 Greetings and welcome to the Introduction to Astronomy. In this lecture, we will talk about ancient astronomy and that is astronomy of ancient time. So we will start off with some of the very earliest, even the prehistoric astronomy and what astronomy was used for at the time. Astronomy is really one of the oldest sciences and has been considered since people have looked up at the sky thousands and thousands and millions of years ago. So it is something that has been around for a long time, not a new science, but a very old one, in fact probably the oldest science. So let's start off here with the very ancient, what do we mean by it being an ancient science? So we see astronomy as an ancient science because it has been around forever, ever since people could look up at the sky. And why was it useful to these people? Well, for a number of reasons and one was that this was their calendar. Because as you recall, perhaps we use things like the motion of the sun to measure our day. We use the phases of the moon to measure our month. And we use the motions of the sun relative to the stars to measure our year. So our calendar is based on astronomical events. So they are used to be able to determine the calendar where we are in the year. And some of the other things, such as the Nile flooding, was actually predicted as well. It came at a certain time of year. But how to know when that was thousands of years ago? And one of the things the astronomers noted was that the rising of Sirius would correspond to about the time of the Nile flooding. So when Sirius was first visible, when they were first able to see that in the sky in that time of the year, that meant that the Nile flooding was not far off. And that was very important for agriculture in Egypt at that time because the Nile flooding would then bring water to the desert areas and actually give some water as a way for to be able to grow crops. The sky, ancient sky, was also used for navigation. And the Polynesians used that. One example is Polaris, the pole star, which happens to be located near the North Celestial Pole. Its position changes depending on where you are. The further north you go, the higher up it gets in the sky. The further south you go, the lower it gets in the sky. And that gives you a way to be able to determine your location. So navigation was something else that was very important and used for, used astronomy was used for. Now one of the very ancient monuments and one of the best known ones is probably Stonehenge. Stonehenge is the, in England and you see the great stone structures here that are pictured. That is the ancient monument from thousands of years ago and built over the course of thousands of years. There's actually more to the monument than just what is shown in the image here. Further out, there are holes that remain that were probably other wooden, earlier wooden monuments that have since decayed and gone. But we do see the remnants of even older structures. And what we see on the right hand side here are some of the alignments that we see. So the altar stone here, if you look out from that direction, out on what we call the main axis of it, if we look over the heel stone here, which would be out at this direction, on the first day of summer, the sun would rise over the heel stone. So if you were standing in the center of this monument, you would be able to watch the sun rise on over the heel stone only on the first day of summer. Any other day, it would rise further to the south of that and would not appear to rise directly over the stone only on that one day. Now, there are other alignments shown here. And one of the difficulties with Stonehenge and other monuments like this is that there is no manual. So we do not know how this was supposed to work. We don't know if all of these alignments were planned or if some of them are just coincidental that you happened to line it up to match for the summer solstice sunrise. And maybe some of these others just happened to work as well. But we don't have any instruction manuals for Stonehenge or any of the other great monuments scattered around the world. So there are monuments, not just similar monuments that have alignments with various astronomical events, not only in Europe, but in Australia, in the Americas, all over the world, we see various different monuments that show that probably these were used as a calendar early on and a way of being able to predict the time of year. So let's go a little further forward into the ancient Greek astronomy. So this is really the basis of what we use and this is, we're going to look here at some of the early astronomers and we'll talk about a few more of these in detail later on, but Eudoxus is one who gave us a very early model of the universe and it was what we call a geocentric universe, meaning that the Earth was at the center. He used nested spheres to explain the motions of the planets, so by having spheres, multiple spheres for each planet to be able to explain the various motions. So you would need a sphere to explain the rising in the setting of that object, of that planet, for example, but you'd also need to be able to have other spheres that would explain how it moved through the stars. So it would take multiple spheres to be able to explain that. Aristotle took and built on this, built in this geocentric universe and also knew that the Earth was round and there were a number of different proofs of this. One big astronomical one is the shadow of the Earth. How do we see the shadow of the Earth? Well, during a lunar eclipse, we can see the shadow of the Earth on the moon and it was noted that it was always circular. The only object that always casts a circular shadow, no matter how it is oriented, is a sphere. So they knew very early on that the Earth had to be spherical and in fact, Eritostanese