But that was impossible. Never before had stargazers found a planet with moons. It had been established by Ptolemy more than 1,000 years ago that everything in the universe orbited the Earth. Sure, there was that upstart Copernicus, who said that the Sun was actually at the center of things. But few people believed him. If Galileo was a Copernican, he was a closeted one.
It only took Galileo a week to complete the observations that would rearrange our solar system, and two more months to get them published in a pamphlet he titled "Sidereus Nuncius" or "the Starry Messenger." By the end of 1610, with those and other observations, "Galileo had killed off the Ptolemaic system," said science historian David Wootton, a professor at the University of York and author of a Galileo biography.
"It was the beginning of the modern understanding of the universe."
Four centuries later, scientists are returning to Jupiter to deepen that understanding. NASA's space probe Juno, which slipped into orbit around Jupiter as the glow of the last fireworks of July 4 faded away, carries instruments aimed at elucidating the history of our solar system. Spectrometers will seek out chemical signatures of the gases that existed when the planets were newborn; a microwave radiometer will measure the presence of water — seeking hints at the origins of this essential ingredient for life.
But there are also several non-scientific pieces of cargo onboard the probe, including a tiny, aluminium Lego figurine of Galileo and a plaque bearing his first observations of Jupiter's largest moons.
"The revelations that occurred from that discovery are still with us," Scott Bolton, principal investigator for the Juno mission, said at a press conference when Juno was launched five years ago. "Understanding that we are not the center [of the universe], in fact there are things going around Jupiter, another planet, certainly affected all of us both technologically as well as philosophically."
"My guess is that he would have thought [the Lego figures] were pretty kitsch," he said.
Perhaps he would have preferred an alternative idea floated by mission scientists back in 2010: sending a fleck of his own bone onboard the spacecraft.
Either way, the astronomer probably would have demanded some sort of recognition of his discoveries.
"He thought they were the greatest thing ever," Heilbron said. "Galileo was not a modest man. Nor had he any reason to be."
But on that night in 1610, when he first directed his gaze toward Jupiter, Galileo was pretty much a nobody. He was employed as a mathematics professor at the University of Padua — a rather lowly position in an era when philosophy was considered the queen of the sciences. In the world view of Renaissance Europe, math was only useful as a tool for astrologers, who needed to predict the positions of the planets in order to divine the best days for weddings and wars.
Scientists at the time believed that "philosophy described the world as it really is," according to Wootton. "Whereas math just told you what is possible."
Regardless, math is a lot more useful than metaphysics when you're trying to build a telescope, which is exactly what Galileo was working on at the turn of the 17th century. By day, he taught the dubious science of astrology and Ptolemaic astronomy to his students; by night, he struggled to gain an ever-sharper view of the stars. The telescope he'd perfected at the start of 1610 was the best of its time, and it revealed things never before seen by a human eye: craters on the Moon, individual stars that made up the Milky Way.
"But that which will excite the greatest astonishment by far, and which indeed especially moved me to call the attention of all astronomers and philosophers, is this," Galileo wrote in "Sidereus Nuncius." "Namely, that I have discovered four erratic stars, neither known nor observed by any one of the astronomers before my time, which have their revolutions round a certain bright star, one of those previously known, like Venus and Mercury round the Sun, and are sometimes in front of it, sometimes behind it, though they never depart from it beyond certain limits."
Those were the first four moons of Jupiter (we now know the gas giant boasts more than five-dozen satellites).
The findings reported in Galileo's slim little pamphlet were met first with incredulity, then with awe as more and more astronomers verified Galileo's claims. According to Heilbron, the English ambassador to Italy had a copy sent to his king, with an accompanying note calling it "the strangest and most wonderful thing that had ever been announced anywhere."
"Very quickly there were people calling Galileo the Columbus of the heavens," Heilbron said. "Independent of the question of whether the Earth goes round the sun or vice versa, he had achieved the most remarkable reworking of the heavens since the time of Adam. These were things that were not available to the naked eye."
Galileo himself was mostly silent about the true significance of the discovery: that Earth was not the only thing in the universe around which objects could revolve. Historians still debate whether he already believed Copernicus's theories — if he did, he was not ready to say so publicly.
More time and further discoveries would solidify his support for the heliocentric system, and by 1633 he was the gutsy dissident of popular legend, uttering "but yet it moves" sotto voce before the Catholic Church inquisitors demanding he recant.
But censorship and imprisonment could not stop what Galileo helped start. Wootton argues that Galileo's discovery of Jupiter's moons launched not just modern astronomy, but modern science as a whole.
"It was the beginning of a new understanding of what science is supposed to be about, which is making discoveries using new techniques, and developing those techniques," he said. This has been a guiding principle of everything that came after, from the development of microscopes to examine Earth's smallest creatures to the construction of spacecraft that can probe the secrets of the solar system.
When Wootton heard about the aluminum Galileo figurine onboard Juno, he laughed with delight.
"I think that's wonderful," he said. "When Galileo writes that his book is a 'messenger from the stars,' he's imagining space travel. He sees the telescope as something that allows you to do virtual space travel."
Regardless of the aesthetics of a little Lego character, "he would have been thrilled at the idea that his own name would have been carried to the stars."