There was a recent news item about Star Trek's Kate Mulgrew in a geocentric movie. The geocentric model states that the Sun and the planets move around the Earth instead of the heliocentric model with the Sun in the center. That's just silly, right? Obviously the Earth orbits the Sun.

Sure, the textbooks all say that the solar system is heliocentric. But how do we know that? More importantly, how can YOU tell which is the better model?

A Case for the Geocentric Model ——————————-

The geocentric model is no more crazy than saying that a tennis ball is made of protons, neutrons and electrons. Sure, we all (most of us) believe there are these particles like electrons - but how do normal humans know this? In fact, the evidence in our everyday lives doesn't make it obvious that there are protons and electrons (yes, you could argue the mere fact of things like computers says these have to exist). The same is true for the heliocentric model.

Find a human that has never looked at a science book and doesn't know anything about the solar system. Now ask this human if the Earth moves around the Sun or the Sun moves around the Earth. I would bet most of these isolated humans would pick the geocentric model. It just doesn't feel like the Earth is moving.

There is another very convincing argument for the geocentric model - stellar parallax (or lack of). What is parallax? Parallax is the apparent change in position of objects due to a change in observation location. You might have noticed this with the iOS background motion. You can also easily see an example of parallax by holding your thumb out in front of your face. Using just your left eye look at where the thumb lines up with some object in the distant background. Now look with just your right eye - the thumb moved, right? That's parallax since your two eyes are at different locations.

The ancient Greeks claimed that if the Earth is moving around the Sun then the stars should shift their positions due to this orbital motion (called stellar parallax). Guess what? The stars don't shift. Well, they don't shift enough for you to notice, but they do indeed shift. This is essentially the same reason the moon appears to follow you around when you drive - it's too far away for any apparent shift due to your motion.

Historical Development of the Heliocentric Model ————————————————

For me, the sequence of events that brought most humans to a heliocentric model is just a great story. It's a great example of the scientific process. Humans make a model and then change the model after collecting more data. This brings in many useful concepts that are covered in an introductory astronomy course.

The first big problem with the geocentric model was the retrograde motion of planets like Mars. If you looked at the location of Mars each night, it might sometimes do this.

Image: NASA/JPL-Caltech

How does the geocentric model deal with this new evidence? Here are some of the highlights.

* Ptolemy develops a geocentric model that has the planets moving around the Earth. However, in order to account for retrograde motion, he put the planets on circles that move in circles.

* Copernicus suggests a heliocentric model. His model has the planets moving around the Sun in circular orbits. This can explain retrograde motion, but his model doesn't fit all the planetary position data that well. Really, it's no better than Ptolemy's geocentric model.

* Kepler proposes that the planets do not orbit in circles. Instead, they have elliptical orbits. This agrees with the observational data very well.

*Galileo gets a telescope and looks at the sky. He see stuff that suggests the Earth orbits the Sun. I'll discuss these in a bit.

* Newton develops a model for gravity that also says planets would have elliptical orbits.

This change from geocentric to heliocentric took a long time. It's silly to think that people just woke up one day and said "Ah ha! Let's put the Sun in the center!"

How Can You Tell the Earth Orbits the Sun? ——————————————

No one likes to just trust the textbook. You don't have to. Here are some things you can do to determine for yourself what orbits what.

Phases of Venus. The next time Venus is visible in the sky, take a look at it with some binoculars. It will probably look something like this.

Image: Rhett Allain

That's not the best picture, but the only one I could find. The brighter object is Venus. If the image had a better resolution, you would be able to see that Venus shows the same kind of phase that we see with the moon. What does this mean? It means two things. First, We can see Venus because it reflects light from the Sun. Second, as the phases change, Venus is sometimes closer to us that the Sun and sometimes farther away. You would see a "full phase" Venus when it is on the other side of the Sun. How can both Venus and the Sun orbit the Earth but also have Venus move farther away? Oh, this is something that Galileo saw with his telescope.

Moons of Jupiter. This is something else that Galileo did that you can repeat: see the moons of Jupiter. Again, you just need binoculars. Look at Jupiter it will look something like this:

Image: NASA/JPL/Malin Space Science Systems

Well, it won't look quite like that. You will probably just see Jupiter as a dot without any details. But you WILL be able to see the 4 big moons of Jupiter. So what? The idea that all the planets (and the Sun) orbit the Earth isn't as strong once you show that there are objects that orbit another planet. These moons of Jupiter clearly orbit Jupiter and not the Earth.

Size and Angular Size of the Sun. Hold your thumb close to your eye (but not so close that you can't focus on it). Now hold your thumb out at arm's length. It looks smaller right? Of course it's still your same old thumb, but the angular size of an object changes as it moves farther away. In general, there are three things to consider: the length of the object (L), the distance from the observer to the object (r), and the angular size of the object (θ). These three things are related in the following manner.

Now, what about the Sun? If you look at it (which you should never do or you will hurt your eyes) you would see it has an angular size of 0.5°. With this angular size, you can determine the size of the Sun for different distances. If the sun were the same size as the Earth, it would have to be 1.46 x 109 meters away. If it were the same distance that Venus is at it's closest approach, the Sun would be 26 times the width of Earth. Of course, the Sun is actually much farther than that as well as much bigger.

In fact, the Greeks even tried to measure the distance to the Sun based on their measurement of the Earth and moon (here are some other cool things the Greeks did). Aristarchus found a Sun distance of 40 times that of the distance to the Moon. This would make the Sun also 40 times the size of the moon. His calculation was way off, but he still suggested that the Earth orbits the Sun because the Sun was so HUGE.

The View From Mars. Here is the sunset as seen by the Mars Pathfinder.

Image: NASA/JPL/University of Arizona

First, you might notice that the angular size of the Sun is smaller when viewed from Mars than when viewed from the Earth. This means that the Mars-Sun distance is greater than the Earth-Sun distance. Also, if you looked at the Sun throughout the orbit of Mars, it would mostly be the same angular size.

Ok, I get it. You think this is silly because there is no way to get a lander on Mars without a heliocentric solar system model. Yes, that's an excellent point. But still, this IS evidence of a heliocentric model that you can figure out for yourself.