 Greetings and welcome to the Introduction to Astronomy. In this video we will talk about the origin of modern astronomy, and in this case working through the time of the Renaissance, and up through for quite a time after that, and discuss how our understanding of the universe changed. So in the origin of modern astronomy, we want to look at how everything changed. Remember that the Greeks had a very specific view of the universe, that everything was centered on the earth, and that everything moved in circles and at uniform speeds. We will now come to see how that has changed over as we get into the Renaissance. So let's start off looking at what we call the Copernican Revolution. So this is quite a change over what we had, and what Copernicus gave us was the idea of a heliocentric universe. So that everything was centered on the sun, and that's what heliocentric means, the prefix here refers to sun, and that everything was centered on the sun. This had been suggested by one of the Greek astronomers named Aristarchus, but it was really not seriously considered, because parallax could not be measured. Parallax is the apparent motion of a nearby star relative to a much more distant star. It is a prediction that would occur if the earth is moving, and since it could not be seen that was an argument that the earth could not possibly be moving. It turns out that the stars are just so far away. Now Copernicus suggested really for the first time that the earth was a planet, just like the other five planets known at the time, and that all of the planets, and that included the earth, orbited the sun. This was a simpler model. It was a simple way to explain retrograde motion or the backward motion of the planets. And that would be that the planet appears to go backwards, and the planet was traveling in one direction, and then stopped, and made a loop going the other way, and then continued on. In order to explain this, we needed, remember that we needed epicycles under the Ptolemaic theory. However, under the Sun-centered theory, or the heliocentric universe, this would be explained naturally when one planet passes the other. Now the other question is could it eliminate the problem of epicycles of having to have epicycles? It didn't need them for retrograde motion, but it did need them to be able to explain the exact motions of the planets for reasons that we'll see. So let's look at what we had here. The heliocentric model, this is a sketch from what Copernicus' book, this is the drawing that would have had the Sun at the center, here, and then Mercury, Venus, Earth, Mars, Jupiter, and Saturn orbiting around it, and then the sphere of the stars. So the Sun in this case was the center of the universe. The Earth and the planets all revolved around the Sun. It did make that prediction, it predicted parallax of stars, and it simply explained retrograde motion, so we did not need epicycles here, we're not needed to explain retrograde motion. And let's look a little bit more detail at what we mean by parallax. So I've mentioned it before, but let's take a look at it here, and let's say that parallax is the apparent motion of a nearby star compared to a more distant star. This is a prediction of the heliocentric model. So parallax is the key word here. What is this parallax? Well, if a star is closer to us than the other stars, then when the Earth is at one point here, say in January, and then it's over here in July, six months later in January it's going to appear in one direction in the sky, and in July it's going to appear in another direction. How close this star is will tell us the amount of the shift, and the more that shift is, the easier it would be able to see. So to the Greeks, if this was not able to be observed, the Earth could not be moving, or the stars had to be so far away that it was inconceivable that that would be possible. The further this star is away from us, the smaller the parallax angle will become. This was predicted but was not actually detected until 1838. So in 1838 we measured the first parallax of a star and were able to really see that the Earth must be moving because that was one of the predictions that the heliocentric model made. Now what was retrograde motion? Well we talked about that perhaps previously. Let's look at what we mean here, and here's a little animation showing how the retrograde motion occurs in each of these two examples. So retrograde motion is the key term to look at here. What it is is an apparent backwards motion of the planets. Now this can occur in one of two ways. It can occur because of epicycles in the geocentric universe, and that's shown here on the right hand side that you have the planet, you have the Earth here in blue, you have the sun moving around it in yellow, and you have a planet say Mars in red. And its net orbit as it moves around its epicycle would then give it this backward motion. So even though the planet moves in multiple circles, when you put those circles together it gives it this unusual shape that accounts for the retrograde motion. In terms of the heliocentric universe that we see here on the left hand side, it occurs when one planet passes the other. So again you have the sun in yellow, the Earth in blue, and Mars in red. And as the Earth comes up and passes Mars it will make Mars appear that it is going backwards relative to the more distant stars. So it is an apparent motion but they both can explain the retrograde motion. So what do we know about the heliocentric model? Let's take a look here. And the heliocentric model gave us a couple of different predictions. Was it immediately accepted and that is no. It was not immediately accepted as the model of the universe. One big reason is because parallax was not detected and there was no sign that the Earth had to be moving. So parallax was a prediction it made but it was still undetectable at this time and the values for parallax were just too small and in fact the first parallax to be measured was less than one arc second and one arc second the full moon has a diameter of 1800 arc seconds. So 1800 arc seconds for the diameter of the full moon less than one so we're trying to measure less than one two thousandth the diameter of the full moon. Well beyond the technology of the time. Now it turns out that the heliocentric model was no more accurate in terms of predictions than the geocentric model. So in terms of predicting where the planets were going to be it was no more accurate but it was more realistic. So it's now that it is realistically what happened but the problem was that it continued the use of circular orbits. So the fact that it was using circular orbits this is what the problem was. We were constantly using circular orbits and that then required the use of epicycles not to explain retrograde motion but to explain small deviations in the orbit from a perfect circle. So it was really no more accurate in terms of predicting the planet positions of the planets. It was of course far more accurate in terms of what reality is. Now one of the other great astronomers of the renaissance was Galileo and Galileo gave us really was in a way the first scientist in terms of experimentation. So Galileo was experiments