Jason Kendall's Introductory Astronomy Course

Adjunct faculty in Astronomy at CUNY Hunter (2015-2018) and William Paterson University (2011-2020)
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Geocentrism is False

Wherein we further discuss the Aristotle's arguments for a stationary Earth, and the construction of Ptolemy's Geocentric Model that ruled the roost for almost 2000 years, until it was dethroned in 1610.

Lecture Notes for This Video

Supplement the videos with "OpenStax Astronomy":
Geocentric Model
Aristotle
Ptolemy
The Almagest
Retrograde Motion
Stellarium

Module 1: Foundations of Observational Astronomy: The Moon, the Seasons, and Mapping the Sky

Video Transcript

00:00:00:00 - 00:00:24:09
Hello, this is Jason Kendall and welcome to the next of my introductory astronomy lectures. Today, we're going to be talking about the motions of the planets around the sun, around the sun. Wait a second. We haven't known that forever. In fact, the object of looking and discovering that the earth actually goes around the sun is one of the central ideas to all the physics and frankly, modern thought.

00:00:24:11 - 00:00:46:05
The discovery of the Earth's motion around the sun and the and the development of thought that led us to that are probably some of the most important things in what we call Western thinking. So and in fact, it traces the beginnings of what we call science. All right, so what are these motions? All right. Well, first, let's actually start off with some basic things.

00:00:46:05 - 00:01:03:07
Well, when we look up at the night sky and we look year after year after year, we see that the stars rise in the east and set in the west, and they go at a certain angle, depending on your latitude. But they always do that and they always seem to be staying fixed with respect to each other in the sky.

00:01:03:09 - 00:01:28:16
And those are the stars. But then we find that there are wandering stars, some stars that don't stay fixed with respect to the others. And in the classical terms of the wandering stars, we have the sun, of course. Moon Wow, it's up there. We have the sun, the moon, we have Mars, Mercury, Venus, Jupiter and Saturn. So those are the classical planets, the planets and planets.

00:01:28:16 - 00:02:04:05
And Greek indicates wanderers, which means wandering stars. So the word planet, the oldest meaning for that word is wanderers, wandering stars. So what are the wandering stars? The wandering stars also appear to rise in the east and set in the West, just like all the other stars. However, they steadily drift eastward every day. So over a very short period of time, over many days, if you watch them day after day after day, you will see them slowly drifting eastward and occasionally they will drift westward on what we would call retrograde motion.

00:02:04:07 - 00:02:25:28
So Prograde motion is the drifting slowly to the east of the planet. And occasionally it does a backward loop that we would call retrograde motion. And Mars shows this quite explicitly. The Mars Mars's retrograde motion was one of the great important things to study in terms of the Earth, the motion of the planets around the sun. All right.

00:02:26:00 - 00:02:49:29
We also know or at least appear to know, that the planets reflect light. They don't have their own light of their own. Now, that is an interesting statement. And of itself, and it's hard to really determine that from the sky. But the ancients thought and noticed that the slower moving planets were also the dimmest, so they reasoned that, hey, if they're the dimmest, they must be the furthest.

00:02:50:04 - 00:03:09:17
And if they're reflecting light from the bright sun, which they know they knew was happening with the moon, if the moon was reflecting light, then the other planets may be reflecting light too. And they're very close with respect to the stars. So therefore, if they're farther from the sun or farther from the earth and therefore they would be dimmer.

00:03:09:19 - 00:03:35:26
So that's an old argument that the ancients knew about as well. So this led to the direct trying to understand the influence of these things and what they meant, because there was a difference between the motions of the planets and the stars and the things on the earth, because the ancients knew and everybody's known that the stars have always risen in the East and set in the West, and they always have done this.

00:03:35:29 - 00:04:07:17
And the planets motions are extremely regular. So there's a difference between what happens in the celestial sphere and what happens in the meteor world, the material world we call the earth. And so two ancient philosophers are and scientists of the day. We wouldn't really call that science trademark science, but science or what we would call the beginnings of science, have the idea that you actually had to do the operating principles for the Earth were different than that for the stars.