actually measured the circumference of the Earth so they knew that it was spherical and not only that, but they were able to measure how big it was. Aristarchus was one early astronomer who suggested that the Sun might be the center of the universe. That would be a heliocentric universe. So a heliocentric universe would be another possibility. At the time this was discarded, how could the Earth possibly be moving? We would know that the Earth is moving and one of the things that was missing is that they could not detect parallax. Parallax is the apparent shifting of a nearby star relative to a more distant one and if the Earth were moving around the Sun, it should be something that was detected and the Greeks were unable to detect it so that meant that the Earth could not be moving. Of course, we now know that the parallax is just so small that it would not have been possible to measure it. Hipparchus will talk about in more detail. He gave us the magnitudes, measuring the brightness of stars and discovered procession and how the pole is moving over the course of time. And then Ptolemy kind of put everything together in his great work, the Almagest, which gave us a formal mathematical model explaining how the universe works and our understanding of it at the time. So let's look at some of these in more detail. First, we'll look at Aristotle. Aristotle gave us two things here. He gave us the concept of circular orbits and uniform speeds that everything in the heavens moves in a circular orbit. That's the perfect shape and everything moves at a uniform speed that there's no variations in speed. Everything in the heavens was perfect and that was very important there and really led to the basis of our understanding of planetary motion for over a thousand years. This was the basis of it. Now these are what we call postulates or assumptions. We did not know that they were true but they were assumed to be true because they made sense. We use postulates today and in fact Einstein did in his special theory of relativity saying that the speed of light in the vacuum is the maximum speed that anything can attain. So if that turns out not to be true, there's no proof of it other than that it has held up to experimentation over time but it is something that Einstein's theory was based on. If we ever find something that can travel faster than light that throws out Einstein's theory of relativity just like when we found that things did not follow circular orbits and did not follow uniform speeds, it threw out Aristotle's model of the universe but for over a thousand years this was our understanding of how things worked. Now we did not only do we know the earth was spherical but Eratosthenes actually measured the size of the universe. So Eratosthenes was able to do this by making some measurements and actually making note of the fact that at Syene in Egypt on the first day of summer that the well cast no shadow meaning that the sun was straight overhead. So that's this image right here. This object would actually cast no shadow. If you look down a well you'd be able to see straight down it because the sun is shining straight overhead. A little further north of this at Alexandria on the same day the sun was not quite overhead but was at an angle of seven degrees. So you'd have that slight angle shown here and the difference between those allowed him to determine the circumference of the earth. You could measure the linear distance between the two cities here and you knew this angle this was seven degrees. You know that the entire circle is 360 degrees and you can set up a ratio that you have now measured. You know that this is seven degrees out of 360. Well then this distance if you can measure that is the same proportion of the entire circumference of the earth and he got a very accurate measurement based on the numbers of the time. If our understanding of their units of measurement is correct they may have gotten this correct to within a few percent or so. So it could have gotten a very good measurement of the size of the earth thousands of years ago. Now one of the other astronomers we mentioned was Hipparchus. Hipparchus gave us a couple of things here. He told us about magnitudes. He developed the magnitude system that we use today and procession. First looking at magnitudes he divided the stars into six bins. This was based on their apparent brightness how bright they appeared in the sky and his first magnitude stars were the brightest. So that was the brightest groups those were stars of the first magnitude and then down to the sixth magnitude stars that were the faintest. That was just barely what you could see. That was barely what you could see with your naked eye those were just visible. Now that led us to the fact that magnitude system is backwards because as we've used it today a star with a magnitude 1.5 and a star with a magnitude 3.5 this is the brighter one and this is the fainter one. So there's a difference in that but the bigger number means a fainter star whereas the smaller number means a brighter star and that's the opposite of what we usually do in terms of measurements. But everything is still based on what Hipparchus gave us thousands of years ago. Now procession is another example and that is how the earth spins. The earth spins much like a top. If you've ever watched a top spin you notice that it spins very quickly on its axis just like the earth spins once a day on its axis but that the direction of that axis changes at a much slower rate. The earth axis does the same thing so where the North Pole is pointing in the star in the sky is changing over time. Right now it's pointing towards the direction of Polaris. Over time that changes and thousands of years from now, 2,000 years it'll be closer to another star here and thousands of years after that it'll be closer to