It seems that it would be difficult to observe from the earth that the earth goes around the sun instead of vice-versa. What did he observe that led him to the notion that the earth goes around the sun? I suspect that he was looking at the stars in the background, so to speak, but that still seems hard to understand.
In Copernicus’ time people were creating mathematical models of the movement of the Sun, the planets and the Moon. Everything was done in circles, in part possibly because that’s straightforward math* and definitely because circles were perfect. The apparent retrograde motion of the planets required a bunch of epicycles, smaller circular orbits nested to give the same result.
Putting the Sun at the center simplified the models significantly. Exactly how, or whether, Copernicus argued why this was better I leave up to someone who has read his work, or a summary with more detail than Wikipedia.
*I don’t know if anyone tried elliptical orbits before Kepler, and if no one did, if it was because it was difficult to work with or considered less perfect. My guess would be that elliptical orbits didn’t offer an advantage without heliocentrism, so accepting that was an important step to realizing ellipses were the way to go.
Ptolemy’s model put the Earth at the center of everything. An Earth-centric universe has some problems, especially with the motion of planets not making much sense. Everyone knew that there were a lot of issues with Ptolemy’s model, but no one had anything better.
Copernicus was using the same observations that everyone else was, namely the odd motion of the planets under Ptolemy’s system. Stars in the background didn’t factor into it. They were envisioned as moving on a large sphere, and their motion made perfect sense. It was the planets that royally screwed with Ptolemy’s theory. The whole reason that they were called “planets” is the Greek word πλᾰνήτης (planetes), which means “wanderer”. The five “planets” (Mercury, Venus, Mars, Jupiter, and Saturn) wandered across the sky in patterns that did not make much sense.
There was also an Islamic theory that the Earth wasn’t fixed in position and did in fact move, though I don’t think they had placed the Sun at the center of our solar system. Copernicus was likely aware of this theory (that the Earth moves).
Copernicus’ observations of the heavens were made with the naked eye. He died more than fifty years before Galileo became the first person to study the skies with a telescope. From his observations, Copernicus concluded that every planet, including Earth, revolved around the Sun. He also determined that the Earth rotates daily on its axis and that the Earth’s motion affected what people saw in the heavens. Copernicus did not have the tools to prove his theories. By the 1600s, astronomers such as Galileo would develop the physics that would prove he was correct. Copernicus died on May 24, 1543.
https://starchild.gsfc.nasa.gov/docs/StarChild/whos_who_level2/copernicus.html
As discussed above, his biggest “observation” was that the current theories were incredibly complex, and became more complex every time anyone really looked at it closely. Putting the sun in the center of the universe made everything easier, without losing any accuracy, which seemed like a good idea at the time.
As with a lot of theories like that, confirming evidence came about much later. But part of the reason we went looking for that evidence was to resolve which theory was more correct.
One of the biggest problems that everyone knew about was the issue of retrograde motion of the planets. This was something you could see with the naked eye, so long as you kept accurate records over the course of several years. Explaining it is relatively easy with the heliocentric model of the solar system, but really, really hard with a geocentric model.
When you are going around in a merry go around, you can always come up with a model that describes complex motion of everything around you and a model where you are stationary. But such a model is complex and needs many adjustments - like things appear to move towards you and then away from you (the retrograde motion of planets).
A simple model where you are going around in a merry go around makes everything simpler.
That’s essentially what Copernicus realized. And for the record, Copernicus was not the first : Heliocentrism was discovered and accepted centuries earlier in India: Heliocentrism - Wikipedia
Yes, and we can turn the OP around in one sense, and wonder why the heliocentric model wasn’t arrived at by more people sooner. When you have weird, spirally motions of the planets, and relatively straightforward motions of the sun and moon, surely more people would think to try a model with the sun (or moon) at the center of all the motions?
But of course, not only does it feel instinctively as though we’re stationary, but seeing a “shell” of apparently static stars, and the planets moving within that shell, seems to confirm that view. It takes quite a leap to think otherwise.