00:04:07:20 - 00:04:24:19
In fact, they didn't seem to behave the same as all. So, for example, you take an object like an apple and you go up a tree, and if you take an apple and drop it out of a tree, guess what happens? It falls towards the earth and it's free. So it falls. Well, look at Mars in the sky.

00:04:24:24 - 00:04:51:15
Mars must be free of the stars. And if it's free of the stars, why doesn't it fall to the earth? That's a really interesting question. Same with Saturn, same with Jupiter, seen with the moon. Why doesn't the moon simply fall to the earth? It stays up there for whatever reason. So whatever principles act on the earth, appear to be acting different, have a different set of principles that work with respect to the planets and stars.

00:04:51:18 - 00:05:12:17
And so therefore, people who studied the stars weren't called weren't deemed these wonderful notion of we call the astronomer. Well, they weren't even called scientists, They weren't even called physicists. They were called mathematicians. They were mathematicians who calculate odd things, who calculated the length of the day and the length of the year. And the principal reason for studying the stars was calendars.

00:05:12:19 - 00:05:36:07
So you knew when to plant things and when to do things, and when important annual events would occur and when important monthly events would occur. And so you would also then if you were making calendars, you would notice that there were important events that happened in certain times. So therefore, if those important events coincided with with the apparent positions of the things in the sky, then they might be related and thus was born astrology.

00:05:36:09 - 00:06:08:29
Astrology is the apparent or supposedly link between the motions of the planets and the stars. And what happens down here on earth. Well, just got to go to show you that astrology is simply a construct. It is a way that people have looked up in the sky and looked up there and said, Wow, that's happening now. That must be related in the same way that when you look up in the sky on a cloudy day on a summer, wonderful day, looking up, you see faces and dogs and cats and kittens and shapes in the clouds.

00:06:09:01 - 00:06:34:20
You always see faces in the clouds, but you never see clouds in faces. That's because we're really good. Humans are extremely good at finding patterns and things, so we found this pattern that seems to be there. But it isn't. So astrology is just simply a trick. It's a way of saying when things are when things are happening. And so, yes, you, you were born when Mars is descended.

00:06:34:27 - 00:06:53:25
Well, you can't argue with that. But there is no meaning to that. There is no influence that. So today we now know that if to astrologers, you ask five astrologers your opinion, you're going to get 17 opinions. So no, they don't agree with each other. They have no way of agreeing with each other. But that's where astrology was born.

00:06:53:28 - 00:07:21:18
Astrology was born by the extensive study of the behavior and movement of the planets in the motions of the star of these wandering stars. So let's give it some credit where credit is due because it kept people looking at the sky and it was very important to determine calendars. All right. So if we look at the sky and we try to determine what's up there and how it moves, we naturally want to come up with some sort of explanation for it.

00:07:21:20 - 00:07:46:11
And that explanation that seems to be the most rational is the geocentric model, not rational by what it really is, but rational by our experience and rational by experience means we simply don't feel the earth moving underneath us. And there's other arguments as well. But the ancients used to actually justify geo centrism and actually beat down Heliocentrism in its nascent in this nascent beginnings.

00:07:46:13 - 00:08:06:25
But let's actually look at that for just a second because this is a very interesting set of arguments. All right. So in a geocentric model, meaning the earth is at the center of the cosmos, this philosophy held that the Earth was at the center. Well, why did Aristotle, who was around from about 380 B.C. to about 320 B.C., why did he come up with that?

00:08:06:25 - 00:08:30:23
The Earth, the very Earth must be still, that it must not be moving. Well, he used a number of arguments, and his first argument was, Where's the wind? The wind? If the earth was moving, well, where's the catastrophic winds of us moving through the cosmos? All right, that's a good argument. Where is the wind? Second, let's say you take an arrow and they had archers back then You take an arrow and shoot it straight up.