other stars and there will be times when it is not close to any star. So a couple thousand years ago there was no star near the North Celestial Pole. The fact that we have one now is just a coincidence and this changes over a 26,000 year cycle. So Polaris is now near the star near the North Celestial Pole and it will be again in 26,000 years. In between that we'll take turns with other stars and times when there is no star near the North Celestial Pole. Now let's look a little bit about how this works. So here's kind of a diagram that shows how that pole changes over time. It's near Polaris right now but later on as this moves it will actually go around and we'll watch it here again in 12,000 years or so. Vega, an even brighter star will be somewhat close to the North Pole. Won't be near as close as Polaris is but it will actually be a very bright star near the North Celestial Pole. And that change will change consistently over time due to procession and that ends up changing the entire astronomical coordinate system over time. So how we measure where the positions of stars are is constantly changing because we measure them relative to the North Celestial Pole. Now this was all put together by Claudius Ptomey and what he did was give us really a formulation of the geocentric universe. So looking at things as a geocentric universe and some of the terminology that we use here is he gave us the epicycle, which epicycle is A in this image. So the epicycle is this small circle. The deferent is C, that's the orbit that the center of the epicycle takes and the equant is B. I'll come back to that in a minute. Let's first look at how this worked. The epicycle, here's the Earth, very close to the center and the center of the epicycle moved around the Earth and while it did that, the planet moved around on the epicycle. So the planet was actually orbiting empty space here and while it did that, this moved. Now this was important because this accounted for retrograde motion and this was important because the planets would appear to move backwards relative to the stars from time to time and we had to be able to make that prediction to be able to have our model explain why that happened and this was one way to explain how retrograde motion occurred. We'll later see with the heliocentric universe that there was another simpler way to be able to explain this. However, in this case, it does explain retrograde motion and Ptolemy was able to make predictions of the positions of planets that were about as accurate as the observations of the time. So very relatively accurate and worked for the predictions and was able to give us pretty good predictions of how things worked. So the epicycle, this part, the deference, the big circle here, the equant is actually point B here and that is directly opposite the central object. So the center of the universe is actually here offset from the Earth a little bit and the equant is the point on the other side. So the object, not actually going around the center of everything or around the Earth, but actually around the equant, a pretty nice on the opposite side of the center from the Earth. And that accounted for things getting a little bit closer to and a little bit further from the Earth and changes in their apparent speed. So it was able to account for some of the observations that occurred. And as I said, this lasted for a thousand years, well over a thousand years as our way of explaining how the universe moved, how objects moved within the universe. Now, what happened after Ptolemy? Well, we had the fall of the Roman Empire and that led to a decline in Western science. The great library at Alexandria was burned and a lot of the information that was lost, a lot of the information from some of the astronomers that we talked about ended up being lost. So there's probably a lot more that they knew and that they did that simply has not been recorded. However, not everything was lost at that time. Much of the work was advanced through Indian and Arab astronomers who had copies of some of these works and continued to build on them. So they translated them and then refined these Greek works and developed the mathematics, especially of algebra that was needed to be able to do these calculations. The Greeks did everything with geometry. So they built everything geometrically. Algebra was then needed to be able to do some of the calculations to explain these motions. So their refined works then eventually ended up returning to Europe at the time of the Renaissance and then were continued and expanded and we'll see eventually led us with more accurate observations to a heliocentric universe that we have today that the sun is at the center of the solar system and not the earth. So let's summarize what we've looked at in this lecture and we started off, we talked about how this guy was important for timekeeping and navigational purposes, being able to know what time of year it was and how to be able to travel was very important and the stars were an important way to do that. We used Stonehenge that we talked about and other ancient monuments that were used for the timekeeping process to be able to tell us when summer was starting or when winter was starting. We knew when those seasons were. We looked at the Greek astronomers and kind of went through how they developed a geocentric model of the universe that lasted for over a thousand years and during the Middle Ages, not much was going on in Europe but Indian and Arab astronomers preserved and improved upon this work that later returned to Europe and helped lead to the Renaissance and our updated understanding of the universe. So that concludes this lecture on ancient astronomy. We'll be backing in next time to discuss another aspect of astronomy. So until then, have a great day everyone and I will see you in class.