so he did experiments to study the motions of objects gave us the idea of the concept of inertia that would eventually become Newton's first law and found that all objects accelerate at the same rate due to gravity it didn't matter whether it was a less massive object or a more massive object it would accelerate at the same rate. Galileo also used the telescope. He did not invent the telescope but he was the first to use it to observe the sky that we know of and certainly to record and publish his observations. So he gave us the first observations of solar system and objects outside our solar system that were made with the telescope so he did not invent it it was actually invented a year or two before he heard about it and was able to make a telescope and then use it to look at the sky. Now what did he look at? Well let's take a look at some of his observations here and he looked at things like the sun, the moon. Now what did he learn about these two? Well what he really learned was that the sun had spots, sunspots and that it was rotating. Having sunspots meant that the sun was not perfect and that was in contradiction to what the Greeks had told us that the heavens were perfect and that everything moved in circles. Well here we have evidence that the sun and the moon having mountains and craters again were not perfect. He observed Venus. So what are you going to look at with this telescope? You're going to look at the brightest objects you can see. And he looked at Venus which was a very important one because he found that Venus has a complete cycle of phases. This is extremely important because it means that it must orbit the sun. That is the only way to explain that you can see its complete cycle of phases. And in fact this one observation threw out the Tome's idea of a completely geocentric universe. It did not prove that everything orbited the sun, only that Venus orbited the sun. But it did throw out Tome's model because Tome's model would predict that Venus was only visible as a crescent phase. Since we could see Venus as a full phase as well now we knew that it was not possible for Venus to orbit the sun. He looked at Jupiter. In the case of Jupiter he saw that Jupiter had four satellites orbiting it and that showed that not everything had to orbit the earth. So one of the problems with that was one of the problems was that how could an object move and not leave its moons behind? So this was one example here of what Galileo saw. He also looked at Saturn. He saw that it had two lobes on either side. He couldn't see the rings but again Saturn was something unusual, not this perfect spherical object. And he saw the Milky Way and saw that the Milky Way was made up of countless stars. There are not a finite number of stars and the stars are probably a lot further away than we originally thought. So let's look at a couple of Galileo's sketches here that we see. And what Galileo showed us, these are some examples of what we see in terms of the craters in the moon and the features on the surface of the moon. Here is the phases of Venus, crescent phase to a full phase. And again, seeing these thicker phases, more than half full, was one of the problems and ruled out Tomey's theory. And he also saw the moons of Jupiter. And here you can see Jupiter is a little disk. And then the little stars, or in this case what we now know of his moons, that orbited around it. So these two especially, the moons of Jupiter and the phases of Venus were very good evidence to help push the heliocentric theory. They didn't prove that the Earth moved but they gave us some very good circumstantial evidence. Now what happened with Galileo? Well Galileo had some issues with the inquisition, as you may have heard. He published his observations, his earliest observations, his earliest 1610. And just a few years later in 1616, any books supporting the heliocentric theory were banned as heretical. So his books were banned at that point. He had worked at that point, he had been working on motion and discussing motion and how objects moved. But in 1632, he published his book, The Dialogue Concerning the Two Chief World Systems, which was written as a debate between supporters of the geocentric and the heliocentric universes. And because of that, he was actually tried, found guilty of heresy, foreholding, teaching or defending the heliocentric view and was placed under house arrest for the rest of his life until he died in 1642. He was eventually vindicated. And in 1820, his books were, his books and that of Copernicus were removed from the index of banned books. And in 1992, he was formally cleared by the Catholic Church. So what really, what happened here? What happened between with Galileo and the church? There's a lot of things that we will never know for sure. But some of the ideas that we've discussed are, one of the problems was that Galileo could not prove the heliocentric theory. He could prove that Venus orbited the sun, but that did not necessarily mean that the earth orbited the sun as well. That proof did not come until really 1838 with the discovery of parallax nearly 200 years after Galileo's death. So there was no absolute proof while he could rule out one theory. He couldn't absolutely prove his theory and there were other models that would work with the earth still being the center of the universe. Some of the other problems that were going on is that the time the Reformation was not that far removed from this time and the church was losing power. So trying to hold onto power, anything that was seen as trying to diminish their power such as supporting the heliocentric theory would be looked at in a bad light and would try to be removed. Internal politics were probably important in this as well and Galileo was not maybe the friendliest personality. He could be quite abrasive at times and his book that we talked about, The Dialogue Concerning the Two Chief World Systems, portrayed the defenders of the geocentric theory as simple-minded people and many thought this was a implementation of the pope, that the pope was the one who he was using as the defender of the geocentric theory and making him to seem to be a simple-minded person did not sit well with the church. So again, he eventually was vindicated but was placed under house arrest for the last few years of his life. So let's look at what we've covered here, finishing up this section. Just to summarize a little bit, we'll look at the summary here. Copernicus proposed the heliocentric model of the universe in which the earth moved around the sun in a circular orbit. The heliocentric model did easily explain retrograde motion but it required parallax which would not be observed for hundreds of years. We talked about Galileo who made and published the first telescopic observations of astronomical objects and found some circumstantial evidence for the heliocentric model but was not able to prove it. He was tried and convicted by the inquisition but eventually in 1992 was cleared by the church. So that concludes our study of the origin of modern astronomy. We'll be back again next time to talk about another part of astronomy. So until then, have a great day everyone and I will see you in class.