Nicolaus Copernicus lived from 1473-1543, a time when rebellion against the Church was at its height. And unfortunately for the astronomy of the time, it had gotten inextricably tied up with Christian teachings.

In that time, heaven and hell weren’t just parts of our personal religions and beliefs—the Church held some of the highest authority over the land, and questioning heaven and hell just wasn’t done.

The way astronomers of the time pictured the universe fell right in sync with the heaven and hell geometry—Earth was the imperfect center of the universe, with hell nested deep below. Heaven was a place of perfection, and it was where all the heavenly bodies—the moon, sun, planets and stars—all moved.

Problem was, the Ptolemaic model of the universe really couldn’t explain detailed observations. It used epicycles to explain why the planets moved backwards sometimes. And no matter what people tried, they couldn’t get it to be accurate…

But then along came Copernicus, who would be the first to challenge the Ptolemaic universe and be believed.

I’m not going to go into detail about Copernicus’s life before he figured out the sun was in the center of the solar system.

What’s surprising is that he was well connected to the Church. The orphaned son of merchants, he was raised by his uncle, an important bishop, and he actually got appointed as a Church administrator at one point.

But Copernicus’s passion was astronomy, and he didn’t let his connection to the Church stop him from trying to explain the real reason the planets go round.

Copernicus’s idea was revolutionary. We don’t think of it as such today, but imagine if you were stuck in the paradigm that the heavens orbit around you, and not the other way around?

He proposed that the sun—yes, the sun, not the Earth—was the center of the universe.

This diagram shows the Copernican model. “Sol” at the center is the Latin word for “sun,” and the planets going outward are also in Latin. It shouldn’t be too hard to tell which is which—they’re in the same order we know today.

The Copernican model was correct, with a couple exceptions—Copernicus insisted on staying true to uniform circular motion, and as no one had figured out that most of the points of light in the sky aren’t actually in our solar system, there was still a sphere of “fixed stars” beyond the planets.

Uniform circular motion, as I’ve written a little about before, is the concept of all the objects in the heavens orbiting in perfect circles at uniform speeds—never speeding up, never slowing down. It was all part of the perfection of the heavens.

And it was wrong.

Copernicus may have come up with the revolutionary heliocentric (sun-centric) universe, but he would never break out of that paradigm…and as a result, his model couldn’t predict motion in the skies any better than the Ptolemaic one.

But there was one thing it did explain, simply and perfectly, and that alone made it “stick” within the astronomical community.

It explained retrograde motion.

Retrograde motion is the tendency of some planets to actually move backwards in the sky, like so:

Okay, I know what you’re thinking. How on Earth—wait, excuse me, how in the world—can planets possibly move backward?

They don’t, they just appear to.

Imagine that you’re walking in a circle. It’s that simple. You’re walking in a circle, and a friend is walking in a circle just outside of yours. Except that you’re moving a bit faster, so every once in a while, you pass your friend by.

You’ll notice, if you try this, that your friend looks like he or she is actually moving backwards for a bit.

You might also notice this on the highway. Ever been in the fast lane, and passed a car just one lane over? Ever noticed that even though it hasn’t even slowed down, it looks like it stops for an instant and then moves backwards?

This is basically what happens with the planets that orbit outside of the Earth.

What Claudius Ptolemy had tried to explain with epicycles, Copernicus explained simply and elegantly. Astronomers of the time recognized that it was the more likely explanation, since it was the simplest.

You’ll notice that the path Mars traces across the sky in this diagram is a bit different from what we see in the image above. That’s because the planets don’t all orbit in exactly the same plane.

Some orbits are tilted a bit…and the shape of the retrograde motion depends on the angle between Earth’s orbit and whatever planet we happen to be passing.

Copernicus managed to explain orbital motion without epicycles, and that alone was a huge accomplishment. But because he was trapped in the paradigm of uniform circular motion, he still couldn’t make his model work right.

In fact, he delayed publishing until he was practically at death’s door for this very reason—his model couldn’t predict the planets’ motion.

Why? Because planets don’t move in circular orbits, as Johannes Kepler would soon find out. But that’s a story for another day…

Since Copernicus couldn’t get his model to work properly, he actually abandoned its simple elegance and tried adding epicycles to get it to work.

These epicycles were super small, hardly as important as they were in the Ptolemaic model, but they were meant to explain the way planets slow down at one end of their orbit and speed up at the other end. But they could never quite get accurate enough…

And that set the stage for a certain unlikely pair of astronomers to pull classical astronomy out of the dark ages and into the modern age.