Was the Geocentric Model correct at all?

  • It's easy to find resources stating that the heliocentric model is right and geocentric is wrong.

    But how wrong was it? Was it correct in any way?

    It was built on incorrect assumptions, but despite that - was it of any use to describe the apparent motion of celestial bodies? Was it more accurate for some things, but less accurate for others? Or was it altogether a flop and astronomers couldn't get anything out of it either way?

    I can only find multiple articles proving the heliocentric model, explaining the geocentric one or claiming that it was simply wrong - but I can't find anything about its accuracy and usefulness.

    (1) (2) (3)

    Edit 1:

    I incorrectly used geocentric model when it seems I wanted to say Ptolemaic model - the one with deferents and epicycles, with Earth as its origin.

    ptolemaic model

    Ptolemaic model (click for full size)

    Thank you for clarifying, and sorry for the confusion. The answers provided regarding any other geocentric models are still valid and useful, so this is just a minor errata.

    Well, they got the relationship between the Earth and the Moon basically right. Other than that...

    I highly recommend the nine-part series of blog posts by Michael Flynn (not that one) titled The Great Ptolemaic Smackdown. It presents a lot more of the arguments that were made for the geocentric model and against the heliocentric model than most other popular-level histories do.

    Related, but not a duplicate, at the physics.SE: Are all reference frames equally valid?

    It's all about Occam's Razor. Both models are usable, but the geocentric model requires many extra complications such as epicycles.

    I would also recommend The Coperncian Revolution from the Inside; it helped me to get a good understanding of the reasons behind some of what was going on.

  • ProfRob

    ProfRob Correct answer

    2 years ago

    Ptolemy's epicyclic, geocentric model, in use until the Renaissance, was very accurate in terms of predicting the positions of planets and the times of eclipses. What it couldn't account for were things like the correlations between apparent size and phase of Venus, or to properly account for the variation in brightness of the planets.

    Thus the reason for discarding the geocentric model was not really because it lacked precision, but that it failed to explain various other observational facts, especially after the development of telescopes.

    No doubt you could tune the Ptolemaic system even further (more epicycles?) to iron out some of the small errors that were revealed by Tycho's positional measurements at the turn of the 16th century, which had a precision unavailable to Ptolemy. However, the advent of Kepler's laws and subsequent explanation by Newton, rendered the geocentric model obsolete.

    As you can judge from (well written) articles like this one, geocentrism is actually quite hard to kill-off observationally, if you are prepared to accept that the universe is arranged "just so".

    And with the theory of relativity, all we can say is that we find the heliocentric frame of frame more useful. The geocentric frame of reference is valid, just unnecessarily complicated.

    Would you have some sources to support the first paragraph? Excellent answer Rob - speaking about accuracy precisely is exactly what I needed - thank you!

    @Accumulation - I don't think that you need either of SR or GR for the basic insight that there are multiple possible frames of reference. It would have been equally obvious to Newton. Perhaps Earth tended to be the sole considered frame of reference before it became obvious that humans and/or their instruments could in principle travel all the way to celestial bodies?

    From the perspective of accuracy and simplicity, even a heliocentric model doesn't quite cut it. Developers of modern ephemerides use a barycentric model because third body effects vanish in such a frame. Those third body effects result in an implicit second order ODE. Using Newtonian gravity, the equations of motion are fully explicit from a barycentric perspective. Relativistic effects do make the equations of motion implicit, even from a barycentric perspective, but the relativistic effects are small enough that the Newtonian acceleration form a very good approximation.

    @Acccumulation Is that so? Is an object in orbit not in an accelerated frame?

    @kutschkem one of the key insights of GR is that free-falling frames are the proper generalisation of inertial frames to situations with gravity. Objects in orbit are in free-fall so, from a properly relativistic pov are inertial

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