00:08:30:26 - 00:08:52:05
Where does the arrow fall? The arrow falls straight back down at your feet. If the earth were moving, then the earth should move out from underneath the arrow and fall to the ground. That's an interesting thought. Why doesn't it do that? It doesn't do that because the Earth's not moving. Or so Aristotle thought. Third, we can turn around the other way.

00:08:52:05 - 00:09:18:11
Build a tall building, go to the top of a tall cliff. And if you drop a rock, the rock. If the earth was moving or rotating, for that matter, because everybody at the time knew it was round, it didn't know how big it was until about 100 years later in Aristarchus world and Aristotle's world. If the Earth should then move away from it and the rock would not plummet all the way to the ground, to the base of the cliff, but will drift either towards the cliff if the cliff is to the east or away from the cliff.

00:09:18:11 - 00:09:41:28
If away from the cliff is to the east, if the earth is rotating. All right. Now, that's a very interesting set of argument based on Coriolis, the next argument against geo centrism or for geo centrism, which is interesting in itself, is parallax to Aristotle reasoned that if we don't see parallax in the sky, then therefore the earth cannot be moving.

00:09:42:00 - 00:10:03:19
And this is, as we've described before, an extremely difficult measurement to make and wasn't even accomplished until the 1830s. Well over 2000 years after Aristotle's death. So we can forgive him for not seeing it because the stars are extremely far away. All right. The next thing that he does is some funny little arguments. One argument said that that the Earth was the heaviest thing.

00:10:03:21 - 00:10:17:24
And so if it's the heaviest, it must go to the lowest point. And the lowest point must be at the center of everything, because that's where the heaviest thing should lie is at the center. It's kind of a woo woo argument. But we can we can live with that. And then he also said, Now here's an interesting thing.

00:10:18:01 - 00:10:53:29
The stars move. Well, we see the stars moving. We don't see the earth moving. So therefore the stars are moving. We're not moving. So we can look at Aristotle and think there's a smart guy who came up with a lot of reasons in 360 or 30 and 40 B.C. as to why the Earth doesn't move. Now, remember, technology that we have today did not exist for him, so his ideas carried a huge amount of weight and tended to actually give tended to hold progress back for a very long period of time.

00:10:54:01 - 00:11:10:18
All right. So then let's actually look at some other things. Why didn't people say that the sun was at the center? Didn't we just learn from the last lecture that the sun is 20 times bigger than than the moon? And while the truth is 20 times bigger than the moon and the moon is half the size of Earth.

00:11:10:18 - 00:11:32:23
So it means it's it's actually it's a huge, huge object. The sun's really big. If the sun is really big, shouldn't it be at the center and people supported it. Such as Paris Tigers. Sorry. Pythagoras of Pythagorean Theorem. Yes. He posited that the sun should be at the center. Well, why did he say that? It's because fire is nobler than Earth.

00:11:32:25 - 00:11:55:24
So therefore, fire should be at the center. Well, it's not very scientific. So the Tigris was known to be a little bit of mystical woo woo. And if we look up at the Pythagorean solids, they were actually kept secret because they thought that these things were were powerful, mystical forces. So Pythagoras wasn't so much a scientist as a myth, as a mystic and a mathematician.

00:11:55:26 - 00:12:20:13
So Eris models ideas held deep sway because he actually tried to reason through the nature of the cosmos that he got it wrong. We can excuse because of technology and because we have modern technology that assists us in understanding things a little bit better. But his models worked and models that work stay put in science until there's something that breaks the model.

00:12:20:19 - 00:12:48:14
And that's the important thing that occurred later. But let's actually find how we can build a model in 140 A.D., about 500 year, almost 500 years after 400 years after Aristotle, the Greek philosopher, mathematician Claudius Ptolemy wrote his almagest. And the Almagest was an enormous, enormous undertaking where he plotted the charts of the heavens and tried to determine how things actually moved.

