A Crash Course On Planetary Orbits

As you may or may not know, the Supermoon just happened this week, for the first time since 1948.

The Supermoon occurs where the moon is the closest to earth in it's orbit, and so, I decided that this week's post could be about (as you could probably guess) orbits.

Here we go!

The earth's orbit around the sun is not circular

It's not just our orbit that isn't circular, it's almost all orbits. Some orbits do come quite close to being circular, but no orbit is absolutely, perfectly circular (the closest I could find was Asteroid 113474's orbit around the sun). Sure, a circular orbit isn't impossible, but it is extremely improbable.

This is the premise of Kepler's first law.

Mercury has a weird orbit

Newton, one of the most famous scientists of all time, wrote up some pretty important laws about gravity, that worked in most cases, but not one.

Mercury's orbit.

This was a pretty huge deal. You can't have a law of the universe only apply to some things and not others, that's useless.

So, Einstein decided to revise the theory of gravity, and remade it to adhere to all observations in the universe. This was his very famous "theory of general relativity" (I actually wrote an article about this). Clearly, planetary orbits are pretty important, even beyond the more obvious reasons.

The rule of equal areas in equal times

This one is actually quite impressive, much more so than the previous two.

Here's the rule:

No matter where in the orbit the planet is, in equal amounts of time, the area of the sector the planet draws is exactly the same. This is because when the planet is closer to the sun it travels faster, when it's further away it's slower. Because of all those variables, this results in the areas of the sectors being the same. If you're having trouble visualising this, look at this.

The Harmonic Law

This one is quite incredible (hence the pretty beautiful name).

This one, just like the law before, was discovered by Kepler, also known as Kepler's Third Law.

Here's what it says:

p = The period of an orbit (how long it takes)

a = the average distance of the planet from the sun

P squared / A cubed is the same for all bodies orbiting around the same body.

For example, P squared / A cubed is the same for Earth as it is for Jupiter.

The full equation can be found here, have a go at deriving it if you're more mathematically inclined (although it will require some knowledge of Newton's laws of motion).

And that's all!

http://renpkepler.weebly.com/the-law-of-areas.html

https://www.boundless.com/physics/textbooks/boundless-physics-textbook/uniform-circular-motion-and-gravitation-5/kepler-s-laws-56/kepler-s-third-law-267-11197/

http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html

http://www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/astronomy-general/orbit

https://commons.wikimedia.org/wiki/File:Artist%E2%80%99s_impression_of_the_planet_around_Alpha_Centauri_B.jpg

And Mayako for inspiring this post