[jkot]

Earth-Centric model was actually scientifically better at that time. With Occam's razor you would prefer it.

- Even church agreed that Earth is not static, but is rotating.

- Nobody observed star parallax, major proof for Copernican model was missing until 19th century.

- Ptolemaic model with its epicycles provided better predictions.

- Copernican model is also wrong, planets are orbiting around center of gravity, which is outside of sun..

[lmkg]

The Ptolemic model was not better at the time, but it wasn't really worse. The Copernican model provided roughly the same observational accuracy, and was very slightly simpler. Occam's razor would probably prefer the Copernican model in a vacuum, but the differences were too slight to prefer unseating the incumbent.

> Copernican model is also wrong, planets are orbiting around center of gravity, which is outside of sun..

Well, a bigger shortcoming is that Copernicus still insisted that orbits be circles instead of ellipses.

> Ptolemaic model with its epicycles provided better predictions.

Not only did the Copernican model include epicycles, it actually included more epicycles than the Ptolemic model (modern myth notwithstanding). Both models could improve accuracy by adding more epicycles, and for the same amount of computation, both models produced predictions of roughly equal accuracy.

The one improvement the Copernican model provided, was that it removed something called the equant (and replaced it with more epicycles). The equant allows for non-constant orbital velocity around an epicycle, by instead having constant angular velocity around a point that is not the center of the circle. When Copernicus' work was published, the removal of the equant was considered by many mathematicians to be the main argument in favor of heliocentrism (although some considered it just a computational model, with geocentrism still being correct with respect to reality).

[colechristensen]

>planets are orbiting around center of gravity, which is outside of sun..

The only planet with a barycenter with the sun outside it's radius is Jupiter. And that only slightly. (with an altitude of 0.07 of the sun's radius)

The earth-sun barycenter is 0.0006 times the sun's radius from it's _center_

[jkot]

Heresy! There is only one true barycenter in solar system! :-)

[jahnu]

Even then the definition of the surface is different than the definition of the surface of, say, the Earth, so I'm not sure what "outside" the Sun means precisely.

[ekianjo]

The sun is roughly a sphere of a fixed size. What is there not to understand ?

[jahnu]

Plenty as it happens. Look up the definition of the surface of a star. It's not as simple or intuitive as one would expect.

[lojack]

I believe he's referring to the "fixed size" part of it, really only an argument a true pedant would make.

[jahnu]

Pedant? Or maybe just making conversation about the interesting differences between human experience and understanding of terms and what definitions astronomers use. Why do so many people on HN react as if every comment is a competition?

[24gttghh]

Just to be pedantic, the barycenter of the Solar System [0] is "sometimes" outside the Sun, but is also often within the limb of the Sun. [0]https://commons.wikimedia.org/wiki/File:Solar_system_barycen...

[0xfeba]

Interesting. Does the barycenter then have a barycenter over time?

[carleverett]

By definition a barycenter is a center of mass, so since a barycenter itself has no mass, it would not have it's own barycenter.

It could have a "mean location" though!

[0xfeba]

Ah, that's what I mean!

[klodolph]

To put it the other way around, that's either the position of the barycenter relative to the sun, or the position of the sun relative to the barycenter. If you're going to pick one and say it's not moving, the barycenter is a good choice. The barycenter only moves relative to the rest of the galaxy.

[nkrisc]

My understanding was the sun is orbiting around a constantly (though subtly) shifting point because of the complex interactions of all the planets that orbit it. Each planet orbiting the sun alone would have a different barycenter. Put them all together and you get a much more complex picture. Correct me if I'm wrong.

[klodolph]

It's not that the barycenter is moving, it's that the orbit of the sun is not even remotely circular. The barycenter only moves in response to forces outside the solar system (the galaxy).

On the other hand, if you look at the barycenter of, say, Earth + Sun, then that barycenter will move around because there are a bunch of other planets involved. But the barycenter of the solar system is more stable, due to the conservation of momentum and all that, and the distance from the solar system to the nearest objects that would influence it.

[nkrisc]

Thanks, that is a better explanation.

[24gttghh]

I'm sure there is an average center that is implied by the repeating pattern of the plot I posted.

[DigitalJack]

It's barycenters all the way down.

[jcranmer]

- The Copernican model held uniform circular motion around the sun, which is at odds with even the crude experimental evidence of the time.

- If the Earth was a high-speed ball flying around the sun, shouldn't we be thrown off the planet? (Note that this is before Newton's law of gravitation)

- To be visible from such a distance, the stars would have to be >1AU in diameter according to the best measurements of the day.

- Some of the more esoteric portions of Copernican model were dangerously close to actual heresy, for example about the nature of matter in an infinite universe.

It should be noted that what saved the heliocentric model was not so much Kepler's elliptical motion but Newton's gravitational theory.

[mannykannot]

What the church believed is neither here nor there.

Failure to observe stellar parallax is not scientific evidence for geocentricity, as there is an explanation, obvious to anyone who understands parallax (and, as it happens, correct), for the non-observation. Consequently, Occam's razor (which is a methodological assumption, not an axiom of either logic or the scientific method) does not apply.

Kepler had fixed the prediction issue.

I do not believe the center-of-gravity issue was considered at that time, and if not, could not be a reason for preferring geocentricity at that time (and by the time it was recognized, geocentricity was already scientifically untenable.)

[jostmey]

Occam's razor is not science, but it is rational. Science is about running physical experiments to verify your understanding, and to include proper controls to validate the experiments.

A lot of research does not have a component of science to it, but the studies are rational. Rational != Science

[daveguy]

> With Occam's razor you would prefer it.

> - Ptolemaic model with its epicycles provided better predictions.

These two statements are contradictory.

[bbctol]

How so?

[eternalban]

OP thinks the assumption of "epicycles" tips the balance of "least assumptions" to the contending theory. (He did not elaborate the full count of assumptions of either.)

[eternalban]

I think this is a good test case for The Razzor. (tm) /g

(Not concerned here with the meta question of why even accept aphorisms as fact.)

Epicyles are observable phenomena and noted from antiquity. The Ptolemic System was an answer to the cosmic riddle that incorporated a reasonable, integral and coherent, explanation for this naturally observable phenomena. The Copernican System also explains the phenomena per its internal logic.

So Epicyle, imo, is not an assumption in either case, since it was not 'introduced' into the model. It is an inherent property of the system P and subjective side-effect (observer relative) phenomena in the system C.

The Copernican System assumes the following:

1 - Earth is not flat.

2 - Earth is rotating around its axis.

3 - Earth is rotating around the Sun (just like any other body in Solar System.)

We know these are facts today since we have proof that explains these "counter intuitive" and "unfamiliar" phenomena. We (1) experience a flat ground, (2) do not feel as if we're on surface of a moving body, and (3) clearly see :) that everything rotates around Earth!

That is the 'factual' state of affairs at some point in the Middle Ages.

[icegreentea]

Practically no one in the time period you care about thought that the Earth was flat.

[cfmcdonald]

Both systems (Copernican and Ptolemaic) had epicycles.