 Greetings and welcome to the Introduction to Astronomy. In this video we are going to be talking about ancient astronomy, some of the earliest times, going back to the ancient Stone Age and then through the ancient Greek times, and then heading towards modern times as we will pick up in the next lesson. So we will be looking at some of the very earliest eras of astronomy that we know of. So let's go ahead and get started here and what we see is, first of all, astronomy is really the earliest science and one of the earliest things that was studied and why was it so important? One of the big questions can be why? Why was this so important to the early astronomers? Well, a couple of things that were important for this one is that astronomy gave us the calendar. So a way to keep track of time by following the motions of the sun, planets, and stars. And that gave us a way to find out when the seasons were coming. And you have to think back thousands of years ago, this was not something obvious as to when the first day of summer or the first day of fall was, and that was very important for things like agriculture. When was it time to plant? Plant everything so that you could actually get the crops ready for the new year. And one thing that relates to this was the Egyptians which used the rising of Sirius. Sirius, the brightest star in the sky, rose for the first time early in the morning when it was visible and that coincided with the time that the Nile began flooding. Now Egypt is of course in a desert area so the Nile flooding was a way to bring water to the areas around it and make them more fertile. So it let us let them know that it was time to be able to begin the plantings. Another thing that was used is for navigation and the Polynesians were big with this and especially because when you look at the area out there in the South Pacific, it's a lot of islands. So you have to be able to navigate from island to island and that's very important. It's very much easier to walk around on land than it is to travel between the islands because once you're out of sight of an island all you have is ocean around you. So it was very important to be able to know the patterns of the stars and be able to use that to guide you from one area to another. Now some of the very early things that we look at in terms of ancient astronomy and one of the very prominent ones is Stonehenge. Now Stonehenge is well known ancient stone section here of materials built up in a specific pattern and you can see the image of Stonehenge here on the left hand side and on the right we have a schematic showing how everything is lined up. So there was the great stone circle around and what lined up. When you looked at the altar stone here at the center and compared that to what was the heel stone out here in the distance, when you looked at that that gave you the summer solstice sunrise. So you could see when the sun was rising that would give you the summer solstice sunrise by that timing when the sun when you'd stand here by the altar and look out over the main axis of Stonehenge that told you the first day of summer when sun was rising furthest to the north. But that's not the only thing that has been found now. We have found far more because we have been able to calculate other things that we find which includes different moon rise, moon set and sunrise and sun sets at different times of year and especially timed to the winter solstice and the summer solstice. So it was very useful as a calendar to be able to track those times to let the ancient peoples know when things would with time of the year it actually was. Now one of the difficulties with Stonehenge and the many other monuments scattered around the world that are similar to this is that there's no records, there are no instruction manual for Stonehenge to tell us what it is supposed to do. There's nothing that says stand here and see this. This is what we have learned from looking at it and from doing studies and computer simulations to figure out where everything would rise or set. So whether these were all the intentions, that is a good question and not something we can easily answer. But it is unlikely that you would have so many alignments occur just by chance. Now to things that we know a little more and we actually have records, we go back to the ancient Greeks and the ancient Greeks, there were a number of them, I've listed a few here and we will talk about some of these over the coming sections including we look at Eudoxus and really what I wanted to look at for his was he gave us one of the very early models of the universe to explain how things work and give us the first methods of being able to predict the positions of the planets. Aristotle gave us two, a couple basic postulates, one that said that the earth knew that the earth was round and the other was that the earth was the center of the universe and we call that a geocentric universe, meaning that the earth is at the center and Eratosthenes gave us a measure of the circumference of the earth, so he was able to measure the circumference of the earth, so we already knew at this point that the earth was not flat, so it wasn't later that we figured that out, the ancient Greeks knew that they were able to measure the circumference of the earth and able to determine that the earth was definitely not flat and was spherical and in fact one ancient Greek astronomer Aristarchus gave the idea that the sun was the center of the universe, so first thought of this that we know of at least the first recorded record of someone suggesting that the earth is actually moving and it is the sun that is the center of the universe instead. Hipparchus gives us a couple of things that we still use today, magnitudes, a way of measuring the brightnesses of the stars and the idea of precession, the way the earth's orbit, the earth's tilt, changes over time and then finally that all culminated with Ptolemy in the second century and that was he gave us a mathematical model in his great work called the Almagest that was a way of explaining how the planets moved and being able to predict their positions. So let's look at a couple of these, I'm not going to go through all of them but we're going to look at a few of these in a little bit more detail. So let's go ahead and start with Aristotle and Aristotle gave us two things here. He first of all said that the heavens are perfect so that everything in the heavens was going to be perfect and that meant that things moved in circles and everything had to move in a circular orbit. A circle was the perfect shape so everything in the heavens would move in a circular orbit and also that they would move at uniform speeds. The speed would never change so however fast a planet was moving it always was moving at that speed. So he gave us these two postulates to explain planetary motion and these were really the foundation of planetary motion for over a thousand years. This is what was used. It took more than a thousand years to get rid of these two things not until the time of Kepler that we'll talk about in a future lecture did we overcome these two ideas and in fact Kepler's laws of motion, his first two laws of motion are what finally got us out of these. But for over a thousand years this is what we used for being able to predict the positions of the planets. Now as I said the ancient Greeks did know that the earth was spherical so we could do that and we could measure then the size of the earth and Eratosthenes was able to do that and what he noted was that at Syene, well in Syene here, that there was no shadow on the first day of summer. So that would correspond to the first position over here. So this position here the Sun is coming straight down and there is no shadow visible and if there's no shadow then the Sun must be straight overhead at that position. Whereas at Alexandria here we note that there was a slight angle and that a small shadow was cast. So here at Alexandria we would note it's a seven degree angle and that's what we see in the little inset image up here that there would be a slight angle and that