Episode 78: Historic and Modern Geocentrism
Recap: Ancient astronomers searched for evidence and found none that Earth is not the center of the universe. Thousands of years later, we found that evidence, but there are still people today who point to other reasons and claim heliocentrism is wrong. This episode explores those and some independent ways to show why geocentrism doesn't work.
Puzzler for Episode 78: All orbits are ellipses. Some are more elliptical than others, but there are no perfectly circular orbits. Why is that?
Answer to Puzzler from Episode 77: There was no puzzler for Episode 77.
Q&A: This episode's question comes from Donovan W. who asks: "Is there any reliable way to judge our absolute velocity in relation to universe as a whole?"
The answer is yes and no. Since there is no absolute reference frame, and our current thinking of the shape of the universe is a hypertorus with no center and no edge, then a motion relative to "the universe" is meaningless.
What we can do is determine our motion relative to other objects. We know our motion on Earth's surface relative to the center as it spins. We know our motion around the Sun. We know to a pretty good accuracy our motion around the galaxy, as I discussed in a few episodes about why we weren't going to be in the center for 2012. This is done by lots and lots of measurements of motions of other stars nearby.
By very careful measurements of doppler shifts, we can also measure our speed towards or away from other galaxies. Measuring horizontal speed is much more difficult because the objects are so far away and don't move much in a human timescale, or the life time of a grant.
We can also measure our velocity relative to the Cosmic Microwave Background radiation, again based on doppler shifts. Our entire local group of galaxies appear to be moving around 370 km/sec relative to it.
- Logical Fallacies / Critical Thinking Terms addressed in this episode: Affirming the Consequent
- Relevant Posts on my "Exposing PseudoAstronomy" Blog
Claim: This is a simple idea and by disassembling the term, we know what it is: Geo = Earth, centrism = center. Earth-centered. Either an Earth-centered solar system or universe, or various gradations in-between.
I spoke during the last episode's Q&A about how, in science, we tend to start with the most simple model to explain something and then layer on complexity when the simple model breaks down. Only when the model cannot possibly be changed to accommodate new data do we usually head to a completely different one. Geocentrism if a very good example of this overall philosophy, though one that some people today still don't accept ... more on that in a bit.
Possibly the reason for geocentrism is that it was easy and it fit with basic observations and every-day experience. First off, as I sit here at my desk recording this episode, it does not feel as though I'm moving at all. It seems like I'm stationary. And yet, when I look outside, I can easily tell that the sun has moved over the last hour. And at night, I can clearly tell that the stars also move over the course of an hour or so. These two very basic observations would lead anyone, who doesn't have any other information, and who's assuming the most simple explanation, to think that Earth is stationary and the rest of the universe revolves around it.
That was the thinking in most ancient civilizations, such as Ancient Greece by the 6th century BC, proposed by Anaximander. Two centuries later, Plato and Aristotle also used the geocentric idea and also proposed that everything went around Earth on circles or resided on spheres. The philosophy of circles and spheres being perfect, and the heavens being perfect, was very important and adopted by several religions in that era and shortly thereafter, most notably Christianity.
Before I move on to the problems with this and how it was eventually supplanted, I think it's important to also mention that the Greeks weren't stupid, they actually DID try to determine if Earth was moving through space. The thinking was that if Earth did move, then the stars should noticeably shift position relative to each other over the course of a year. This is the basic idea of parallax, which can be demonstrated by closing one eye, holding your finger in front of a distant object, then opening both eyes and closing the other, and the distant object moves relative to your finger. Unfortunately, the stars were MUCH farther away than the ancient Greeks thought, and they couldn't detect any parallax in the stars -- in fact, parallax wasn't discovered until the 1800s. Faced with this choice - either the stars were MUCH farther away than they thought, or Earth was the center, they chose the latter.
While Aristotle and Plato were influential, it was the Greco-Roman astronomer Claudius Ptolemaeus who developed the Ptolemaic system that was based on several previous civilizations' observations. His work was then adopted by European and Islamic astronomers and it's usually what's thought of when people think of geocentrism.
I'm not going to go through how this system works because that's actually not important to the narrative I want to tell. AstronomyCast has an excellent five-episode arc, where Parts 2 and 3, Episodes 184 and 185, go through the specifics. I'll link to them and of course Wikipedia in the show notes for people who want more information.
