Geocentrism: Was Galileo Wrong?

“I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.” –Galileo Galilei

While practically every scientist (and 79% of Americans) accept that the Earth and the other planets go around the Sun, it isn’t quite everyone. In fact, a number of people have recently pointed out the following conference to me.

That’s right, this November, a group of people are going to get together and try to put together as convincing an argument as possible for geocentrism, or the model that the Sun (and all the other planets) revolve around the Earth.

This is in stark contrast to our standard, heliocentric (Sun-centered) model, where the Earth is just one out of many planets, all of which orbit the Sun in elliptical orbits.

Both models allow for the same very reasonable observations, all visible with the naked eye. This includes the following:

  1. The explanation of days and nights.
  2. The explanation of Moon phases.
  3. The explanation of seasons.
  4. And finally, the observation that planets appear to “wander,” or shift their position across the night sky, relative to the fixed, background stars.

But the greatest puzzle, back in the time of Copernicus, was the problem of retrograde motion. What’s that? If you observe the planets, night-to-night, you’ll find that they move across the sky in roughly a smooth line. But if you look up at Mars during the right time of the year, you’ll find that it seems to stop in its tracks, reverse direction, go backwards for a little while, stop again, and continue forward in roughly the original direction.

This happens for all of the outer planets, by the way, and has been confirmed hundreds of times over the centuries.

Both models — heliocentric and geocentric — had a way to explain this by time Galileo came along. The geocentric explanation came first, by way of Ptolemy.

Image credit: Nick Strobel's Astronomy Notes.

Rather than making a perfect circle, each planet moved about the Earth on a “circle upon a circle.” When the planet passes by closest to Earth, it appears to reverse its motion! This worked remarkably well for predicting the positions in the sky of the planets, and was the most accurate method we had by a lot. In fact, by even the late 1500s, 50 years after Copernicus, this geocentric explanation was far-and-away the most superior way to predict what the positions of the planets would be.

But the heliocentric explanation was a little more elegant, and didn’t need these epicycles or “circles-upon-circles” at all. Instead, the inner planets, in theory, moved around the Sun more quickly than the outer ones! When an inner planet passed by an outer one, the outer one would appear to move backwards for a brief period of time, and as the inner one continued to move forwards, eventually the outer planet would resume its original course.

It wasn’t until the early 1600s, when Kepler finally figured out that planets moved in ellipses (and not circles) around the Sun, that finally allowed the heliocentric model to catch up with the geocentric one in terms of predictive power.

So which was better? Was Galileo, the most passionate of all the heliocentric proponents, justified in his adamant rejection of geocentrism?

Up until the early 1600s, I would have said “no.” But right around 1609, something remarkable happened that scientifically settled the issue.

The telescope was invented! (Note, you can repeat any or all of the following observations with even the simplest and cheapest of telescopes.)

And one of the first things that people looked at through a telescope was the planet Jupiter. In a single night, check out what you can see!

This video is a time-lapse of about 3 hours condensed into 10 seconds. (And it is taken with an amateur telescope, albeit a good one.) And you’ll notice that there are definitely moons orbiting Jupiter, not Earth!

But perhaps even more striking is what you see when you look at the planet Venus. Taken over the course of a few months, you can see the individual phases of Venus, which go all the way from crescent to full.

Both heliocentric and geocentric models will allow Venus to run the full gamut of phases. But you’ll notice something with a telescope that you’d never notice without one: Venus appears much larger when it’s at its “new” phase and much smaller at its “full” phase.

This is different from the Moon:

and from the Sun:

which vary in size only slightly as seen from Earth.

Believe it or not, this observation is a killer for geocentrism! Why?

Because in the old geocentric model, sometimes Venus is on the same side of Earth as the Sun (and appears new), and sometimes it’s on the opposite side (and appears full).

Notice how the closeness of Venus to Earth (which determines how big Venus appears) has nothing to do with what its phase should be! And yet, a crescent Venus is always huge, and a full Venus is always tiny!

But, in the heliocentric model…

Venus is huge when it’s a crescent because it’s closer to Earth, and small when it’s full because it’s on the other side of the Sun!

And while there are many, many, many other, subsequent observations that support the heliocentric model and contradict the geocentric model (such as the laws of gravity, accelerometers on spacecraft orbiting other worlds, the orbits of comets, the discovery of other stars and extra-solar planets, etc.), this is enough!

In other words, once we were able to make observations that were more sophisticated than the simple “position of the planets over time against the background stars”, the geocentric model gives wild predictions about the apparent sizes and accelerations of the planets, inconsistent with observations.

But the idea that “Galileo was wrong” is now 401 years out of date, and very, very easy to disprove. The geocentric model has yet to come up with an explanation for the apparent size of Venus in its different phases, and the scientific conclusion is that’s because it’s wrong. But perhaps someone out there knows better… any ideas?