00:12:48:16 - 00:13:12:16
And so he created a model of the cosmos, the geocentric model that held sway for almost 6000 years, actually 1500 years, all the way up through Galileo's time. And beyond that, the Earth was at the center. So this is an incredibly important thing. So let's see, what did he do? Okay. So first, Ptolemy posited, along with Aristotle, that the Earth was at the center.

00:13:12:16 - 00:13:35:11
All righty. So then he he wanted to see how things move. So he created a coordinate system, the the equatorial coordinate system, which is the right ascension and declination. And he used the right ascension and declination of the stars and planets to determine the positions. Yeah, that's where that coordinate system comes from. It comes from 140 A.D. That's an old, old, old coordinate system.

00:13:35:13 - 00:14:04:23
Anyway, so what did he do? He posited that around that, around Earth is at the center. And in order to see both the progress motion and the retrograde motion of the planets, here's what he posited must occur. Every planet must be embedded in some or above, and the orbit must be made of some ether. And this ether is in a circle that surrounds the earth and embedded in that orb of ether.

00:14:04:25 - 00:14:30:06
The the planet rides on a circular orbit, called it different. A different. The different is actually the orbit that it rides upon, and it's a perfect circle. Now, at the start, the planet itself doesn't actually ride on the different inside of the or the ethereal orb. But what it does is it rides on an epic cycle on top that's sort that centered on the desperate.

00:14:30:08 - 00:14:49:02
So you've got this epic cycle that's on the difference of this. As the planet goes around the earth, it does these little loop de loops all the way around. So we get these little loop de looks. In addition to that, the Earth is not exactly at the center but offset. And so the offset is offset by a little bit of distance.

00:14:49:02 - 00:15:10:11
And each each of the planets has a different offset from center of the earth. So the earth isn't truly at the center, according to Ptolemy. Well, it's actually the center of it is actually between the earth and a place called an excellent and an excellent is and is a place that's equidistant from the center. So he posited a little of the earth here, equities there.

00:15:10:18 - 00:15:31:00
There's the center of the deferred. It rides around this deferred like this and then it rides on epicycles around the different. It's a fairly complicated system. And every single planet had its own set of epicycles difference and and and excellence, every single one of them. And the earth was offset from the at once, from each one of them.

00:15:31:03 - 00:15:50:22
In fact, to in order to to see the patterns of Mercury and Venus. Mercury and Venus had to have their their deference right in between the sun and earth. So there's an even odd thing because Venus was between the earth and the sun in order to always stay in that position of the sky, that that it was seen it.

00:15:50:24 - 00:16:20:25
All right. So this is a very complicated system. We would look at this and see. It's very complicated. Extremely complicated. And the reason why it's almost nobody back then was a public speaker about astronomy, because it took years and years of work to really understand what the almagest actually says and how to actually predict things using it. Remember, they didn't have mathematical equations that we do today, and so everything was done both geometrically and with huge tables.

00:16:20:27 - 00:16:41:28
So they actually had to do tabular data of the positions of objects in the sky and note where they were over time. So it actually was an extraordinarily difficult undertaking in order to actually do this math. And Ptolemy's work was actually very important and still stays important, stayed important all the way up through the latter part of this of the 17th century.

00:16:42:00 - 00:17:13:07
In fact, the only with the advent of Tycho Brahe data in the 17th century did we actually get to something where we could actually say, Hey, this is really important and how we can actually break away, but how did we break away? All right. So fast forward through the dark ages, through the the loss of of of the of the fall of Rome through the fall of Greece, the knowledge of the Library of Alexandria propagated out through Western from Western culture all the way through Islam.

00:17:13:14 - 00:17:43:13
And the end it came back. Ptolemy's writings went from Greece into translated into Arabic and came back to back to the West from those Arabic translations. And that is why it's called the Almagest, because it just means it's the table. The Almagest, I believe in Arabic means the great tables. In any event, this tabular data that was created for the geocentric material proof proved to be its own undoing because of the nature of what it predicted and how it predicted it.

00:17:43:16 - 00:17:52:08
And next time we'll talk about the heliocentric model and how the sun was actually shown to be at the center. So you say.