Given the observational data available at the time of its acceptance, was geocentrism a valid theory?

iorgfeflkd · 7 months

The main issue was stellar parallax. If the Earth is moving around the sun then the stars should be in slightly different places depending how far away they are. The ancients mentioned this, but could not observe it.

The issue is that the parallax is very, very small. It wasn't observed until Bessel I believe in ~~the eighteenth century.~~ 1838.

Heliocentrism implies stellar parallax, and if the observed parallax is zero, that's a case against it.

zeug · 7 months

Given that the entire point of the model at the time was to predict the position of the planets in the sky - often in order to construct horoscopes - and not to determine their actual position is space, the geocentric theory worked remarkably well for nearly two thousand years.

With the geocentric Ptolemaic model, one can accurately predict the positions of Mars, Jupiter, Mercury, Venus, and Saturn - the five planets visible without a telescope - to about as good a resolution as one can get with the naked eye in most circumstances.

The heliocentric Copernican model was not any more accurate, or less computationally complex than the Ptolemaic model. It was Kepler who did a first systematic study of which one predicted the position of Mars better, and found that they had similar accuracy.

Kepler's geocentric system of elliptical orbits was found to be more accurate, although Kepler had been chastised by his professor for using the principles of terrestrial physics to try to explain the motion of the celestial bodies.

A good discussion of the actual history of astronomy in this period which debunks many of the persistent myths - such as the idea of epicycles within epicycles - is The Book Nobody Read by Owen Gingerich. Unfortunately, I don't know of a good online reference for this topic.

The_Counselor · 7 months

The phenomena of retrograde motion was also a significant hurdle for the geocentric paradigm, but it was resolved through Ptolemy's inception of epicycles, equants, and deferents. Primarily, the goal of these concepts was to preserve the uniform circular motion of heavenly bodies postulated by Aristotle. Eventually Copernicus expressed his dissatisfaction with the equant and proposed the heliocentric model. However, Copernicus still required some use of the epicycle with his model because he himself tried to preserve circular motion with circular orbits. Of course we now know that the orbits of the planets are elliptical thanks to Kepler and Newton, and retrograde motion is a consequence of the earth's motion relative to the motion of the other planets.

cassander · 7 months

Historian here. Pre-copernican astronomy was remarkably accurate. Astronomers had spent literally thousands of years working out a system of plotting the planets on circles, and circles within circles(called epicycles), to the point where the geo-centric model was actually a lot more accurate than Copernicus' first efforts, though much more complicated. Copernicus insisted that planets move in perfect circles for philosophical reasons, so he couldn't ditch the epicycles, and it wasn't until much later that astronomers accepted elliptical motion, and not until Newton that they understood why the planets moved that way.

Benutzername · 7 months

There is no "correct" reference frame. The heliocentric one just makes it easier to describe the movement of the planets on the sky.

cscrwh · 7 months

There was an article about 2-3 months ago in Physics Today on the "Martian catastrophe", which is a 5' error every 32 or so years that can only be resolved with a Keplarian heliocentric system. The error was +5' or so for Copernicus's system and -5' or so for the Ptolemaic system. So the available evidence would have pointed to a correct heliocentric system.

Actually (IMHO) the geocentric systems fail because they are overly complex. Good scientific theories are no more complicated than they need to be. If one system requires many more adjustable parameters than another to achieve the same accuracy, then it is more than a bit suspect.

hesperidisabitch · 7 months

If you're curious about those types of questions, I highly recommend a book by Bill Bryson titled "A Short History of Nearly Everything." Its alternate title could be "Science discoveries of man and how we thought we've been right about alot of things and it turns out we were wrong."

Really a great read.

Gecko99 · 7 months

The Ptolemaic system had a very complicated system of epicycles and other movements that did a good job of predicting planetary motion. However, all this complexity was needed because the reality is that the planets are not all perfectly aligned on the ecliptic, the plane in which the planets revolve. Their orbits are all a little bit tilted. Another problem necessitating all the complexity was that the planets do not revolve in circular orbits. Early heliocentric models expected them to go in circles, creating inaccuracy. Kepler worked out that they go around in ellipses, and that's one of the big ideas that started to make heliocentrism start to become a valid explanation for the carefully observed phenomena of planetary motion.

pmam · 7 months

It must have been deeply concerning to geocentrists that Mercury was never very far away from the Sun, Venus too to a lesser degree. I suppose the differences in the very nature of the Earth as opposed to the planets was a powerful argument that the Earth belonged in the centre.

tedrick111 · 7 months

Your eyes fail to detect distances greater than 20 miles or so. Everything farther than that looks to be at about the same distance because long story short, they're not far enough apart. During the period you're talking about there weren't many city lights at all, so access to the full view of the night sky was a luxury those people were afforded.

