Talk:Deferent and epicycle

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[edit] Epicycles on Epicycles

I ran across someone elsewhere (slashdot) mentioning that the notion of epicycles upon epicycles being added in the middle ages is bad history, and that this is one that Britannica had some very bad articles on at one point. So someone (unlike me) who does know this well may want to double-check this entry and maybe comment on other mis-information floating around...

Yeah, I have mentioned that on slashdot a couple of times. And indeed, I think this article is still inaccurate. In fact, I think we have some good ammunitation against Britannica here, because Britannica carries a lot of the responsibility for the exaggarations :-) . I haven't worked on this subject for the last few years, and some of my books are packed down somewhere, but let me take something from memory.

The most fundamental thing here is that Owen Gingerich recomputed the Alfonsine Tables, which was computed in the 13th century, and found that they were based on a purely Aristotelian model. Still, Alfonse lamented the complexity of the calculations, and that has often been interpreted as complexity had been added. I think we can safely say that if epicycles were added, they had no practical influence whatsoever.

The Alfonsine Tables is not the only piece of evidence. We have several textbooks from the 13th-16th centuries confirming that the astronomical models were almost the same as Ptolemy's. Not to get into the details of what was changed, but nobody was adding epicycles. Anyone care to guess what's the first western book to have epicycles-on-epicycles? I'll give you a hint: it was published in 1543. The story about Alfonso saying that he could have given God some advice, that is apocryphal. I think Gingerich wrote a short piece on it, which would be a good thing to cite here.

So, to the question of whether it is correct to say that epicycles were added upon epicycles. There is a short discussion in a book by J. L. E. Dreyer where there is also a figure of a world system with circles within circles. I guess you could point to that and say "see, they did", but there is very little record of this having any substantial influence on astronomy, and so, I still think that this article needs a good rewriting. I'd like to do it one day, I just need the time... :-) Kjetil Kjernsmo 00:10, 5 November 2005 (UTC)

I assume you mean his History of Astronomy/World Systems? I would like to know what this could be. Sounds like a cosmology section showing the order of the planets, not epicycles. Maestlin 21:59, 11 March 2006 (UTC)

You have added far too many (disputed - see talk page) comments - surely one would be enough?? I think that this should be changed. I will modify it a little.

Regarding the last part:

"The switch to the Sun-centered model removed epicycles for a while, but the original versions insisted on circular orbits for the planets. (disputed — see talk page) Better observational data from improved telescopes once again showed data counter to the model, and epicycles were brought back to plug the holes. (disputed — see talk page) It wasn't until Kepler developed the elliptical orbital model that epicycles were finally eliminated."

I have some disagreements here too. Who is disputing that Copernicus gave the planets circular orbits? Also, I have Never heard of epicycles being added to his system - where is the evidence for this? There is some discussion on the Copernicus site but is there a translation of his book availiable to firm up what he did or didnt say? Copernicus' model was disputed by the Church, but one of their objections was that his system gave predictions for planetary positions that was as bad as, or worse, than Ptolemy's system.

Kepler realised that the planets had elliptical orbits and this straight away removed the errors with the Copernican system. I'll modify some of the text and come back at another date to discuss some more. Adrian Baker 15:10, 10 December 2005 (UTC)

Kjetil is right about the myth of epicycles-upon-epicycles (Gingerich discuss it in The Book Nobody Read, pp 58-60). Also, Copernicus did in fact add epicycles to his system (Gingerich called them "epicyclets" as they were considerably smaller than the previous ones; p 265)--this is well-known among history of astronomy scholars. And to say Kepler "straight away" removed replaced circles with elliptical orbits is absurd; he published his first book in support of them, and only with Tycho Brahe's data did he realize circles couldn't be matched up to observations.--ragesoss 18:05, 14 January 2006 (UTC)


Thanks for the source by Gingeriich. I'll try and get a copy and read it. This would help clarify some of the points above. Regarding your comment about "Kepler 'straight away'.... ...is absurd", you have misread what I have written. I did NOT say that Kepler straight away hit the right solution, I said that "Kepler realised that the planets had elliptical orbits and this straight away removed the errors..." - two very different things. Adrian Baker 23:16, 15 January 2006 (UTC)

My mistake. Sorry.--ragesoss 01:08, 16 January 2006 (UTC)


With regard to "epicycles upon epicycles": it has been pointed out in Ptolemaic System (referred from Equant) that "The eccentric in the figure below (the figure of eccentric) is fixed; it could also be made movable. In this case the center of the large circle was a point that rotated around the Earth in a small circle centered on the Earth. In some constructions this little circle was not centered in the Earth."

