all right so today we're gonna continue going through the history of astronomy um focusing mainly on the ideas people had not necessarily every historical event that happened in people's lives so when we left off we had already talked about how ancient people used astronomy mainly for navigation and telling the seasons to help them live more stable lives but again we said that you don't necessarily need to know why the stars move the way they do in order to be able to do those things and so it took a long time for people to start actually trying to apply a scientific method to figuring out why the Stars do what they do and it was about 2,000 years ago a little over 2,000 years ago that the ancient Greeks started really looking into this question and they started trying to figure out okay if this is what we see what's what is the universe actually like what's it shaped like where is everything where are we where are the stars where's all this stuff okay and so what we're gonna do is we're gonna start just building up let's just start drawing a diagram of how they thought the solar cycle universe worked and we'll start with what they thought was in the center okay so the ancient Greeks were trying to figure out what was in the center of everything and they actually came up with two ideas they came up with two major ideas for what was in the center of the entire universe and does anybody know what those two ideas are what were the two possible things the ancient Greeks thought might be at the center of everything give me some guesses on that what's at the center of the universe according to the ancient Greeks or what were their two major ideas yeah one was the earth and one was the Sun okay so they actually a lot of people know that the ancient Greeks thought that the Sun sorry the the earth was in the center of the solar system but they actually did come up with the idea that the Sun was in the center as well so those two ideas are called a geocentric system and a heliocentric system there we go so geocentric and heliocentric and which one of these is for having the earth in the center of everything which one of these is Earth centered geocentric or heliocentric yeah geocentric means earth in the center because Geo right here that means earth so Earth Center and Helio actually means Sun so anytime you see Helio and we're talking about the Sun so heliocentric means Sun in the center okay they had both of these ideas does anybody know which one they decided to go with ultimately when they tried to apply the scientific method to this when they tried to put up tests which one did that yes they went with geocentric okay so even though they had both of these ideas they decided to go for geocentric instead of heliocentric and there's two main reasons for that why did they pick that one two main reasons one of them is a good science reason and the other one is a bad science reason okay in fact one of them is a scientific reason and one of them is not a scientific reason so let me explain the bad reason that they went for geocentric first heliocentric idea who came up with it the heliocentric idea was mainly thought up by a guy named Aristarchus so yes or no real quick Aristarchus give me a yes or no have you heard of this guy before how many of you have heard of Aristarchus yes if you've heard of him no if you have it no no no no exactly probably you don't if you probably have never heard of this guy because they didn't ultimately go with his ideas but the main person who was pushing the geocentric idea was a guy named Aristotle so show of a yes or no who here has heard of Aristotle exactly and if you had asked random ancient Greek people on the street they would have also known who Aristotle was they would not necessarily have known as random people wouldn't have known Aristarchus Aristotle was a very famous scientist back then AP was famous for a lot of things but one of the things he was famous for was science and he actually had done a lot of really good science he had come up with a lot of good well supported ideas but that made him more popular and so basically a lot of people decided that he must be right because he was a famous smart guy right it's kind of like he was sort of like the Einstein of his day like if Einstein Stein figured this thing out you're kind of likely to think he was right but even Einstein wasn't right all the time right so one reason people went were there with the geocentric model is that it had good popular people setting it up that's not scientific at all you can't just trust a scientist you got to try it you got to look at the work right so what's the better reason they went with geocentric well again they applied the scientific method they used observations and tried to figure out which one of these more sense okay and here's the thing the ancient Greeks knew that the Sun was really really really far away because even if you went to the top of a mountain the Sun didn't look any closer so that tells us that must have been really far away right and they also knew that the patterns the Stars made in the sky took one year to come back to the same spot that's what we were talking about a couple of times ago a couple of classes ago so they knew how long it would take either the Sun to go around us it would be one here or for us to go around the Sun that would be one year and if the Sun is really far away let's assume that the Sun is in the center of the solar system let's assume the sun's in the center if the Sun is in the center and we're really far away from it and we have to go all the way around it in one year what does that tell us about the speed of the earth is the earth gonna have to be fast or slow to get around the Sun in one year if it's really far away from the Sun fast or slow fast yes yes it would have to be fast right and that means that if the earth is moving fast we must have some kind of evidence that the earth is moving really really fast so we're standing on a thing that's moving really really fast and remember it right now I'm talking through the ancient Greeks line of logic okay which we'll see is a little bit flawed in a second so check this out did ancient Greek people go really really fast not really they didn't they didn't go really fast what's the fastest an ancient Greek with the fastest thing in ancient Greek has probably ever done it's probably writing something right what what were they probably writing to get make them go fast yeah probably a chariot