The logical conclusion, looking at the dome of sky without factoring planetary motion, was that the earth was at the center. Only an astronomer with a lot of data (for the time) would even be in a position to doubt it.

I say yes, because of the lack of instruments and information technology to spread data.

thang1thang2 · 7 months

You have to remember that back then, "swept under the carpet" meant that the church said that geocentrism was the TRUTH, therefore anyone who didn't say so was a heretic and could be burned at the stake. If anyone had any valid arguments, they probably kept them to themselves and studied on their OWN without ever making any formal inquiries

TheBoomBoom · 7 months

Let's try an academic exercise here... prove to me that the Sun is the center. Don't use evidence of the contrary (retrograde motion, parallax) - use positive statements that would prove to ten year old that Earth goes around Sol, and not vice versa.

Not sure it can be done.

kybernetikos · 7 months

The other opinion - followed by the Pythagoreans, Aristarchus of Samos and others long before Aristotle or Hipparchus or Ptolemy came into this world and returned to center stage by Nicolas Copernicus some one hundred years ago - is still approved by the most excellent mathematicians. It places the sun at the center of the universe as the soul of the world and governor of the universe, from which the earth and all the planets borrow their light. There they believe it to remain stationary, as we can see in the second figure…

Now it is not our intention here to specify which of these opinions is consistent with truth and best befits the natural order of the world. That is a question we must leave to those versed in the science of celestial matters. And for all that the earth, according to the Copernican hypothesis, moves annually around the sun in a circle whose diameter would be two million leagues from Germany, and that the sphere of the fixed stars would be of so vast an extent as to be utterly incommensurate with that of the sky around the earth, there is, nonetheless, no noticeable difference between this theory and the earlier one (according to which the earth is the center of the universe) in respect of the apparent rising and setting of the celestial bodies, the changing duration of days and nights and the other things that follow from this. But since the hypothesis of a fixed earth seems generally more probable and is, besides, easier to understand, this introduction will adhere to it, without for the present considering the other theories.

Introduction to Geography, Joan Blaeu, Atlas Maior, 1665

drockers · 7 months

Yes at the time Tycho Brahe used science to observe the lack of a parallax. It would have been impossible to see at the time, and so from his observations Tycho believed the galaxy was geocentric. He did however prove the heavens weren't immutable and that the universe was bigger then the tiny little solar system we thought it was. Kepler his assistant believed the solar system was heliocentric and went on to be an advocate for the theory after Tycho died.

Brodecahedron · 7 months

This reminds me of the time where a friend and I struggled endlessly to explain the Heliocentric model. We (two high school seniors) argued factually and functionally; this did not change the man's (a well-learned college graduate) mind. We continually explained the innumerable evidence against the geocentric model, yet he continued to counter with, "But why is it that when in an airplane, the earth does not spin beneath you because it's going much faster?". We told him that it was due to earth's gravitational field, the same reason why the earth doesn't fly under you when you jump. He was a nice guy, knew most of the material. He just decided to not believe scientific evidence (he blatantly stated this).

TL;DR: Argued with a man about the heliocentric model vs. geocentric model. Made sense (I think) and he still completely bought geocentrism. He wasn't the only one there who agreed with him.

CorpulentCorpuscle · 7 months

No and bizarrely the idea that the sun went around the earth was a fairly new idea from around the 1600s (ish, someone will come and correct the details).

SirBlackness · 7 months

I've always wondered if the universe extends "infinitely" in every direction, how are we sure that Earth isn't the center of the universe?

zyzzogeton · 7 months

Wait, this sounds like homework... interesting homework, but a homework question just the same.

That or the premise of an alternate history fiction... which would be interesting. What first principle, if discovered markedly earlier in history, would have fundamentally changed it?

rmxz · 7 months

Isn't it still a valid theory?

Isn't it just as valid to say you are the center of the universe as any other point.

If by "center of the universe" you mean the "observable universe", you are indeed - almost by definition the center of your observable universe.

If by "center of the universe" you mean the "whole universe" most evidence suggests it's infinite; so you're just as free to pick yourself as the center as any other point.

bl4ckduck · 7 months

In the 200s BCE, Eratosthenes estimated the circumference of the earth using a model that would refute the need for your question.

Ignorance is eternal.

marishtar · 7 months

I still consider Earth to be the center of the universe, and calculate everything relative to it. This doesn't make me stupid; it makes me a masochist when it comes to calculating orbits.

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