The next major figure is a Titan in the history of science, Galileo Galilei. Born in Pisa, Galileo was the first modern physical scientist conducting experiments, developing theories, and generally an all-rounder. He followed the scientific method to the t, and defines physical science for succeeding generations. As Galileo approached the study of astronomy, he was aware of three objections to the Copernican model, rooted in Aristotelian physics. The first was the objection to the Earth being in motion, because no motion is apparent, even though we should all be moving at hundreds of miles an hour as the Earth spins on its orbit once a day. The second objection, is based on the perfection of circles and spheres. The fact that imperfections in the heaven would not be countenance, in the Greek view of the cosmos. The third objection relates to stellar parallax. If the earth is suddenly in motion, then why do the stars known appear to change their brightness and relative positions or angles, as the Earth goes around the sun? Two thousand years passed before a revolutionary breakthrough was made by a mathematics professor in the ancient maritime republic of Venice. In 1609, Galileo Galilei demonstrated an instrument that would soon be called the telescope. From the tallest build towers, he showed that this device could spot approaching ships hours before their sails were visible to the naked eye. Later, when he aimed his telescope at the night sky, Galileo discovered that the moon was a world with mountains, Jupiter had its own moons, and the Milky Way was a band of countless stars. Galileo overcame the first objection by coming up with a precursor of the modern physical idea of inertia; the idea that the Earth as it spins and rotates, carries it's atmosphere with it. So for someone standing on a surface, a large motion of rotation is not felt. He extended this into what's called the Galilean theory of relativity. He conceptualized a situation where someone might be below the deck of a ship sailing on a smooth body of water, and in this sealed environment with no window to the outside, might not be aware that they were in motion. Any experiment they could conduct with falling or dropping objects, would look the same as if they were stationary on the surface of the land. That was enough to convince him that the Earth indeed could be moving, and carrying its atmosphere with it at the speed required by its size and rotation. He also recognized that an object in motion, might stay in motion, which is against the ideas of Aristotle and also was a precursor to one of Newton's theories of motion. We think of Galileo as the first physical scientist for these insights. In fact, he did conduct many experiments with falling objects. The story of him dropping bocce balls and wooden balls from the Tower of Pisa is unfortunately apocryphal, was written by a student of his after he died. In his laboratory, he conducted many experiments with falling or dropping objects, often using inclined planes to slow them down so he could see their motion better, and timing the situation with his pulse. These observations convinced him that the way an object fell had nothing to do with the nature of the object, that so-called light and heavy objects would fall at the same rate of acceleration. It's perhaps a little expensive to spend $50 billion and $2,014, to go to the moon to test out Galileo's idea, but it was indeed done on one of the Apollo missions. Well, in my left hand I have a feather, and my right hand a hammer. I guess one of the reasons we got here today was because of a gentleman named Galileo a long time ago, who made a rather significant discovery about falling objects and gravity fields. We thought that where would be a better place to confirm his findings than on the moon. So we thought we'd try it here for you, and the feather happens to be appropriately a falcon feather, for a falcon and I'll drop the two of them here and hopefully, they'll hit the ground at the same time. How about that? That proves that Galileo was correct in his finding. The second objection to the heliocentric model, was rooted in the Greek idea, that spheres and circles represent a perfection, and so should apply to the celestial realm. But Galileo showed with his telescope, that the sun was imperfect, that it had sunspots. He was also able to show by the differential rotation of the sun, that it could not be a solid object. He's telescope also showed the mountains, valleys and geological features of the moon in extravagant detail, making it clear that the moon was a geological world like the Earth. This observation alone removed the distinction between the Earth as a special world in space, and other worlds in space. This is the beginning of what's called the many worlds idea. He was able to detect moons around other planets. So not only were the Earth and the other planets going around the Sun, but there were objects orbiting other planets. Once again, removing the specialness of the Earth as the center of everything. In particular, his series of observations of the four brightest moons of Jupiter, the so-called Galilean moons, although he named them the Medicean moons after his patron the Duke of Medici, are exquisite scientific observations used to show objects going around another object, while Jupiter goes around the Sun, also confirming Kepler's laws applying to the moons of the giant planet in the outer solar system. Galileo also was an innovator in the way that he published. Until the time of Galileo, scientific publishing occurred exclusively in Latin, and there were no scientific journals. So manuscripts were painstakingly copied, or reproduced and passed around small communities of scientists. Galileo did two things. He published in Italian, the language of the common people, and he wrote his science in pamphlet form, or in short quantities. He also gave public lectures. So he made his science digestible to a larger audience, because he wanted to get these ideas out. The third objection to the heliocentric model was a lack of stellar parallax. The brightening and dimming of a star or the change in the angle between stars as the Earth moves in its orbit of the sun. This had never been observed with the telescope and even Galileo could not observe it. Stellar parallax of even the nearest star was not first detected until 1824. But what Galileo could do with this simple telescope was resolve the milky gauze or the milky way into the pinpoint light of thousands or millions of stars. He could calculate that