Last but not least...

Hello readers. I have already written blog entires about the first two laws of planetary motion, and now the wait is over. I will be sharing with you, today, the third and final law of planetary motion. Let's not hold the suspense any further and dive right into it, shall we?

The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

There you have it, the third law. I know what you must be thinking: What? Actually, it's not that complicated. Especially after that long explanation about ellipses, I'm sure you guys can handle anything. What is fascinating about this law is that it doesn't just apply to the planets in our solar system, but also their moons, dwarf planets and asteroids, satellites going round the Earth, and more. It doesn't work, however, when the mass of the secondary body is a significant fraction of the primary body. If this is the case, then the law has to be tweaked a bit. But for most things in our solar system, like the planets orbiting the sun, this 100% works.

What the third law says is that the amount of time that a planet takes to revolve around the sun is directly related to the semi-major axis. Forget what the semi-major axis is? It's the distance of a planet from the Sun as it orbits. Therefore, the amount of time that a planet takes to make a full orbit is directly proportional to how far it is from the sun. For example, it takes Mercury, the closet planet to the sun, about 88 days to make one full orbit, while it takes Earth, the third closest planet to the sun, 365 days.

And that, my friends, concludes my three laws of planetary motion. I hope you enjoyed my explanations, and look into it even furthur!