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Trying to observe the night sky for a few weeks, the motion of the Sun and the stars pretty much fits into the Geocentric Theory i.e. All of them move around the Earth.

What then, which particular observation, made us think that it could be the other way around, that all the planets move around the Sun?

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  • $\begingroup$ It doesn't; they both happen to rotate (though not necessarily in a circle or constant angular velocity) about their center of the mass (if you ignore the rest of the universe). In practice, however, the rest of the universe is negligible and the sun is so much more massive that the center of mass is actually contained within the sun. $\endgroup$
    – Jonathan Gleason
    Commented Feb 28, 2013 at 14:46
  • $\begingroup$ This made be of use: en.wikipedia.org/wiki/Deferent_and_epicycle $\endgroup$
    – OmnipresentAbsence
    Commented Feb 28, 2013 at 15:09
  • $\begingroup$ To expand on your question, the reason why the geocentric model is reasonable is because the stars are so far away that there is almost no parallax. Their distance makes it seem as if they are literally attached to fixed locations in a celestial sphere. $\endgroup$
    – amr
    Commented Mar 2, 2013 at 0:12
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/10933/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Aug 12, 2013 at 21:35

3 Answers 3

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which particular observation, made us think that it could be the other way around

Retrograde motion must be a prime candidate.

enter image description here
As seen from Earth against star background, Mars occasionally slows down and goes backwards. Our moon doesn't.

It probably became clear to people constructing orreries that heliocentric models were enormously simpler and more convincing. They also tied in with simple inverse square laws of gravitation and planetary motion.

The discovery by Galileo Galilei of Jupiter's moons also provided firm evidence of the existence of heavenly objects that, perversely, did not orbit the Earth.

enter image description here
Photo: Thomas Bresson (Galileo probably didn't have a Nikon / mobile phone handy)

enter image description here
Luckily, he had available a corner of a napkin, a goose and some soot (or equivalents)

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  • $\begingroup$ Footnote: they tried to 'fix' this observation into the geocentric theory by inventing so-called 'epicycles' (see Wiki), but this was horrendously complicated. Epicycles were so bad in fact that they leave quite a nice note to later generations of scientists in that they show how not to do science - i.e. don't pick a theory and try to prove it - let the evidence tell you what's going on, and look for simple solutions, which is how Copernicus, Kepler, Newton etc. blew epicycles out the water. $\endgroup$
    – user12345
    Commented Mar 13, 2013 at 10:32
  • $\begingroup$ Although Copernicus did use epicycles. Indeed because he wanted uniform motion he introduced them so as to avoid equants. Just saying. $\endgroup$
    – Francis Davey
    Commented Sep 6, 2014 at 14:22
  • $\begingroup$ The fact that other planets orbit the Sun would not imply that the Earth must do so as well; an article I read some time back in Scientific American suggests that one of the competing astronomical theories, which has planets orbiting the Sun and everything else orbiting the Earth would actually be more consistent with evidence that was available until IIRC the late Nineteenth Century. $\endgroup$
    – supercat
    Commented Nov 29, 2014 at 17:56
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It would seem so, but there are a couple of subtleties involved.

Firstly, if you time the rise and set of the sun everyday, and the rise and set of your favourite stellar constellation, you'll find that the constellation rises four minutes earlier and earlier everyday. This is easily explained if the Earth is revolving around the sun, but not so if the other way around.

Also, if you observe the motion of comets you'd have to device some really fancy trajectories for them to be revolving around the Earth, rather than the Sun. The same goes for planets, which don't follow the same path "around the Earth" as the stars do. It just made more sense overall to allow for the Earth to be revolving around the sun.

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  • $\begingroup$ Your definition of "a day" is changing as the Earth moves the Sun ... or more specifically your timing of the start of the day is changing. Hence your "time of day" is rotating. Use "a day" that is based on a point much further away, outside of the Milky Way Galaxy. $\endgroup$
    – Skaperen
    Commented Mar 2, 2013 at 16:47
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The observation of the phases of Venus through early telescopes by Galileo Galilei were key to discrediting the geocentric model. Search the web for "phases of Venus in geocentric model" for many good summaries of this history. Paraphrasing what I've read:

Galileo Galilei used his telescope to observe that Venus showed all phases, just like the Moon. He thought that while this observation was incompatible with the Ptolemaic system, it was a natural consequence of the heliocentric system. Ptolemy placed Venus' deferent and epicycle entirely inside the sphere of the Sun between the Sun and Mercury. He could just as easily have made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun, such as placing the center of the Venus epicycle near the Sun.

Under the Ptolemaic system, and if the Sun is the source of all the light: If Venus is between Earth and the Sun, the phase of Venus must always be crescent or all dark. If Venus is beyond the Sun, the phase of Venus must always be gibbous or full.

But Galileo saw Venus at first small and full, and later large and crescent.

This showed that with a Ptolemaic cosmology, the Venus epicycle could be neither completely inside nor completely outside of the orbit of the Sun. As a result, Ptolemaics abandoned the idea that the epicycle of Venus was completely inside or outside the Sun.

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  • $\begingroup$ Maybe you could expand on this? $\endgroup$
    – Manishearth
    Commented Mar 1, 2013 at 15:34
  • $\begingroup$ @MarkRovetta All the historical accounts I've encountered agree with this. The phases of Venus were an unignorable problem for the geocentric model. $\endgroup$
    – amr
    Commented Mar 2, 2013 at 0:26
  • $\begingroup$ Galileo is my favorite physicist. Just is. $\endgroup$
    – Mark Rovetta
    Commented Mar 2, 2013 at 2:17
  • $\begingroup$ @amr: One could (and according to a piece I read in Scientific American, many people did) hold a geocentric view of the universe in which planets revolve around the Sun, but everything else in the heavens, including the Sun, revolves around the earth. Such a theory would avoid a problem with the heliocentric theory: it implied that stars must be mind-bogglingly big and mind-bogglingly far away, but there was no other evidence that such huge objects and distances could possibly exist. $\endgroup$
    – supercat
    Commented Nov 29, 2014 at 18:01

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