Indeed, one reason for opposition to heliocentrism early on was due to the fact that stellar measurements showed (with the equipment of the time) no parallax. Thus, either the stars were a truly huge distance away, or they were moving around the earth. With no independent concrete data to establish such great distances, geocentrism was not completely untenable.
The heliocentric view had been generally accepted by the time solid stellar parallax measurements were available in the 19th century, and distances to planets had been obtained e.g. via diurnal parallax (measurements made in different places on earth at the same time), so the long distances implied by the stellar parallax measurements could be justified.
Observations of the planets reveal a number of facts:
1: Two of the planets, Mercury and Venus, are never far from the Sun in the sky. Sometimes they’ll be a little ahead of the Sun and hence be morning stars, sometimes they’ll be a little behind and hence be evening stars, but either way, they’re never far. More specifically, Mercury is always within about 30º of the Sun, and Venus is always within about 45º.
2: All of the other planets (aside from the Sun and Moon, which were included in the definition of “planet” at the time) would usually move in the same direction relative to the background stars, but would occasionally, on a regular basis, move in the opposite direction, what’s called “retrograde motion”.
3: While a planet is in retrograde motion, it is always in the spot in the sky directly opposite from the Sun.
4: While a planet is in retrograde motion, it is always at its brightest (the brightness of planets varies on the same regular period as everything else about them).
The Ptolemaic model, with the Earth at the center, could reproduce all four of these facts, but only arbitrarily: There was no inherent reason why any of them should be true, and one could just as easily construct a similar model where none of those four points would be true. But in the Copernican model, with the Sun at the center, all four of those facts naturally emerge, and couldn’t be any other way.
It is sometimes pointed out that, for either the Copernican or Ptolemaic model to exactly match the observations, a great deal of further complication was needed (epicycles and equants and so on), and it was about the same amount of complication in either model. This is true, but at just the most basic qualitative level of those four facts I mentioned, Copernicus manages to succeed with none of those complications at all, while Ptolemy already needs epicycles. That’s the sign that Copernicus was closer to correct.
(a simple model that matched observations exactly, to the limits of experimental error, had to wait for Kepler’s laws explaining Tycho’s data, and a physical explanation for why those laws held had to wait for Newton)
Yes, the seven Ptolemaic planets are, in order, Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. These are the permanent heavenly objects that wander among the fixed stars. I’m not sure if comets or meteors were classified as planets or not, but I believe not, as they are not permanent.
At the time in question, it wasn’t even certain that comets were astronomical bodies. Some thought they were atmospheric phenomena. It wasn’t until Edmund Halley predicted the return of a comet which actually happened that they were confirmed to be astronomical.
This is the point in the thread where I encourage everyone to read The Great Ptolemaic Smackdown, an entertainingly written history of the transition from the geocentric to the heliocentric system. Pull quote:
Before you laugh at your ancestors, [the author] invites you to prove that the earth is, contrary to your senses, in wild and careening double motion: spinning like a top and whipping around the sun without (somehow) leaving the Moon and Air behind, and without everyone stumbling around like dunkards. You are not allowed to appeal to authority or to the success of NASA, or suchlike things. You’ve got eyeballs and armillaries, and that’s pretty much it. Go. [The author] will wait here.
Plus there’s the issue of “classics”. The Europeans emerging from the dark ages had the knowledge that a great mighty empire full of bright scholars, grand engineers, and amazing militrary might had preceded them. They essentially saw themselves standing on the shoulders of giants, and it was a big step to question the giants of established “science”. A matter of “it was written by the Greeks and Romans that this is so, who are we to question them?” IIRC the same could be said of medicine and physics.
I mean strictly speaking the Heliocentric model is wrong, since the Sun is also orbiting the supermassive Black Hole in the Galactic center
The geocentric/heliocentric debate was about the Solar System and ignored everything else. So I don’t think failing to account for Sgr A* is a valid criticism.
