A funny thing called an ellipse...

An explanation of the first law of planetary motion.

As you can probably recall, my previous entry was a brief intoduction to the Laws of Planetary Motion, three laws that I made known to the world after years of laborious work. That aside, I intend to go more in depth about each law, beginning with the first one: the law of ellipses.


I have always been a supporter of the Copernican Theory. That is, I believe in the heliocentric model of the universe; heliocentric meaning that the planets, including the Earth, revolve around the Sun. I strongly disagree with the once widely accepted model that Ptomely developed, which cemented the geocentric theory for several centuries. Geocentric means that the universe revolves around the Earth, as opposed to the Sun. Copernicus's theory, however, was no better at predicting the positions of the planets in the sky than the Ptolemy. I knew that something had to be missing, and so my research began.
Using Tycho Brahe's observations of Mars (you will be hearing much more about Tycho, I can assure you), I noticed that the orbit of Mars did not fit Copernicus's model, in which the planets orbited in perfect circles. I did not want to believe that my discoveries were true at first, because they would contradict the ideas put forward by both Copernicus and Aristotle. However, finally I was forced to realize the truth. While Copernicus was correct in placing the sun in the center of the universe, the planets did not orbit in a circular motion.
They orbited in an elliptical motion.

This picture is fairly clear in demonstrating the elliptical movement of the planets. Notice how the shape is like a flattened circle, this is the elliptical orbit.

Now, the properties of an ellipse are very complicated, and I will try to keep this as simple as possible. There are two points for an ellipse called foci (singular: focus), and the sum of the distances to the foci from any point on the ellipse is a constant. You might be asking yourself, how does this relate to the position of the Sun? Well, the Sun is at one focus of the ellipse.

See? It's really pretty straight-forward. The distance between the planet and the Sun is constantly changing as the planet orbits around the Sun.

We're not done quite yet, so hold tight. I'd like to introduce an important term, the term being eccentricity. As you can see below, as eccentricity increases, the ellipse appears to flatten. So, if the eccentricity of an ellipse was equal to 0, then it would be a circle. This is what threw many people off, including Copernicus; The orbit of most planets have such a small eccentricity that they cannot be determined by first glance. Careful measurements must be taken in order to prove that the orbits are, in fact, elliptical.

Lastly, an ellipse has two axes called the minor axis and the major axis. Half of the major axis is called the semimajor axis, which is equal to the distance of a planet from the Sun as it orbits.

There you have it, a fairly simple explanation about ellipses. Keep a look out for my next blog, which will be about my second law of planetary motion.