allowed us to determine the circumference of the earth. We could then figure out a ratio because this angle between those two cities was seven degrees. The Greeks knew that a full circle was 360 degrees so this was some fraction of the complete circle. So all they had to do was to measure the distance between these two cities, not an easy feat to do back at that time, but you almost had to walk off the distance between those two cities. But once you did that you could then figure out what fraction seven is of 360 and figure out then what fraction the distance you measured is to the entire circumference of the earth. We don't know exactly how accurate he was. It really depends on the measurements that were used and of course measurements were not standardized back in ancient Greek times the way they are today. So the measurements that were used there's some question as to exactly how accurate he was but it's possible that he was able to measure the diameter of the earth, the size of the earth, to within about 10%. So Hipparchus gave us a couple of things here and one that we look at is the idea of magnitudes. Magnitudes are a way of determining the brightness of the star and it's based on their apparent brightness, how bright do they appear in the sky. And he did, he divided the stars into six groups. The first magnitude stars were those that were the brightest, the grandest stars in the sky and the ones that could barely be seen were stars of sixth magnitude. Now our measurements of magnitudes, which we'll discuss in a future chapter in more detail, we're all based upon Hipparchus' setup and essentially what he did was give us backwards magnitudes. In most things that we measure bigger numbers are brighter or hotter or longer. Here in magnitudes the smaller the number the brighter the star. So everything is backwards so it would be like a smaller temperature meaning a hotter object and that's the way magnitudes are still work today. Now the other thing that he determined was procession and in fact what he found is that the earth spins like a top which causes the pole star to change. So the pole star, Polaris here that we see up towards the top here is currently very close to the north celestial pole. But the orange line here is showing the path that the pole will take. So Polaris stays at the same point but the pole slowly moves along so that in 2000 more years the pole will be pointing towards this direction, another 2000 it will be over here, and so on. So the pole of the earth does not always point toward the same star. It can point to different places and in a regular pattern. And this occurs because the earth spins like a top and actually can change what we have here. And here we see it as it would spin like a top and we can see the earth and how its axis points right now towards Polaris. However if we look at it and let the clip run here we can see that over time it will slowly change its position. And it will start and we'll let that fade and look at it one more time as we start and it will point towards Polaris and then it will go around and back to Polaris again. So right now Polaris is our pole star and it will begin in 26,000 years. But 10,000 years ago or 5,000 years ago Polaris was nowhere near the pole. So things are constantly changing because of procession and this is very important to astronomers because their coordinates that they use, things like right ascension and declination are constantly changing because the earth's pole position is changing what changes our entire coordinate system. So let's look at how we can put all of this together then. And what we find is that Tolmi put all of this together in his work called the Almagest and he put together a model of how everything works. We had the earth here at the center. So let's look at the earth here. There is earth, the blue dot. That is earth. And actually if you notice it's not quite at the center. The center portion is this little line right here. The earth is off-centered slightly and then there are other sections. The actual orbit is centered around this point B which is the equant. So the equant is point B in this image and that is what everything is orbiting around. Now the main orbit of the planet was called the deferent. That's letter C down here. That's the large circle of the orbit. But the planet did not actually move on the deferent. It moved on an epicycle labeled A at the epicycle. So it would go around the epicycle here while the epicycle would go around the deferent. And that would account for the motions that were seen in the sky. So we're able to then explain the positioning of them. And all of these things were set up to account for the positions changes that we saw. So why is the earth not at the center? Well that was needed to account for different speeds. The fact that things would move slower and things would move faster at different times. Since the planet couldn't change its speed the earth had to be off centered which could account for that. And the epicycle was needed to explain what we call retrograde motion, the apparent backward motion of the planet relative to the stars. So why did the planets go backwards? Well there actually had to be some kind of backward motion. Now this is what was used for over a thousand years to be able to predict the positions of the planets. And it actually worked relatively well to the accuracy with which things could be measured. It was it would give you very correct answers. So why did we believe this for such a long time? Well there was no reason to change it when it was still working. And it wasn't until more instruments were developed that gave us more accurate measurements that we were able to find that there were some deviations that simply could not be explained in this type of model. But for over a thousand years this is actually what we used to be able to explain the motions of the planets. Now after Ptolemy the Roman Empire fell and there was kind of a dark the dark ages descended on Europe. So we lost a lot of things that had been going on a lot of material that had been going on and a lot of materials were lost. So in Western science declined the library of Alexandria in Egypt was burned and a lot of work was lost. So many of the Greek material that we have a lot of it was actually destroyed so we don't actually know all of their materials that were written not everything survived as it would more likely today. However even during this time work advanced through Indian and Arab astronomers who did a lot of work who translated the works of Ptolemy and others into their languages and then refined them making them better and they continued to develop different types of mathematics. The Greeks everything was done geometrically and algebra was developed by Arab astronomers as better ways of being able to calculate and this work then returned to Europe during the Renaissance which is what we'll look at in the next video where we will talk about how the Renaissance astronomers were able to put this all together and come up with a new model that did not have the Earth at the center but instead put the Sun at the center of the universe. So let's finish up here as we do with our summary and what we find is you know what have we learned well we looked at how the study of the sky was important for a couple of things for timekeeping for navigation as a couple of ways of being able to keep these. We looked at Stonehenge as one ancient monument but there were a lot of others around the world that did similar things and were used for timekeeping. We looked at the Greek astronomers who gave us the geocentric model of the universe and that was our model that explained planetary motions for over a thousand years and finally Indian and Arab astronomers preserved and improved upon the Greek work which was then which then came back to Europe with those modifications and with those improvements during the Renaissance. So that concludes our discussion of ancient astronomy. We'll be back again next time for another topic in astronomy. So until then have a great day everyone and I will see you in class.