However, Copernicus’ model was not correct because he had circular orbits. Which meant the orbits’ centers were not actually in the Sun, but offset somewhat. And also required epicycles. But Ptolemy’s model didn’t have the orbits’ centers at the Earth for the same reason.
Two things:
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Actually the sun doesn’t orbit Sag A*. Though the supermassive black hole is at the center of our galaxy (as virtually all SBHs are), it makes up a minute fraction of the galaxy’s total mass, quite unlike the sun’s dominance of the solar system. If we could magic away Sag A*, very little would change in the orbits of the milky way’s stars.
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The milky way itself is also being pulled by other masses, so by that token we cannot say it is stationary either. But of course, all motion is relative anyway.
AIUI heliocentrism is still a better model than geocentrism because the change to the earth’s velocity due to the sun’s gravity, is much greater than the change to the sun’s velocity due to the earth’s gravity, and that is something verifiable (for example by looking at the orbits of the planets).
Hmm, is Chronos in the house to confirm if I’ve got that last part right?
(This post is brought to you on the basis of 40-plus-year-old memories of my studies of Ptolemy, Copernicus, Kepler, et al, in college, so keep a few grains of salt handy.)
One of the other difficulties with the Ptolemaic model not previously mentioned here can be seen in the picture on the left. Note that some of those loops that result from the epicyclic motion bring the planet in very close to earth, relative to the outer sphere of the fixed stars.
Here on earth, when things get closer, they appear larger and brighter, and this might be expected to be visible in some way with those planets, especially since the ratios of their apogees and perigees are so great. (For the one that comes closest, it looks to be around 9:1.) But no such changes in their size or brightness are observed, at least not in a magnitude that remotely corresponds to that presumed change in distance.
IIRC (see note above), the Ptolemaics handwaved this problem away by claiming that the planets were perfect celestial bodies that didn’t behave like corrupt terrestrial objects, or that the distances were infinite, so the ratio between one infinite and another meant that there would be no apparent change.
When I was studying this stuff, this objection seemed one of the greatest unresolved difficulties of the Ptolemaic system, since it flew so blatantly in the face of ordinary observations and experience. The whole system was intended to “save the appearances,” i.e., explain the observed behavior of the celestial bodies, but it couldn’t account for this major discrepancy without an appeal to some arbitrary assertion about their nature. Circles, epicycles, equants, all that stuff would have been fine with me if they had really explained everything without such an obvious contradiction between prediction and observation.
Looks about right.
Oh, and to add: Galileo bought Copernicus’s arguments, but he also added some of his own, thanks to his use of the telescope (he wasn’t the first to make a telescope, but he does seem to have been the first to use it to make and record observations of the heavens).
First, he observed that Venus and Mercury have phases, like the Moon, which means that they’re sometimes on the same side of the Sun as us and sometimes on the opposite side.
Second, he also observed that there were objects that appeared to orbit Jupiter, and if one non-Earth object could have things orbiting it, why not another?
Third, and this one is a bit abstract, part of the motivation for the Ptolemaic model was the idea that heavenly bodies were “perfect” and unmarred. Oh, sure, the Moon has visible features, but that was explained away by the Moon being the closest object to the Earth (they got that part right), and so its near side was corrupted by constant exposure to fallen Earth. But Galileo with his telescope was able, first, to determine that the “blemishes” on the Moon were just mountains, not unlike the mountains we have here on Earth, and second, he was able to detect dark spots on the Sun, which he (reasonably but incorrectly) interpreted as clouds, thus establishing the notion that the heavenly objects were, fundamentally, of the same nature as Earth.
I have to report this: some 20 years ago I was camping in southern Wisconsin and on a moonless night and I decided to play with my binocculars. Looking at bright objects in the sky while bracing my hands against a post, I looked at Jupiter and saw three moons. I have never before or since had such a thrill. I saw what Galileo saw. And, while I was at it, I was looking at another planet and its moons, actually out in space. I can’t describe the largeness of the moment. Like being at the birth of an idea.