In defense of geocentricism

During their lessons on science, school children are eventually taught about two models of our Solar System.

The geocentric model, they are taught, was devised by the silly, irrational people from our distant past. These ancient peoples thought that planets were “wandering” in the sky, or following complex epicycles. They were either too superstitious, or they simply lacked the imagination and intelligence, to understand that planetary motion was much simpler and more elegantly explained by a better model of the solar system.

The heliocentric model, children are taught, is what our solar system is Really Like. The planets have elliptical orbits around the Sun, which is a “star” and not a “planet”, and the Moon orbits the Earth, and is not a “planet”, but instead “a moon”. Only brave iconoclastic geniuses of the Renaissance were willing to stand up against the oppressive and irrational orthodoxy of their day, children are taught.

This is probably a fine story to teach children; the persecution of Galileo Galilei by the Church, for instance, is particularly informative. Every child should understand that people in power will inflict cruelty even (maybe even especially) when they’re clearly wrong.

But it’s a bad way to think about the solar system — a better model acknowledges that all of these bodies are acting upon each other (better still, that all of the particles comprising these bodies are acting upon each other). The heliocentric model is a good model, because the relative sizes and distances make the particular arrangement of matter in space that is our solar system behave as though planets are orbiting the sun and moons are orbiting planets. But it’s just a model. It’s a not actually very good at predicting the lunar orbit, and people struggled with this until computers were invented. It ignores that this whole system is moving around space relative to the center of our galaxy, and that the galaxies in our local group are interacting with each other. It ignores the need for relativistic corrections, and the fact that space itself is expanding.

The statistician George Box has a famous quote:

All models are wrong, but some are useful.

The heliocentric model is a useful model. You can predict celestial mechanics fairly well with fairly simple math. But it’s wrong — so it can’t be The Truth. This needs to be part of the lesson for children.

Let’s get back to Ptolemy and his predecessors. If the heliocentric model is wrong, the geocentric model is definitely wrong. Is it more wrong? Maybe... but I don’t know if that’s a good question, or if it’s possible to answer. Is it useful? Well, actually, I think so.

During Covid (especially during the early days, when everybody was taking the pandemic seriously and everything was closed), I have gotten in the habit of paying closer attention to my world. I know the names of more of the little plants that grow in sidewalk cracks around my neighborhood, and I can differentiate a few dozen more. I finally see the fun architectural details that they put into buildings built more than a century ago. I know the species of bird that frequent my neighborhood, and what they sound like. And I notice the night sky.

The sky at night here is washed out with light pollution, and nothing dimmer than Saturn can be seen with the naked eye. So I’ve gotten into the habit of recognizing the position of the planets and phases of the moon. I walk my dog around the same time every night, so the “wandering” of these bodies is readily apparent. In the winter of 2020, I watched Jupiter creep up on Saturn and then shoot past it in the most spectacular conjunction of these planets in nearly 800 years. I watched the event unfold over the course of weeks. I didn’t even know to look for it at the time, it’s just something I happened to notice. I still feel lucky for that.

And let me tell you, as a newly-minted planet-spotter: if you go outside every night and try to find the planets (without consulting any resources first), the heliocentric model is useless. Sure, it’s fine if you want to sit with pen and paper and figure out where the planets will be in a few months. But that’s not planet-spotting, that’s math homework. You can spot the planets fairly well by just starting your search in the same place they were last night. If you want to be a little faster, you pay attention to how they’ve been moving night-to-night. Which ones are speeding up? Which ones are in retrograde? Eventually you know exactly where to look, without even giving it much thought.

That is the geocentric model at work.