In defence of the Ptolemaic Model (sort of)

Continuing with my fascination with using historical astronomy to discuss problems in science, today I will be looking at how science can believe silly things for a really long time, before finally everything clicks into place.

From antiquity until the late 16th century, the dominant belief was that everything went round the Earth. The Sun, the moon, the stars, the planets, all of it was whizzing round a completely static Earth, that doesn’t even spin on its axis. Oh, and everything zooming around the Earth is made out of a special substance called aether, embedded in spheres which are also made out of aether.

Now, this might all sound rather ridiculous to our 21st century ears, but 500 years ago, this was not just what the uneducated peasants believed, but what the key scientific thinkers of the time would have used as a theoretical basis for everything astronomical. And the reason for this was because the Ptolemaic Model worked.

The way we’re usually taught the story went is that everyone believed bullshit for a long time, and the church was to blame for this, and when Copernicus came along with his model, it was only the Catholic church that really got in the way of it being immediately widely adopted, by virtue of doing shit like locking up Galileo.

However, this is only partially true, and it definitely wasn’t as linear as that.

The Ptolemaic Model seems absurd from our perspective, because it goes to almost comical backflips to explain observable phenomena which can easily be covered by “it all goes round the Sun in kind of elliptical orbits, duh.” For example, planets sometimes appear to switch direction, and start moving “backwards” across the sky (this is known as retrograde). This is pretty easy to explain if you grok that everything is moving round the Sun at different distances: we’re overtaking them, or they’re overtaking us. However, if you whack the Earth in the middle, like we did for millennia, this requires things to be moving in small circles in their bigger circle around the Earth. I want to try and explain this better, but to be quite honest, I’ve tried and tried to get my head around epicycles, equants and deferents, and I can’t. Basically, lots of circles are moving around in a really fiddly fashion.

However, here’s the thing: it worked. It worked really well at accurately predicting where everything would be in the sky. It worked so well that it took over a century from Copernicus’s publication for a heliocentric model to become dominant, because for most of what they were doing, using the old model worked just as well, and this was, after all, the model that the scientists of the day had learned from their own training.

Firstly, critiques of the Ptolemaic Model existed pretty much from the time Ptolemy’s model was developed. From pagans in Carthage proposing that Venus and Mercury went round the Sun, to the Islamic scientist ibn Al-Hatham who literally wrote a text titled Doubts on Ptolemy, there were always, well, doubts on Ptolemy. However, observations that contradicted the Ptolemaic Model were usually explained away in terms of a model which was still geocentric.

When Copernicus published his theory, the most influential astronomer of the time, Tycho Brahe, acknowledged that Ptolemy had a lot of problems, but also acknowledged that Copernicus had a whole bunch of problems, too. He therefore created the Tychonic system, which agreed with Copernicus that the Earth rotates, and the planets go round the Sun… but that the Earth is stationary and the Sun goes round the Earth. Again, with our modern sensibilities, this sounds on a par with Hollow Earther beliefs, but at the time it was quite neat, explaining many of the observations which had put holes in the Ptolemaic Model. It remained reasonably popular as a theory for another century or so, peacefully coexisting with the Copernican model, and indeed being really good for astronomers who didn’t want to get persecuted by the Vatican, and only fell out of favour when the weight of observations meant that the Earth had to be in motion around the Sun. This, by the way, was in the 18th century.

Oh, and while we’re at it, the Ptolemaic Model is still used these days for certain applications. Ever been to a planetarium? Their projectors are built with the innards resembling the Ptolemaic Model, with little circles moving round bigger circles. For an ancient theory, it was surprisingly robust.

So why did the notion that the Earth is the centre of everything persist so long? Mostly, it just made common sense. After all, it’s not like we feel the Earth moving around, when we look up at the night sky, it certainly looks like we’re in spheres within spheres. And, of course, it really does flatter our egos to think we’re the most important special snowflakes, rather than some insignificant little specks on a pale blue dot at the arse end of nowhere. It went mostly unquestioned for a long time because it was a strong theory which just so happened to also fall in with social constructs about our own significance.

Geocentric models were pretty decent, but they were also, at the end of the day, wrong.

It’s worth remembering the hardiness of the Ptolemaic Model when we look at other scientific theories which are taken for granted at present. I bang these drums a lot, but, say, for example, differences between men and women. Or, in fact, the existence of two biological sexes. Just because smart people believed in it since antiquity, doesn’t necessarily make it right.

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