How did people begin to disprove the geocentric model?
In: Technology
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Early evidence: the visible planets follow bizzare paths across the sky that are difficult to explain with the Earth at the center. Mercury and Venus never leave the Sun’s proximity, the others lazily wander through the sky on multi-year cycles that confounded early astronomers.
Eventually (1500s) telescopes got good enough to resolve details about the planets, and astronomers noted that Jupiter had objects that seemed to orbit it instead of Earth and that Venus went through distinct moon-like phases.
This suggested that Venus was orbiting the sun and not simply moving in some bizzare path ahead or behind it, and that it was possible for objects to orbit other things.
Taking that observation and making a model with the Sun in the center produced a much cleaner series of nice circular orbits instead of labyrinthine explanations of retrograde motions, and it soon became the established teaching (despite some infamous early resistance from religious figures)
[Summarized from this really good article here:](http://tofspot.blogspot.com/2013/10/the-great-ptolemaic-smackdown-table-of.html)
The most important discovery involved the phases of Venus by Galileo in 1610. This gave direct evidence that Venus was sometimes farther than the Sun, and at other times closer. This disproved the Ptolemaic model that had all the planets orbiting Earth. However, Tycho Brahe had a geocentric model that involved all the planets orbiting the Sun, and the Sun and Moon orbiting Earth. The important thing is that if you just consider objects within the solar system, this model is pretty much indistinguishable from a heliocentric one. The key problem with a heliocentric theory was lack of stellar parallax.
Kepler several years before Galileo’s work had published his now famous laws of planetary motion, but for whatever reason they were largely ignored for a few decades. However, in 1687, Newton develops his theory of Gravity. This provides a theory in which the Kepler model *ought* to be true. From this point, no one really questions the heliocentric model even though they still can’t find the stellar parallax.
In 1728, stellar aberration is discovered. While not exactly parallax, it does prove that Earth is moving relative to the stars.
In 1806, stellar parallax is observed.
In 1835, an illusory optical phenomena (Airy disk) was identified that causes stars to appear more disc-like in telescopes. This explains the erroneous calculations Galileo’s contemporaries made that suggested stellar parallax should be significantly larger than it actually is (thus they thought that they should be able to see it when it was actually too small).
It was Copernicus who first put forward an alternative idea to the geocentric model. He was able to put his idea together because his mentor Tycho Brahe had spent a good chunk of his life making detailed measurements of the locations of the stars using the most sophisticated technology of the time. Copernicus then took the measurements and made a model of the solar system that explained the movements of the planets. His model was called heliocentric because it had the sun as the center of the universe.
Copernicus did try to prove that the sun was not the center of the universe, or rather he checked to see if it was, by looking for parallax (google it) between stars. He didn’t see any so he concluded that the sun was, in fact, the center of the universe. It wasn’t until glass polishing got better and lenses started being used to look at the stars that the parallax shift was detected and the heliocentric model was replaced
So it’s pretty complicated and ELI5 is not really the best place to get a real explanation for it (this is really an /r/AskHistorians question). I am writing this as a professional historian of science, for whatever that is worth.
The basic thing to keep in mind is that they didn’t _disprove_ it. What happened is that a _better_ model was eventually developed, over hundreds of years, and many observations accumulated that fit with that model better. And so a heliocentric model _displaced_ it.
And along with that, they also developed the physics that was necessary for a moving Earth to even be imaginable — if you don’t have a concept of inertia or reference frames, for example, it’s pretty hard to imagine that the best explanation is one where the Earth is traveling at tens of thousands of miles per hour (while rotating at about 1,000 miles per hour) and somehow we don’t feel it (or fly off).
If one were to track the main steps on the way to a successful heliocentric model, they would be:
* Kepler’s orbital dynamics, which for the first time made the Copernican model actually fit the data, and at the same time eliminated the crystalline spheres concept that was common to both the Copernican and Ptolemaic systems.
* Galileo’s observation of the phases of Venus, which convinced even the Catholic Church that the Ptolemaic system did not fit observational data (they replaced it with the geocentric Tychonic model instead).
* The gradual development of the physics of motion and theories of gravity, which culminated in Newtonian mechanics, which basically made it entirely clear that anything other than a heliocentric system was a bizarre contrivance and not very useful, and provided all of the physics you need to explain why we don’t fall off of the planet, what holds the system in place, and so on.
Which _isn’t_ what your astronomy textbook will say (they’ll say: Copernicus, Kepler, Galileo, done). But your astronomy textbook is not a history textbook and things are more complex than astronomy textbooks tend to let on. There are a lot of observations that are typically mentioned (moons of Jupiter, observation of stellar parallax) which were not especially convincing at the time, at least not compared to the factors mentioned above. They make for a nice story but are bad history.
Retrograde motion of mercury basically. It means in the sky mercury appears to be moving backwards. However what’s happening is that our orbit is passing by mercuries orbit so it only looks like it’s moving backwards.
This made people question what’s wrong with the Sky.
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