What IS important to my story is that it was science as opposed to philosophy that ended up supplanting geocentrism. It was the observations of Tycho Brahe in the 1500s, the mathematical models of Johannes Kepler to fit those observations in the 1600s, the different observations and explanations of Galileo Galilei in the 1600s, and the theoretical backing in the late 1600s offered by Isaac Newton that provided all the evidence needed to show that geocentrism could not possibly explain what we see, but that a simple, basic, heliocentric model could explain them easily with the planets on ellipses and not circles. The only problem for heliocentrism was Mercury's orbit, but that would have to wait for Einstein in the early 1900s and is a story for a different podcast.
As anyone who's familiar with this story knows, probably the biggest push-back against geocentrism was from the Catholic Church. It's not easy to change religious dogma -- just look at attempts to get condoms officially allowed over the last half-century.
And, this was the 1400s-1600s, where in Europe, the church had an extraordinary amount of power, and a lot of this power came from the idea they were infallible. If what they had been preaching about the very structure of the universe was wrong, imagine what that could do to their power structure. Or at least, that's how I imagine it in my head. I'm sure it was more complicated than that, but at a basic level, that's the jist of it.
While the scientific community pretty much agreed by the late 1600s through the 1700s that Earth orbited the Sun, the Catholic Church refused. It wasn't until 1820 that the Catholic astronomer Joseph Settele was allowed to treat Earth's motion as a fact. It wasn't until 1822 that they stopped prohibiting publication of books that said Earth moved. And perhaps more famously, it wasn't until 1992 that the Pope finally apologized to a long-dead Galileo over the treatment that he had received.
That's not to say the Catholics are the only jerks in this story. There is still a sect of orthodox Jews who maintain geocentrism, and there are a few isolated sects in Islam who do, too.
But, most geocentrics today are relegated not to mosques, synagogues, and churches, but rather to the fringes of the internet and even some conferences. For example, there is a very active website called "Galileo Was Wrong" at galileowaswrong.com that has a blog with articles posted nearly daily. Many of them are responses to various people trying to critique their arguments against geocentrism. There's even one about Phil Plait. And who knows? Soon, there may be one to this podcast episode.
Modern geocentrists point to a couple different lines of evidence OTHER THAN religious texts, and, in isolation, at the very least you could say that they can be interpreted either way.
One of these is the Michaelson-Morley experiment, first conducted at my alma mater over a century ago. At the time, it was thought that there was a physical substance called an œther, something that permeated the universe and was the medium through which light waves travel. I said that very deliberately: When this experiment was done, light was considered to be a wave, and it was thought that it, like all other waves, must travel through something.
When this was done in 1887, it was known in the scientific community that we do live in a heliocentric solar system. Therefore, Earth moves. The idea behind the Michaelson-Morley experiment was that if Earth is moving, it must also be moving relative to the œther. That meant that since the œther is carrying light, then light should move a little teensy weensy bit faster in the direction of motion relative to the œther, and a little teensy weensy bit slower in the trailing direction.
The Michaelson-Morley experiment set out to show that by splitting a coherent light beam, sending it in two different directions, and then bringing it back together. If there was any speed difference at all, then they wouldn't recombine quite right and you'd see that when the split beams were brought back together. They didn't. And before I get e-mails, I've simplified it a bit, but that's the really basic idea. But, this is one of the most famous "failed" experiments in modern science, and the conclusion was that there was no œther.
Geocentrists interpret it differently: There is œther, but since Earth is stationary and everything moves around it, then you would expect a null result from the Michaelson-Morley experiment. That's why I say that in isolation, this experiment could be interpreted either way.
Another way that geocentrists argue their case has to do with redshift. Back in the 1920s, Edwin Hubble observed that the majority of distant galaxies were moving away from Earth, no matter in what direction he looked. Galaxies that were farther away were moving faster. He interpreted this to be that the very fabric of the universe was expanding, and it was carrying galaxies along for the ride. And, it's not that we're in the center of this expansion, which is what the geocentrists say, but it's that in an expanding universe, no matter where you are, you would see the exact same thing going on. So in fairness, at the very least I would say that this is neither evidence for nor against geocentrism, but rather just for an expanding universe.
