed a problem. The idea appeared to contradict certain verses in the Bible - such as Joshua's description of a battle in which he commands the Sun (not the Earth) to stand still.

Galileo believed that conflict could be avoided so long as one was careful in interpreting such passages. Perhaps, he argued, they were metaphorical - designed to awe the reader, not to teach one about the workings of the Solar System.

Owen Gingerich, an astronomer and historian of science at Harvard University in Massachusetts, told me that he sees the 'Galileo affair' more as a power struggle - and a local Italian one, at that - than as an ideological battle over the structure of the heavens. What really bothered Catholic leaders was that Galileo had the chutzpah, as it were, to say anything about the interpretation of Scripture: that was the Church's business. And, at a time when Catholics were waging war with Protestant countries north of the Alps over matters of doctrine, the interpretation of Scripture was a serious business indeed. Galileo "was seen as rocking the boat by the Catholic authorities," Gingerich says.

THE TIDE TURNED in Galileo's favour when a new, liberal Pope, Urban VIII, came to power in Rome. Galileo was now able to get the Vatican's permission to write a book about the competing models of the heavens - so long as he treated the Copernican view as speculation, on equal footing with the Ptolemaic view.

The book, titled A Dialogue Concerning the Two Chief World Systems, was published in 1632.
Unfortunately for Galileo, the book clearly endorsed the new astronomy. The following year, he was called before the Holy Office of the Inquisition. It would be his final visit to the Eternal City.

Today, visitors to the Italian capital swarm around the Pantheon, the best-preserved Roman building in the city; a block away, in front of the church of Santa Maria sopra Minerva, one finds a much-loved Bernini sculpture of an obelisk-toting elephant. But few visitors realise that this church was where the darkest chapter in the Galileo story unfolded, as the scientist's encounter with the Inquisition reached its conclusion. The trial took place in the Dominican convent adjacent to the church (now the library of the Italian parliament). In the end, Galileo was convicted of "vehement suspicion of heresy" and forced to recant his belief in the Copernican system.

Galileo's life was spared. In that respect, he fared much better than Giordano Bruno, an Italian philosopher and mystic - and, as it happens, a Copernican - who had run afoul of the Inquisition three decades earlier. Bruno was accused of a litany of heresies, including the belief in a multitude of inhabited worlds in an infinite cosmos. He was burned at the stake in 1600. (His reputation has improved since then: today, a grand bronze statue of Bruno stands in the centre of the Campo de' Fiori - the Field of Flowers - not far from the city's ancient centre.)

Speaking of improved reputations, the Vatican itself is now a strong supporter of world-class astronomical research; the domes of the original Vatican Observatory, founded in 1891, can be seen on a hilltop in the tiny, picturesque town of Castel Gandolfo - the Pope's summer residence - a short train ride south of Rome.

IN THE END, Galileo was sentenced to house arrest. He was allowed to serve out his sentence at his villa in Arcetri, now a suburb of Florence. The villa, with its lush garden and spacious courtyard, was known as Il Gioiello (the jewel). Today, a plaque and a small bust of the scientist can be seen on an outer wall overlooking the street; it marks what was once the boundary of Galileo's world. (Now owned by the University of Florence, the site is not normally open to the public. I was lucky enough to gain access during a Galileo conference held in Florence at the time of my visit; the closing ceremonies were held at the villa.)

On a hilltop within sight of the villa is the convent of San Matteo, where Galileo's two daughters lived as nuns. Virginia, the older of the two, took the name Sister Maria Celeste, and cared for her father until her own death at the age of 33. Her letters to Galileo become the inspiration for Sobel's bestselling book.

In Arcetri, Galileo continued to work, completing another groundbreaking book, Discourse on Two New Sciences. The manuscript was smuggled out of Italy; his friends saw to its publication in the Dutch city of Leiden. Though sick and nearly blind, Galileo received several well-known visitors, including two famous Englishmen - the poet John Milton and the philosopher Thomas Hobbes. Galileo died in his villa during the winter of 1642.

Galileo remains one of the most enigmatic figures in the history of science - a man who changed the very way we conceive of the universe, and paid a heavy price for his boldness. "To this day, Galileo remains symbolic of problems between science and religion," says Sobel, "even though he never wanted to be that person."

Galileo's tomb can be found in the Basilica of Santa Croce, one of Florence's most spectacular churches. The elaborate memorial features a bust of the scientist, flanked by the muses of astronomy and geometry. Michelangelo is buried nearby. But the spirit of Galileo can be found far beyond the walls of Florence. We are confronted with his legacy every time we gaze in awe at the night sky.