What observations did Copernicus make that argued for a heliocentric view of the universe?

[md-2000]

@crowmanyclouds

@md-2000

the planets are moving in circles around the sun

I like that the common Person on the Street would give this as the correct answer despite the more correct answer being ‘The planets Objects in the Solar system are moving in circles orbits around their barycenter with the Sun’.
But at that point we’re in Asimov’s The Relativity of Wrong* land.
*Geocentrism is making a comeback thanks to Flat Earthers and other stupid folks.

The point being at the time of Copernicus the only objects in the sky and moving were either the sun and planets (which included the moon, IIRC) and comets, which as noted were possibly atmospheric. And if the average person today confuses circular and elliptical orbits, imagine the lack of understanding of such nuances in 1400 when few people studied the concepts of conic sections and possibly the language did not allow for such nuances.

Plus - the heavens were supposedly perfect, so anything other than perfect circles would be a non sequitur. this was the thinking that made suggesting an explanation that countered ancient and established knowledge so difficult to begin with.

The story goes that some of the people Galileo showed Jupiter’s moons to, refused to believe their own eyes because it countered the accepted view of the perfect heavens to admit there were heavenly bodies that went around something other than the earth.

[Chronos]

Of course, in principle, the Galilean moons are no more difficult to reconcile than are Mercury and Venus: Give each of the moons its own set of circles and epicycles and so on, some of which just happen to have the same period as Jupiter.

[md-2000]

Yes, but however you “redefine” them, they are orbiting Jupiter. This was something incredibly novel and counter to all previous observations… as was the mountains on the moon, etc.

[Chronos]

But the evidence that they’re orbiting Jupiter is just that they’re always close to Jupiter in the sky. Why is that not equally evidence that Mercury and Venus are orbiting the Sun?

[Paxx]

There’s Ptolemy’s equant:

The equant solved the last major problem of accounting for the anomalistic motion of the planets but was believed by some to compromise the principles of the ancient Greek philosophers, namely uniform circular motion about the Earth. The uniformity was generally assumed to be observed from the center of the deferent, and since that happens at only one point, only non-uniform motion is observed from any other point. Ptolemy displaced the observation point from the center of the deferent to the equant point. This can be seen as violating the axiom of uniform circular motion.
…
Dislike of the equant was a major motivation for Copernicus to construct his heliocentric system.

In 1514, Copernicus wrote the Commentariolus as a brief outline of an early version of his heliocentric theory of the universe. In it, he says:

Yet the widespread [planetary theories], advanced by Ptolemy and most other [astronomers], although consistent with the numerical [data], seemed likewise to present no small difficulty. For these theories were not adequate unless they also conceived certain equalizing circles, which made the planet appear to move at all times with uniform velocity neither on its deferent sphere nor about its own [epicycle’s] center. Hence this sort of notion seemed neither sufficiently absolute nor sufficiently pleasing to the mind.

Therefore, having become aware of these [defects], I often considered whether there could perhaps be found a more reasonable arrangement of circles, from which every apparent irregularity would be derived while everything in itself would move uniformly, as is required by the rule of perfect motion.

So Copernicus was motivated to formulate a model that didn’t violate uniform circular motion. Next came the idea that the Earth moves. In De Revolutionibus, Copernicus writes:

For a long time, then, I reflected on this confusion in the astronomical traditions concerning the derivation of the motions of the universe’s spheres. I began to be annoyed that the movements of the world machine, created for our sake by the best and most systematic Artisan of all, were not understood with greater certainty by the philosophers, who otherwise examined so precisely the most insignificant trifles of this world. For this reason I undertook the task of rereading the works of all the philosophers which I could obtain to learn whether anyone had ever proposed other motions of the universe’s spheres than those expounded by the teachers of astronomy in the schools. And in fact first I found in Cicero that Hicetas supposed the earth to move. Later I also discovered in Plutarch that certain others were of this opinion. I have decided to set his words down here, so that they may be available to everybody:

Some think that the earth remains at rest. But Philolaus the Pythagorean believes that, like the
sun and moon, it revolves around the fire in an oblique circle. Heraclides of Pontus, and
Ecphantus the Pythagorean make the earth move, not in a progressive motion, but like a wheel
in a rotation from west to east about its own center.

Therefore, having obtained the opportunity from these sources, I too began to consider the mobility of the earth. And even though the idea seemed absurd, nevertheless I knew that others before me had been granted the freedom to imagine any circles whatever for the purpose of explaining the heavenly phenomena. Hence I thought that I too would be readily permitted to ascertain whether explanations sounder than those of my predecessors could be found for the revolution of the celestial spheres on the assumption of some motion of the earth.

And, some reasons for putting the Sun in the center:

At rest, however, in the middle of everything is the sun. For in this most beautiful temple, who would place this lamp in another or better position than that from which it can light up the whole thing at the same time? For, the sun is not inappropriately called by some people the lantern of the universe, its mind by others, and its ruler by still others. (Hermes) the Thrice Greatest labels it a visible god, and Sophocles’ Electra, the all-seeing. Thus indeed, as though seated on a royal throne, the sun governs the family of planets revolving around it. Moreover, the earth is not deprived of the moon’s attendance. On the contrary, as Aristotle says in a work on animal, the moon has the closest kinship with the earth. Meanwhile the earth has intercourse with the sun, and is impregnated for its yearly parturition.

