Wednesday, February 18, 2015

A History of our Solar System

The universe.

It is pretty amazing when you think about it.

Either that, or you get a major headache.

Personally, I am in awe when I think about space or anything related to it. Space goes on forever and ever. The concept of space, in its entirety, is impossible for the human mind to comprehend. I guess that is what makes it so interesting.

Have you ever thought about how much you actually know about our solar system? If you are reading this, you probably know a lot more than you think. Here are some examples of things you might know about our solar system:
  •  The planets in our solar system revolve around the sun.
  • Each planet has a special orbit.
  • The moon revolves around the earth.
  • Earth is tilted on its axis.
  • When the moon’s orbit overlaps with the sun, here on earth we see an eclipse.
  • The stars that are not a part of our solar system are all billions of miles away.


How do you know all of this? You probably know most of it, if not all of it, because you were taught in school. You had some sort of Earth Science class that taught you more about astronomy than you probably cared to know. Have you ever thought that maybe you are taking this wealth of knowledge for granted?

What? You say. What is this girl talking about?

Would you like to know? I will tell you, but first ask yourself: If you did not have science classes that taught you about astronomy, would you know anything about it? And if not, would you try to understand what was happening beyond your own town, much less in space?

(In order to explain a detail of some specific models of the solar system, I am going to give you some background regarding Aristotle’s physics.)

If you lived in Aristotle’s time, chances are you would be like the majority of the population in 360 BC. As you can probably guess, most people at that time were not particularly interested in astronomy.

Aristotle was a Greek philosopher and scientist who basically came up with the first recorded notion of physics. Aristotle believed that there were two regions of applicability: the celestial sphere and the terrestrial sphere. The celestial sphere was reserved to all things belonging to heaven, and the terrestrial sphere was for things that were less than perfect, like the earth. In Aristotle’s physics, there were five elements. Earth had four of them: air, earth, fire, and water. And one was in the celestial sphere: quintessence.

In Aristotle’s physics, everything had a natural state. In the celestial sphere, the natural state of things were perfect circles. In the terrestrial sphere, everything was naturally at rest, and everything attracts what it was most similar to, or made out of.

For example, you are a human, and you are always on the ground, close to the earth. Therefore, according to Aristotle’s physics, you are mostly made of earth.

Makes sense, right? Well, you might be wondering why Aristotle’s physics were this way. And if you are wondering that, then I personally applaud you for questioning the theory that stood unshaken for almost two thousand years. Aristotle’s physics was a guide for scientists for nearly two thousand years before it was disproven by Galileo Galilei, but that is another topic entirely.

Now I will begin with the history of solar system models.

There were some small models of the solar system that we know were proposed after Aristotle’s time. However, none of them made an impact quite like Ptolemy’s model did. In about 100 AD in Rome, Claudius Ptolemy presented his geocentric model of the universe.

In this model, the earth was considered the center of the entire universe. Each planet moved on a small sphere or circle, called an epicycle, that moved on a larger sphere or circle, called a deferent. There were speculations in the scientific community, of course, but they were quickly hushed with the added complexities. Earth was considered to have an eccentricity from the center of the deferent, which explained many of the patterns they saw in their observations of the night sky.

About thirteen thousand years later, in the year 1543, Nicolas Copernicus’ heliocentric model seriously challenged Ptolemy’s long-standing model. After studying Ptolemy’s model of the solar system, Copernicus found some serious flaws. The Ptolemaic model could not precisely predict the motion of the planets, and it kept getting worse over time. It also used huge epicycles to explain the retrograde motion of the planets. However, when the sun is placed in the center of the model, it explains the retrograde motion more effectively.

In Copernicus’ model, the sun was at the center. It is important to understand that at that time his idea was considered extremely radical. It was widely accepted that the earth was the center of the universe, and astronomers were extremely hesitant to consider other models. Copernicus did not publish his essay with his heliocentric model until the year of his death.

Up to this point all models had used perfect circles to describe the orbits of the planets, mostly because of Aristotle’s physics. Heavenly things were naturally formed in perfect circles, which made sense up to this point in history. However, our next scientist found something most interesting in his new model of our solar system.

In about 1600, Johannes Kepler had been studying Copernicus’ heliocentric model, when he found some interesting discoveries. Mars and Venus were not following the pattern that they should have been following, according to the Copernican model. After a lot of studying and record keeping, he came up with his own model.

In the Kepler solar system model, the orbits of the planets are not perfect circles, they are ellipses. The sun is not in the exact center, either. The sun is at one foci of the ellipse. The planets also do not move at the same speed around the sun. They move faster when they are closer to the sun, and slower when they are farther away.

This model of our solar system was fairly accurate, and the larger framework basically remains unshaken today. Look how far we have come as a society! From the mystery of how nature works, to the grand scale of our solar system, we continue this pattern of scientific progress every day.

Now you know a little more about the history of our solar system, and hopefully you realize how fortunate you are to know what you do about our place in the cosmos.