Homocentric Spheres
January 17, 2015 9:50 AM   Subscribe

What Eudoxus and Aristotle thought about planetary motion. You'd think there would be nice animated illustrations of this stuff on the Web somewhere, but I didn't manage to find any, so I made my own.
posted by Confess, Fletch (12 comments total) 23 users marked this as a favorite
This is wonderful. I was just trying to learn about pre-heliocentric astronomy the other week as a consequence of reading Adam Smith's History of Astronomy, which is a sort of proto-Kuhnian story of how people come to believe different things over time. Man, the ancient world: they weren't right but they certainly weren't stupid.
posted by sy at 10:18 AM on January 17, 2015 [3 favorites]

An updated version of Ptolemy's Almagest by Richard Fitzpatrick is a related interesting thing:
I have attempted to reconstruct Ptolemy's model of the solar system employing modern mathematical methods, standard dates, and conventional astronomical terminology. I hope that the result will make the full extent of Ptolemy's scientific achievement more manifest to a modern audience.
posted by solitary dancer at 11:07 AM on January 17, 2015

Basically any motion can be thought of as a sum of various cyclical motions. Basically what they were doing was creating a Fourier transform of the planetary motions.
posted by empath at 12:39 PM on January 17, 2015 [3 favorites]

And here is a flash toy that animates empath's comment.
posted by solitary dancer at 12:49 PM on January 17, 2015 [2 favorites]

OK - I totally misread that as Homoerotic... And... well... I was very curious as to how this text would play out, and as I got there, realized my mistake. I was, frankly, a bit disappointed, because it could have been waaaaaaay more interesting.
posted by symbioid at 1:15 PM on January 17, 2015

oh, this is SO COOL! Although I really wish he had added them ALL TOGETHER! this makes so much more sense than my head conception previously.
posted by rebent at 1:36 PM on January 17, 2015

It's an often-denied fact, but still true, that nothing is obvious.

Centuries of astronomers cooked up complicated models of the solar system, all because they refused to abandon their certain fact that the Earth is not the center of the universe. Did that possibility even occur to them? Did one of them, at some point, think: "Huh. If the Earth isn't in the center, why all these complicated things cancel right out." Well we know one of them did, Copernicus, but who else might have mused about that, who didn't take the time to work it through, to publish it? How long was the development of astronomy held back by that one simple insight?

So I ask you to consider, first: What sacred, preconceived notions do we hold on to now that distort our ability to see the universe accurately? And second: 500 years from now, what kinds of things will people be looking back at today, and laughing to themselves at how obviously wrong we were?
posted by JHarris at 1:58 PM on January 17, 2015 [2 favorites]

Wikipedia has an interesting page on heliocentrism and its history.
posted by BungaDunga at 2:18 PM on January 17, 2015 [3 favorites]

How long was the development of astronomy held back by that one simple insight?

Well, if you just adopt heliocentrism but keep the orbits circular, you still need epicycles to account for all the variances in apparent planetary motion - which gets you a system that is better, but still pretty arbitrary and complicated. From BungaDunga's link:

Copernicus moved heliocentrism from philosophical speculation to predictive geometrical astronomy—in reality it did not predict the planets' positions any better than the Ptolemaic system.

It wasn't until Kepler came up with the concept of elliptical orbits that the system really got simpler. But no astronomer until Kepler was willing to consider non-circular orbits: the heavens are perfect, and circles are perfect, so clearly everything in the heavens moves in circles, right?

So really it came down to two insights: the earth is not the center, and the motions are not circular. Basically, the scientific revolution: we need to measure what is really out there, and not just presume that we know what should be correct.
posted by purple_frogs at 9:11 PM on January 17, 2015 [3 favorites]

One of the really important things to consider in why geocentrism persisted for so long is that until Tycho Brahe and his island full of telescopes we didn't really have good enough data to build an accurate model of the heavens. The kind of data we have available very strongly delimits the kind of theoretical frameworks we can support. It's great to say, "hey, wouldn't it be nice if the sun was the center of the universe instead of the Earth", but unless you have solid reason to think that that's the way it actually is, that decision is as arbitrary as any other. Hell, the idea that stars are suns isn't that old, and the idea that the universe is bigger than the Milky Way is barely 100 years old.
posted by cthuljew at 5:10 AM on January 18, 2015 [3 favorites]

Once upon years ago I was trying to explain to a group of students about non heliocentric motion and how it could possibly explain the motion of stars as they related to that other of the ancient sciences, astrology - the various slow and fast cycling planets, the progression and regression of Mercury, etc - and not getting very far. Finally, one of my students snapped his fingers in that eureka moment and said 'Oh, it's like a spirograph!'.

I spent a few days figuring it out as best as I was able, and yes, it's conceptually like a huge, 3D, spirograph.
posted by eclectist at 8:03 AM on January 18, 2015 [1 favorite]


There is nothing homoerotic about Bunn's page about homocentric spheres

or maybe I just lack imagination.
posted by lukemeister at 5:43 PM on January 18, 2015

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