[md-2000]

Hmmm… Pythagoras…

If I recall - a long time ago in another thread far far away…

The discussion about Pythagoreans and their contention the earth moves (in contradiction of some Bible passages) was the issue that got Galileo in trouble. There was a cult/heresy current in Galileo’s time that espoused Pythagorean numerology and also made the claim about the earth moving, and that the Book of Job was wrong. The church in trying to stamp out the dissent, wanted to suppress any voice that said the earth and planets went around the sun, which would bolster the heretical group. (Sort of like the current mask debate, where the crazies pick science articles to say “see, science agrees with us - no masks…”) So Galileo was told by his friend the pope to “cool it with the sun thing” until they dealt with the heretics. He didn’t listen.

[Chronos]

That might have been part of it, but the biggest problem was that Galileo was, personally, a grade A asshole. When he did eventually get permission from the Pope to write a book comparing the two models, he responded by making one of the characters a caricature of the Pope, named “Simplicio”, who was always wrong about everything, and then sock-puppeted all of his arguments for heliocentrism.

[md-2000]

@Chronos

the biggest problem was that Galileo was, personally, a grade A asshole.

There’s that too…

[Voyager]

@MikeS

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.

This could have been taken as is from your standard Flat Earther webiste. So you don’t have to think of an ancestor, maybe a neighbor.

Plus, a geocentric model meshed well with the assumption that Earth was created by whatever deity you believed in, and the sun was created to serve the earth.

[Paxx]

@naita

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.

Copernicus’ model was conceptually simpler (once you got past the unintuitive idea of the Earth’s three motions) but, in detail, it was as complex as Ptolemy’s model. Copernicus insisted on uniform circular motion which, according to Aristotle, was the nature of celestial objects and which Ptolemy’s model violated. As a result, Copernicus had to use epicycles to compensate for what we now know are elliptical orbits.

The exact number of epicycles is subject to dispute. This Ohio State web page says “48 epicycles, compared to 40 in the Ptolemaic geocentric system.” This U of Texas web page says “Copernicus’s model … contains approximately the same number of epicycles as Ptolemy’s.” And The Exact Sciences in Antiquity says “the Copernican models themselves require about twice as many circles as the Ptolemaic models and are far less elegant and adaptable.”

In any case, Copernicus eliminated the annoying equant but it took him about 400 pages to explain the math in De Revolutionibus.

[Paxx]

@engineer_comp_geek

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.

The planets made sense in Ptolemy’s model. Their motions were formulated by the use of the eccentric, the epicycle, and the equant:

With such combinations of constructions, Ptolemy was able to account for the motions of heavenly bodies within the standards of observational accuracy of his day. The idea was to break down the complex observed planetary motion into components with perfect circular motions. In doing so, however, Ptolemy violated the cosmological and physical rules of Aristotle. The eccentric and epicycle meant that planetary motions were not exactly centered on the Earth, the center of the cosmos. This was, however, a “fudge” that few objected to. The equant violated the stricture of perfect circular motion, and this violation bothered thinkers a good deal more. Thus, in De Revolutionibus (see Copernican System), Copernicus tells the reader that it was his aim to rid the models of heavenly motions of this monstrous construction.
From Ptolemaic System

[Paxx]

@Horatius

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.

Actually, Copernicus did not say that current theories were incredibly complex. This is what he wrote:

However, Your Holiness will perhaps not be greatly surprised that I have dared to publish my studies after devoting so much effort to working them out that I did not hesitate to put down my thoughts about the earth’s motion in written form too. But you are rather waiting to hear from me how it occurred to me to venture to conceive any motion of the earth, against the traditional opinion of astronomers and almost against common sense. I have accordingly no desire to from Your Holiness that I was impelled to consider a different system of deducing the motions of the universe’s spheres for no other reason than the realization that astronomers do not agree among themselves in their investigations of this subject. For, in the first place, they are so uncertain about the motion of the sun and moon that they cannot establish and observe a constant length even for the tropical year. Secondly, in determining the motions not only of these bodies but also of the other five planets, they do not use the same principles, assumptions, and explanations of the apparent revolutions and motions. For while some employ only homocentrics, others utilize eccentrics and epicycles, and yet they do not quite reach their goal. For although those who put their faith in homocentrics showed that some nonuniform motions could be compounded in this way, nevertheless by this means they were unable to obtain any incontrovertible result in absolute agreement with the phenomena. On the other hand, those who devised the eccentrics seem thereby in large measure to have solved the problem of the apparent motions with appropriate calculations. But meanwhile they introduced a good many ideas which apparently contradict the first principles of uniform motion. Nor could they elicit or deduce from the eccentrics the principal consideration, that is, the structure of the universe and the true symmetry of its parts. On the contrary, their experience was just like some one taking from various places hands, feet, a head, and other pieces, very well depicted, it may be, but not for the representation of a single person; since these fragments would not belong to one another at all, a monster rather than a man would be put together from them. Hence in the process of demonstration or “method”, as it is called, those who employed eccentrics are found either to have omitted something essential or to have admitted something extraneous and wholly irrelevant. This would not have happened to them, had they followed sound principles. For if the hypotheses assumed by them were not false, everything which follows from their hypotheses would be confirmed beyond any doubt. Even though what I am now saying may be obscure, it will nevertheless become clearer in the proper place.
From Nicholas Copernicus | Calendars

Putting the sun in the center of the universe did not make everything easier. Please take a look at my response above to naita’s post.

[Chronos]

@Paxx

Putting the sun in the center of the universe did not make everything easier. Please take a look at my response above to naita’s post.

I’ve already addressed this, in post 9:

@Chronos

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.

[Paxx]

Of course!

When I formulated my post to naita in my head, I thought about mentioning your post but then I got wrapped up in presenting cites to counter the claims by the other posters and I forgot to reference your post.

[Northern_Piper]

@Chronos

Observations of the planets reveal a number of facts:
salient points omitted

Thank you for this summary. It really helped me understand the debate between Copernican and Ptolemaic systems.