maybe a horse right a horse or a chariot one of those things something that was horse powered right and so let's imagine that you're riding a horse or a chariot and you're going really really really fast like a hundred miles an hour or something which I don't think horses can do that would probably kill that horse but let's just assume you're going like maximum horse pede okay so you're going real fast even if you close your eyes you'd be able to tell you were going fast right you'll be able to tell that you were going really fast on a horse or a chariot even if it was on really smooth really smooth ride like what's gonna happen to your hair if you're going maximum horse speed what's gonna happen to your hair what will happen pretty simple things yeah I'm just going to say it we all kind of know what's gonna happen here it's gonna be blown back oh yeah there you go there you go it's gonna be blown back you might feel it on your face you're gonna feel wind going past so here's the main question do any of us right now feel wind going past us do we feel when going past right now no we don't okay we don't feel wind going past but going really fast makes it so you should feel wind going past so based on that line of logic what does that mean according to the ancient Greeks if the earth was moving really fast we should feel wind but we don't feel wind so what did the ancient Greeks conclude what conclusion would they come to right there again going real fast makes wind happen we don't feel that wind so what did the ancient Greeks assume exactly that we did not move okay so you can see how that is based on observation and it's logical but it's the wrong conclusion because they had the wrong idea because here's the thing here's what the ancient Greeks didn't know had any ancient Greek ever been to space no they didn't know that space was a vacuum like we do and they also didn't understand gravity like we do so they thought that the air that's right here like this air right here that this wasn't a part of the earth this was what the earth was sitting inside of and that space outer space was just this air up going on and on and on and on and on and that if the earth moved they thought the earth would have to move through the air so that the air that's like in the room that you're in would be left behind as we moved now we know that because of gravity earth pulls its air with it so we're not gonna get wind because our win our air is not moving past right but they didn't know that like here's here's the problem I think the ancient Greeks might have figured this out a little bit better if they had cars right I think if ancient Greek people had cars they wouldn't have had this mistake because cars have windows right cars have windows imagine you're in a chariot going 100 miles an hour nothing is blocking the wind right all the wind is coming past but if you're in a car going 100 miles an hour and let's say your friends like driving at a hundred miles an hour in your car and you're in the passenger seat and the windows are up okay do you feel the wind going past no right you don't feel the wind going past so that means that you would know that the car was taking air with it and that's kind of the situation the earth is doing so ultimately even though they actually had the idea that the Sun might be in the center of the solar system which is much more correct they didn't end up going with it for a bunch of reasons okay so geocentric they put the earth in the center so I'm gonna draw the earth in the center right here here's my next question don't worry if you get this one wrong and right what shape did the ancient Greeks think the earth was what shape did the ancient Greeks think the earth was this one's gonna get interesting what shape to the ancient Greeks think the earth was yeah so this is a this is what I'm expecting to hear because frequently this is what you you're taught so a lot of people what you're answering right here is that the earth was flat maybe in the shape of like a flat frisbee or a disc that is actually not the shape that the ancient Greeks thought that the earth was okay it turns out unlike what you may have been taught in school the ancient Greeks were actually the people who figured out that the earth was shaped like a ball okay it turns out that the ancient Greeks were able to figure out that the earth was round okay they were the ones that figure that out they thought it was a sphere which that information has been muddled in in school a lot so check this out did the ancient Greeks travel all the way around the earth did the ancient Greeks travel all the way around the earth and here my head is the earth no no they didn't so how did they figure out that the that the earth was round how'd they figure that out so it turns out it was actually Aristotle who figured it out this guy over here who was wrong about the earth being in the center he was the one that figured out that the earth was round okay and how did he do it he did it by looking at eclipses okay and we'll talk more about what eclipses are now but a type of eclipses called a lunar eclipse when that happens what you can do is you can see the shadow of the earth on the moon every time there's a lunar eclipse and they happen pretty frequently you can see the shadow of the earth on the moon and every time you see the Earth's shadow it's always a circle no matter which direction the moon is in so in other words no matter how the earth is turned you're always gonna see a circle shaped shadow and what's the only thing that's gonna have a circle shaped shadow no matter how you turn it okay what shapes gonna have a circle shaped shadow no matter how you turn it looks like we're getting a little bit of a delay in the stream that's done that's okay yes it's got to be a ball shape right it's got to be a sphere or a ball shape okay so the ancient Greeks figured that out okay and here this is something important it turns out that by analyzing shadows the ancient Greeks were actually able to figure out almost exactly how big the earth was also they checked the shadows at noon on the same day in different towns and by looking at the difference in how far the shadow tilted they were able to figure out how big the earth was okay so the ancient Greeks even though they were wrong about a lot of stuff they were the first people to be right about this and that means ain't flat earth people we figured that out thousands of years ago right Flat Earth is