Notes: Epicycles and Planetary Orbits

So far you're pretty sure that you're place is the centre of the universe, and that means that everything in the sky is circling you, as well it should, god's own image, Joshua 10:13 etc etc. However you found something that didn't quite agree with that. The strange wandering stars were moving in a way that didn't exactly suggest  a nice clean orbit around us. So you did what anyone who's sense of place in creation was threatened would do. You made it fit your theory.

What you decide is that the planets are still orbiting you, calling that orbit the deferent, however they're also moving in small circles around that deferent. You call these little circles epicycles, literally 'on the circle'. Alright, everything is sorted. These circles account nicely for the changes in direction, velocity and what not that these planets take when orbiting you.

Source

Except they'd go out and observe a bit more and it didn't quite fit. So they'd add more epicycles. Still not quite right. More epicycles! Still no? Just keep changing stuff until it fits our established theory gorramit!

These days epicycles is something of a swearword in science. I can tell you this with certainty because two lecturers in entirely different parts of the world have told me just that. If a theory comes along that builds into something really unnecessarily and ridiculously complicated it may well be compared to epicycles. Then someone gets punched.

Incidentally, this is where one of those fables I keep writing about come in. In fact this is the first one in that first lecture series where I got the whole idea. Basically the idea that they kept making things more complicated is true, but they didn't so much keep adding epicycles as they did arbitrarily change the velocities of the planets, or make the epicycles move side to side, anything that might help get it to work. The end result is the same though. An arcane mess of a theory that you just want to curl up and ignore.

Kitten break.

So you do. You decide you're fed up with this nonsense. Clearly something is wrong, and a new theory is needed. So thought a fellow named Copernicus, who had a look at the data and thought "Ok, this needs some work". So he proposed Heliocentric cosmology, and published his theory in a book called De revolutionibus orbium coelestium, On the Revolutions of the Heavenly Spheres. Initially it didn't sell so well. Copernicus has made it really rather technical, so that only the most learned of astronomers could really understand it. This allowed it to disseminate into their ranks before it roused any of the more zealous geocentric types. Eventually however it did become fairly popular, enough so that the pope had it put on the Index of Forbidden Books in 1616. Copernicus didn't really mind so much at the time, though mainly because he was dead. Probably best. If he was still alive when it was banned he might have been taught the error of his ways, as it were. And then died.

Now this theory works, you think, though probably only in your head, lest you be put on trial for heretical thinking or whatever. It fits the data. The planets all orbit in the same direction, the outer planets take ages to get anywhere, everything fits. What this also means is that we can take a decent stab at working out how far away the other planets are:

I'm tired, hungry, and have been at this all day. You be happy with my illegible scribbles.

So finally, we have some proper idea of our universe. There's us orbiting the sun, along with other planets, and the fireflies that got stuck up on that big bluish black thing.

Next time, the uneventful adventures of Tycho Brahe.

Not that one.