And how exactly did we find out how they move?
In: Planetary Science
Because the solution for the possible paths in the 1/r potential motion are the conic sections of which the closed orbits are ellipses and the circle is just a special case of an ellipse.
A good way of thinking about it is to imagine a spaceship on a circular orbit that begins to accelerate, once it reaches a high enough velocity it’ll leave the closed orbit and enter a parabolic or hyperbolic orbit. This transition (as long as you continuously increase your velocity) is smooth so you have to go from a circle to something like a parabola. So the circular orbit gets more and more elongated (its eccentricity increases) until the kinetic energy of the spaceship is great enough at which point the ellipse “snaps” and the spaceship enters an outbound orbit never to return.
How have we figured this out? First a guy called Brahe recorded lot’s of data on where the planets were at any given time. Then Kepler worked some 30 years to organise that data and essentially fitted their trajectories and that’s where the laws of planetary motion come from. Then Newton came up with gravity as a force from which Kepler’s laws can be derived.
You’ll get a lot of explanations here, but if you’re still confused, I would recommend getting Spaceflight simulator (mobile) or Kerbal Space Program (pc/console). You can also look up the KSP2 tutorials on YouTube. They do a really good job of explaining orbital mechanics
We figured out they were ellipses through a lot of trial and error. We used to assume they were circular but whenever we calculated their positions based on them being circles we kept getting the wrong answer; the planets weren’t where we said they’d be based on the math.
Then a guy named Johannes Kepler decided to model them as ellipses seeing as how the previous thing wasnt working out and voila, the paths of the planets matched our observations.
Okay, here’s an actual ELI5 answer – when the planet formed, all of the pieces had their own speeds and directions, and when they all added up, that came out to the spin of the planet and the speed and direction it was going around the sun. That direction wasn’t perfectly aligned with the sun’s location – the sun is actually 3,000,000 miles closer to us in the winter! And so our orbit gets a little too close and a little too far. It is getting rounder, and will someday be a total perfect circle. One side of the planet will face the sun at all times, and the moon will stay over exactly one spot over the earth in the same way. This is called being *tidally locked* and is a fancy way of saying that even if you get bumped a little by the other planets, you still fall back into those shapes. When that happens, it will have finally wound down into a circle.
Orbits are elliptical rather than perfectly circular because circles are just a type of ellipse, but nature is just too messy to create a perfect circle.
So you could create a circular orbit, but it wouldn’t remain circular for long because of interactions with other objects, or even the orbiting object’s own behaviour.
Why are they elliptical in the first place? It’s a natural consequence of the laws of physics and gravity being a force attracting everything towards a centre of mass. They just have to be ellipses. Feynman explains it in his Lost Lecture, but there’s nothing ELI5 about it.
And the fact that they are was observed by Tycho Brahe and Johannes Kepler, through some 30 years of meticulous observations and note taking. Kepler then formulated three laws of planetary motion.
Latest Answers