History of Astronomy Part 3: Copernicus and Heliocentrism

Professor Dave again, let’s rethink the solar system. When we left off with Aristarchus, he had correctly proposed that the sun is the center of the solar system, rather than the earth. But he was way ahead of his time, and the geocentric model remained in place for centuries.

This model was perfected by Ptolemy in second century Egypt. He improved on pre-existing models by finding a way to explain elusive phenomena like the retrograde motion of the planets. This was when a planet would seem to turn around and go back the other way in the sky for a few days, before resuming normal motion.

Now we understand very well that this happens because the earth is passing the planet in its orbit, and will thus appear to move backwards. But back then, with the geocentric model, it needed some serious explaining. He proposed that planets move on epicycles, which are smaller circles that themselves move around the larger circle that is the planet’s orbit, and this model was able to predict the motion of the planets with reasonable accuracy.

But it wasn’t perfect, so it was tinkered with, making things more complicated, requiring a different formula for each planet. This went on for centuries until the 1500s, when it just no longer made sense. It was at this point that a paradigm shift in astronomy was necessary.

The first to suggest a heliocentric model since the time of Aristarchus was a Polish man named Copernicus. He demonstrated with great precision that all of the problems with the geocentric model instantly vanished if the sun is placed at the center of the solar system. Objects closer to the sun orbit faster, and the retrograde motion of a planet like Mars is now simply the result of earth overtaking Mars at a particular time in their respective orbits.

As we pass by Mars with a greater angular velocity, there is a moment where it appears to move backwards, just like when you pass a car on the highway, and it appears to you to be moving backwards, given its slower velocity. Copernicus even estimated the distances from the sun to each planet with great precision, using some ingenious geometric calculations. But there were still some issues to iron out.

Critics of the heliocentric model had one very valid point. If the earth goes around the sun, the apparent position of the stars should shift. It would be several hundred years before we had telescopes powerful enough to see it, but this shift does indeed occur.

It is just incredibly small, because of the staggering distances to the stars. We use this slight shift today to measure the distance to certain objects in space. To understand why this works, hold a finger in front of your eyes.

Then alternately open and close one eye at a time. Your finger will shift left and right depending on which eye is looking at it, because your line of sight is originating from a different location. Keep doing this as you move your finger away from your face.

The shift in location becomes less pronounced with greater distance. If you try this with an object across the room, it already starts to be relatively negligible. Now imagine that the earth being on one side of the sun is your right eye, and the earth being on the other side of the sun is your left eye.

A distant star will change its position in the sky, just like your finger, but the star is so far away that we need mechanically precise machines to measure the apparent shift. But we do have these machines in the form of modern telescopes, and because we know the distance from the earth to the sun, which is one astronomical unit, or about 150 million kilometers, if we then measure this apparent shift, we can cut it in half to get what we call the star’s parallax. Then we make a right triangle with the earth, the sun, and the star, and simply use trigonometry to find the distance to that star.

If you need to, check out my trigonometry tutorials for loads of information regarding triangles and angles. Parallax, among many other phenomena, strongly corroborates the heliocentric model, such that there has been no doubt in its validity in the past few hundred years. The Copernican revolution had a tremendous impact on human culture.

The notion that the earth goes around the sun rather than the other way around is a fairly innocent claim to a scientist. One model fits the data much better than the other, and that’s all there is to it. But the philosophical ramifications of this shift cut very deep.

If the earth goes around the sun, the earth is not the center of the universe. Other astronomers like Giordano Bruno reasoned that if the sun is the center of this system of planets, and all the countless stars in the night sky are suns just like ours, possibly with their own planets, and perhaps even living creatures, then mankind is no longer the pinnacle of creation. The Catholic Church, very much in political power at the time, saw this way of thinking as a threat to its supremacy.

As a result, Bruno was tried for heresy and burned at the stake. It seems that mankind was not quite ready to be dethroned from its status as the only reason the universe exists. Our quest for knowledge has had many detractors along the way, which should serve as a reminder of how privileged most of us are in modern times, to have free speech, to have access to all the information we could ever want.

We must value these freedoms, and defend them from tyranny at all costs. But we haven’t quite made it to modern day yet, there is more of the story left to tell, so let’s see what astronomers did with the Copernican model next.