It seems to me that the cited statement is mathematically equivalent to the following: we add one epicycle on another one. However, it's not usually called that way. Besides, due to the difference in size of these circles, the whole system looks quite different. --Fir-tree 17:47, 22 February 2006 (UTC)

Yes, you could convert the motion described into an epicycle, but AFAIK hardly anybody pointed this out or tried to do it in a serious way until Copernicus. And that small circle that's not centered on the Earth? ...Copernicus. Most modern readers do not realize that the full-blown system of De revolutionibus is beastly complicated, and that the headaches they think of as symptoms of Ptolemaic failure only become major players on the astronomical scene after the 1540s.
I think an earlier version of this is the wingnut article that discouraged me from joining Wikipedia some time back. It looks like the disputed tag has been up for months and nobody seems to be committed to the old text. If it gets fixed, does the fixer take the disputed tags out? Maestlin 21:59, 11 March 2006 (UTC)


[edit] Ptolemy and "orbits"

Is it correct to speak of planetary "orbits" in Ptolemaic astronomy? As I understand it (someone correct me if I'm wrong), before astronomers began to seriously attack that idea of the heavens being eternal (e.g., Tycho Brahe's calculations that comets move through different planetary spheres and are not sub-lunar, widespread observation of novae, etc.), the basic cosmology involved planets embedded within rotating spheres and not actually orbiting anything.--ragesoss 18:52, 19 May 2006 (UTC)

I agree, but because there are so many presentist histories and translations that say "orbit," you might find some opposition unless you dig up a source that does terminology. Maestlin 20:29, 19 May 2006 (UTC)

[edit] Modern Epicycle approximation

I realise that it's important not to confuse the two issues, but it should be pointed out that the epicycle approximation is a useful approximation made by people studying stellar dynamics to this day. I don't know whether it should get a mention here, or in separate page which gets linked from here?

[edit] 1898?

I find this sentence: "The popular total of about 80 circles for the Ptolemaic system seems to have appeared in 1898" rather strange. Could it be a mistake or a typo? Or maybe it suggests that this large number was added in posterior sources (modern encyclopedias)? Hugo Dufort 02:04, 16 November 2006 (UTC)

Yes, it's suggesting that there wasn't ever a system actually in use with 80.--ragesoss 03:10, 16 November 2006 (UTC)

[edit] Fix errors

The last paragraph starts "The difficulty with this account" and corrects some errors in the previous several paragraphs. As it is, the "Epicycles on epicycles" section is confusing and contradictory. I hope someone fixes this. Roger 06:00, 28 December 2006 (UTC)

[edit] Is it a matter of viewpoint

(1) Looking at the java applet (external link to University of Nantes) I'm getting confused by geometry. Epicycles are usually presented as a bad attempt to explain the solar system with the earth at the middle instead of the sun. But surely if this were the only problem, the simplest epicycle model would have been to give all planets exactly the same deferent, with the earth at its middle, and the sun at the point where all the planets have their epicycle. Then each planet would have an epicycle corresponding to its orbit, and everything would be fine. Heliocentricity or Earth-centricity is merely a matter of viewpoint! The Java applet makes this clear, as both diagrams trace out the same relative positions of Earth/Planet/Sun, but one diagram moves around on the screen, the other doesn't.

So surely epicycles should be presented rather as a bad solution to eliptical orbits.

(2) And this brings me to point 2. Is it worth a mention that even after heliocentricity caught on, epicycles were still in practical use. A glance at the page on the "Orrery" will show a mechanism where the moon is on a cranked arm, which presumably rotated once per orbit, in a clockwise direction, to simulate the eliptical part of the moon's orbit, and is obviously a mechanical epicycle in action. I'm no mathematician: is this truly an elipse?

Should there, in any case, be a link from some part of the solar system set of Wiki pages to the Orrery page?

195.92.194.11 (talk) 19:58, 9 February 2008 (UTC)