the difference between the brightest and faintest stars he could see in the night sky, was factors of hundreds of thousands, perhaps millions. Given the way light diminishes in intensity with distance from a source by the inverse square of the distance, that implied a difference in distance of the brightest and faintest stars assuming they're the same kind of star with factor of thousands. So Galileo's observations literally gave three-dimensional depth of the universe and convinced him that we lived in a huge three-dimensional universe with the stars at enormous distances. However, the clenching observation for the Copernican model was the observation of the phases of Venus. In the geocentric model, all the planets orbit the Earth. So Venus would have no change in its phases, no phases at all. But in the heliocentric model, Venus passes between the Earth and the sun in one part of its orbit and is in the opposite side of the sun to the earth and another part of its orbit. So Venus should go through a full set of phases and also change its angular size depending on whether it's between us and the sun or on the far side. Very clear prediction of heliocentric model. To the naked eye, Venus' resolve as a non twinkling patch of light, but phases are not visible. Galileo was able to observe the phases of Venus providing a slam dunk affirmation of the Copernican model. When he wrote this up in his pamphlet form in Italian, this was his center argument. While he was doing this, of course, he was making enemies with the church. The Catholic Church had still stood behind the geocentric model and that cozy pact between humans and their creator. Galileo was violating this with his very public affirmation that the earth was not the center of the universe. To complete the story of Galileo, we need to know a little bit more by Galileo, the man. A brilliant scientist no doubt. But also a somewhat obstreperous and willful individual. He, essentially, was fired from his first university where the physics department's run by Aristotelians who used to wear togas in homage to their idol. Galileo mocked that Aristotelians and their toga wear so mercilessly he was, essentially, fired. At his second university, he was unable to get funding for his ideas. There were no research institutes funding astronomy in those days. So he went to Venice, the City-State, and one of the most rich and powerful places in Europe and got funding for his work through a very ingenious method. To the Doge of Venice, the leaders of Venice, he didn't sell the telescope as an astronomical device. He knew that might have no interest to them. But he took them to the top of the Campanilla and some Mark's Square in Venice and showed them that with this telescope you can see an approaching fleet hours before it docked into port. Why was that important? Well, back in those days, sea travel was dangerous, and any fleet traveling with spices or silk or valuable commodity would have ships lost to hurricanes or piracy or some that just sunk. Armed with a telescope, you could see the manifest and recognize which ships from the fleet had been lost, and say, for example, the nutmeg ship had gone down. Nutmeg being more valuable than gold at the time ounce per ounce, you had a few hours to walk around Venice buying up all the nutmeg when you were sitting on a goldmine. It's essentially the future's market. The Doge of Venice and his confederates were quite well aware of the financial implication of the telescope, and so they funded his research. So as you can see, Galileo was a very savvy man. But he was not savvy in his dealing with the Church. As he went around Italy, and indeed Europe, lecturing about the Copernican model, he lectured as if it was the absolute truth with no doubt. Yet, there was some doubt. It was a model with evidence to support it, but it was not irrefutable at that time. In 1633, he was brought in front of the inquisition and Cardinal Bellarmine, the Pope's right-hand man, warned him very sternly not to teach the heliocentric model as the absolute truth. Just teach it as a hypothesis, he said. But Galileo was not happy with that. He was convinced that it was true. To accentuate the point, Bellarmine before Galileo left, took him into a backroom where the instruments or torture were on display. Believe me. These were not rusted instruments of torture. These had been used barely three decades before Giordano Bruno, Dominican mystic, had been burned at the stake in the Piazza de Fiori in Rome for believing in many worlds and life on other worlds. So Galileo knew what the stakes were. He went off and continued with his work. When he was brought in front of the inquisition the second time, he was forced to recant. In a visit to the Vatican Observatory, I've seen his confession. Apocryphal story says that under his breath, he said, "I oppose the movement, but it really moves." But that's not clear. It looks like he genuinely recanted and he spent the last 15 years of his life under house arrest and not to unreasonable situation and beautiful villa outside Florence. But he was a sad man, blinded by careless observation of the sun, writing up his work on physics, and somewhat isolated except for famous people who visited him in his villa. But the genie was out of the bottle. The Copernican system was adopted as fact, and in part because of the obstruction of the Catholic Church, the locus of science in Europe moved to Northern European and Protestant countries, and therefore, Isaac Newton. The final page in the Galileo's saga with the church took a while to unfold. It wasn't until 1824, the same year stellar parallax was first observed, that Galileo's book was removed from the banned list. Since the church moves in strange and slow and mysterious ways, it wasn't until 1992 that Pope John Paul the II, officially pardoned Galileo for the heresy of being right about the solar system. Galileo cements the Copernican revolution with his observation of the phases of Venus with this small telescope. But that's not all. He also shows that the physical universe is three-dimensional with vast distances between the stars. He shows that the moons of Jupiter orbit Jupiter and that there are other worlds in space. The beginning of the idea that not only is the earth not unique but there might be life on other worlds. There's push back against Galileo's ideas and he comes into conflict with the church. But by the end of Galileo's life, the idea of the earth as part of a much larger universe than the Greeks ever imagined has taken root.