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The Kepler Orrery
blaisepascal
Boing-Boing recently posted an article about "Kepler's Orrery", which turns out to be an animated visualization of the data returned by the Kepler satellite. Kepler is designed to search for extrasolar planets in bulk. The batch of data recently released has about 1200 previously unknown extrasolar planets. So the data is cool, and the visualization is also cool.

But it was an initial disappointment, for I had been hoping that it would be somewhat different, something in line with a different visualization I'd seen recently comparing a a Copernican Orrery and a Tychonian orrery. I wanted more like that, but I believe that visualization got things wrong as well.

What am I talking about? An orrery is a model solar-system, traditionally mechanical and gearwork, which shows the relative position of the bodies in the solar system. You turn a crank (or have a motor do it for you) and indicators move as the planets, moon, and sun moves. It is possible to make a graphic computer program which does the same job as an orrery. Virtual planetarium software have to do the same calculations, even if they aren't designed to show an "outside" view. I played with virtual orrerys over 20 years ago.

What I haven't seen are modern orreries, mechanical or virtual, which accurately portray pre-modern models of the solar system. Ptolemy suggested a system of cycles, epicycles, and deferents centered on the Earth. Copernicus suggested a system of cycles, epicycles, and deferents centered on the Sun. Tycho suggested a system of cycles and epicycles where the sun and moon orbited the Earth, and the rest of the planets were on epicycles centered on the sun. Kepler suggested a variant on Copernicus's scheme.

(Here's the timing: Ptolemy lived from 90-168CE, and his work was the standard model used for centuries. Copernicus published his work on his deathbed in 1543. Tycho Brahe was born 3 years after Copernicus died, and did not agree with Copernican heliocentrism. He used his nobility and royal connections to create the last great naked-eye observatory and collect a massive amount of astronomical data to support his own model. He wasn't mathematically sophisticated enough to analyze the data, so he hired one of the best young mathematical minds in Europe: Kepler. Kepler believe in Copernican heliocentrism, but had his own theory as to how the planets were arranged (in nested platonic solids). Tycho, fearing that Kepler would use his data to prove Kepler's theory and not Tycho's, was reluctant to allow his hired mathematician access to the observations -- a point of some friction between the two. After Tycho died in 1601, Kepler got his data, and discovered that an assumption critical to all four models was fundamentally flawed. No one had noticed before because Tycho's observations were that much better. Kepler published his findings in 1609 and 1619, finally allowing a model of the solar system to be made which is still used today, with only minor tweaks for accuracy courtesy of Newton and Einstein)

I'd love to see an orrery which accurately models the systems of Ptolemy, Copernicus, Tycho, or Kepler, as they themselves fully described it. But I know not of such a thing.

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So write - or build - one yourself.


I have not been able to find good, specific, information as to what Ptolemy, Copernicus, Tycho, or Kepler were proposing. The Almagest is written in Greek or Arabic translations, Copernicus wrote in Latin, I don't know what Tycho published, but it's probably in Latin or Danish, and Kepler also wrote in Latin. English translations are hard to find.

One of the reasons I want to see orreries is so I can better understand how the models work.

I know I saw drawings of the systems involving epicycles, in middlebrow astronomy/science books back in the 1950s. Try an older public library. Try obsolete editions of encyclopedias. Try old biographies of Ptolemy, Copernicus, Tycho, and Kepler (and Galileo, for comparison). For that matter... is there a planetarium anywhere nearby? Even a small, inadequate one might have (or have had) some sort of oddball orreries.


I've looked. It isn't hard to find rough overviews of the systems, including pictures of epicycles, and occasionally deferents. But it's the full systems I've been unable to find.

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