Independent Ways to Show Geocentrism is Wrong: Phases and Sizes of Venus
With that in mind, let's move on from evidence they point to to evidence that I think shows a heliocentric solar system and, more broadly, a non-special, non-Earth-centered universe. I'm going to talk about two methods: One observational, one theoretical.
First is the observational method, and it's something that I did over the course of several months back in 2008 - is to observe the planet Venus. If Venus goes on a circle around Earth, then it should always be the same size as seen from Earth. If the Sun also goes around Earth, and it's such that Venus is closer to Earth than the Sun, Venus can only show new through crescent phases, we can never see it as gibbous or full. If Venus and the Sun go around Earth but Venus is farther away, then we would only see gibbous and full phases, not new and crescent.
To perhaps put it more simply, if the geocentric model is right, then Venus should never ever show all the phases that the Moon does.
But, if Venus orbits around the Sun, then we should see all phases: When it's between Earth and the Sun, then we should see a new phase ... in other words, we can't see any of it because 100% of the lit part is facing the sun, away from us. When Venus is on the opposite side of the Sun as Earth is, then we should see it as a full phase except in the rare case when it would go directly behind the Sun.
When Galileo observed Venus over 400 years ago, in 1610 and '11, he saw that it did show phases -- ALL the phases. But he also saw that Venus changed size. And, he saw that the size correlated with the phases: Venus appeared over 6 times bigger just as it entered its new phase versus when it was in its full phase.
It is not possible to explain that with an Earth-centered solar system. You MUST have, at the very least, Venus orbiting the sun to explain this (which later adaptations of the geocentric model did do for both Venus and Mercury). The reason is that Venus is closest to Earth when it's between Earth and the sun, but it's way on the other side of the sun, roughly 6 times farther away, when we can see it fully lit. Because of the large difference in distance, you get the difference in size. This can't happen unless Venus goes around the sun, and you can also use the change in size to estimate how far away Venus is relative to the sun.
Independent Ways to Show Geocentrism is Wrong: Gravity
Another method for showing that we live in a heliocentric solar system is the Theory of Gravity, though I guess in Louisiana and Texas, gravity, like evilution, is Just a Theory®. Really though, once we figured out gravity, it was game over for geocentrism. Well, gravity and Kepler's Laws.
The basic idea of gravity is that all objects pull on all other objects, and the amount of pull is proportional to their mass. For two free-standing, isolated objects in space not under any other influence, they will come together under their own self-gravity and collide. For them not to collide, then they have to go into orbit about a common center of mass. If the objects are the same mass, then the center of mass will be exactly between them, and they have to move in a direction perpendicular to the line between them through the center of mass. Think of a spinning bolas. Meanwhile, if one object is much more massive than the other, then the center of mass will be closer to the more massive object, and while both will still move around the center of mass, the less massive object has to move in a larger orbit around that center, while the more massive object moves less.
With this in mind, all that remains is to apply it to the solar system. Gravity would hold that even if Earth were much more massive than the sun, Earth would still have to move a little bit. But, since the Sun is in actuality much more massive than Earth, then it's the sun that moves just a teeny bit and it's Earth that moves in orbit around it.
It's this basic theoretical idea that we can apply to many other things, such as, say the galaxy. The mass of all the stars in the center of the galaxy is much larger than Earth or the sum of the mass of the solar system, so the Sun moves in orbit around the center of the galaxy. Our galaxy is one of around 30 in the Local Group, and all the galaxies in the Local Group move around a common center of mass, but not fast enough to remain in orbit. That's why the Milky Way and Andromeda galaxies are going to collide in a few billion years. But to bring it back to the topic at-hand, it's also why, again, Earth is in motion.
Summary: As a poorly thought out way to summarize the issue, geocentrism is perhaps slightly less stupid than flat-Earth beliefs, but it's close. In isolation, their evidence at best can be said to support either geocentrism or heliocentrism, but it's other lines of evidence that put the weight squarely on the shoulders of Earth NOT being the center of the universe.
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8:01am on Thursday, August 8th, 2013
Your examples concerning the Venus phases and the required motion of the smaller around the greater have both been answered by geocentrists in the modified Tychonian model. According to this new geocentric model, Venus does orbit the sun, so the phases of Venus are accounted for.