Comment by throw0101b | Hacker News Reader

Comment by throw0101b | Hacker News Reader

There were seven models floating around in the early 1600s:

1. Heraclidean. Geo-heliocentric. Mercury and Venus circle the Sun; everything else circles the Earth.

2. Ptolemaic. Geocentric, stationary Earth.

3. Copernican. Heliocentric, pure circles with lots of epicycles.

4. Gilbertian. Geocentric, rotating Earth. (proposed by William Gilbert in De magnete)

5. Tychonic. Geo-heliocentric. Sun and Moon circle the Earth; everything else circles the Sun.

6. Ursine. Tychonic, with rotating Earth.

7. Keplerian. Heliocentric, with elliptical orbits.

See:

* https://tofspot.blogspot.com/2013/08/the-great-ptolemaic-sma...

* ToC: https://tofspot.blogspot.com/2013/10/the-great-ptolemaic-sma...

* PDF: https://faculty.fiu.edu/~blissl/Flynngs.pdf

By the mid-/late-1600s people leaned toward Kepler, mostly because the math was easiest.

With regards to evidence for the Earth's motion, the first inkling was in 1728 with stellar aberration with in γ-Draconis:

* https://en.wikipedia.org/wiki/Aberration_(astronomy)#Discove...

The first for the rotation of the Earth (around an axis) was in 1791 by Guglielmini:

* https://en.wikipedia.org/wiki/Giovanni_Battista_Guglielmini

We finally got parallax in 1806 by Giuseppi Calandrelli in α-Lyrae.

Stellar parallax was considered since at least Aristotle, as he mentions in his On the Heavens (II.14), and since it is not observed then it is reasonable to conclude that there is no motion (it took several thousand years to develop instruments to actually measure it).

Daniel Whitten's "Matters of Faith and Morals Ex Suppositione" is an interesting read.

> Tychonic. Geo-heliocentric. Sun and Moon circle the Earth; everything else circles the Sun.

Interesting. Isn’t that (disregarding the Galaxy) functionally the same as a heliocentric system?

That Heraclidean one also seems interesting. At the distance of Uranus or Neptune, it makes little difference whether it actually orbit Sun or Earth

What do you make of the Antikythera mechanism?

Which model does it fit? Or does it point to a different model?

https://www.tiktok.com/t/ZTNFPxAMV/

https://en.m.wikipedia.org/wiki/Antikythera_mechanism

I'd think that the Earth rotating would be obvious much, much sooner, since that's far less work than the entire universe orbiting around the Earth every day, while also having individual objects orbiting on their own monthly, annual and other cycles. And spinning tops are common toys.

Keep in mind that there was nothing indicating that the sun and the stars were of the same nature, or that they were so far away. When you think about it, all stars appear equally distant to the naked eye, so the idea that stars were fiery stones dispersed on a vast celestial sphere was absolutely plausible. This solid, physical sphere was the thing rotating.

Also people had little understanding of the scale of things. There is nothing indicating that the moon is smaller than earth or that the sun is much, much bigger than earth, and both appear equally large in the sky...

> Also people had little understanding of the scale of things. There is nothing indicating that the moon is smaller than earth or that the sun is much, much bigger than earth, and both appear equally large in the sky...

The Ancient Greeks were actually quite clever about trying to calculate things:

* https://physicsteacher.blog/2021/05/31/from-the-earth-to-the...

* https://en.wikipedia.org/wiki/On_the_Sizes_and_Distances_(Ar...

* https://en.wikipedia.org/wiki/On_Sizes_and_Distances_(Hippar...

> I'd think that the Earth rotating would be obvious much, much sooner […]

How exactly could you tell the difference between one moving reference and another? Einstein eventually showed us that it is not possible.

As this US Navy video shows, having the Earth stand still and having the 'celestial sphere' move actually makes celestial navigation using a sextant possible / mathematically easy:

* https://www.youtube.com/watch?v=UV1V9-nnaAs / https://www.youtube.com/watch?v=cun0DGZ6-sk

The Ancients / pre-Moderns concluded that the stars were on one celestial sphere and the wanderers (planetes in Greek) were on others.

Einstein did not show that. The principal you are referring to is, ironically, Galilean relativity, and attributable to Galileo. Einstein's contribution was combining the principle of relativity with a constant speed of light.

Additionally, under both systems, relativity only applies to intertial reference frames. Rotating reference frames are detectable.

You can detect Earth's rotation without any external observations using a pendulum. Although such an experiment would not be done until Léon Foucault about 2 centuries after Galileo.

You can tell the difference between rotating and not, because a rotating reference frame is not inertial. In fact, the geocentrists knew this, because one of the longest-standing objections to the idea that the earth rotated was that you would expect a falling object to be displaced from the apparent vertical due to the rotation, these experiments were done, and the displacement was not observed (it turns out, because the effect was so small, but no-one would have the mathematical tools to work this out until Newton).

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