Why do planets travel in elliptical orbits?

Question

Circular orbits seem more likely to me.

Answer 1

Actually, they don't ... quite. However, there are mathematical reasons why they *nearly* travel in elliptical orbits, relating to the fact that Newtonian physics are a fairly good approximation within the solar system.

First off, planets orbit the sun because they are close enough to it and traveling slowly enough that the force of gravitation bends their trajectories into orbits. However, that does not mean that at any given moment the planet is traveling fast enough (or slow enough) to stay at a constant distance. Mathematically, there is only one speed where they run at a constant radius - the one where the acceleration from the gravitational pull exactly equals the 'centrifugal acceleration'.

So, what happens when they travel (say) slightly faster that that?

Well, first off, their trajectory will still be bent by the force of gravity. However, it won't be bent enough to keep them in the circle around the sun. Thus, the planet will start traveling away from the sun.

When this happens, though, the sun is still pulling straight in. So, while part of the pull goes to making the planet's direction of travel change, another part goes to *slowing it down*. And, eventually, it'll slow down enough that the sun's gravity can make the planet turn in a *tighter* circle than it's current distance out - and it will stop going out and start coming back in.

And, coming back in, it will accelerate. Which will make it start turning less, relative to its radius. Which will make it go out again.

Now, as for why the orbits are (nearly) elliptical, specifically - well, for that, you need to do some fancy math. But the above is why orbits need not be circular.

Answer 2

A perfect circular orbit is so unlikely to me. That would only happen if there was like one and only one sun and one and only one planet orbiting it in the entire universe.

Even then the gravitational pull of the sun would make the once near perfect circular orbit decay, and over time the orbit is really a slow death spiral for the planet.

So I would think that elliptical orbits are a planetary system's gravitational "balancing act".

(btw Kepler spent his life studying planet orbits and at one point found that the orbits were elliptical - (which is in fact true) - but he discarded this result because the whole tradition of astronomy had come to place so much value on the 'perfection' of the circle)

Answer 3

Gravity doesn't play favorites.

The planets are not orbiting the sun so much as barely not falling in. They are attracted by the gravity of the sun and fall toward it. The speed that they are moving at is enough that they can scrape by and try to escape the sun, but the sun won't let them go so they fall back into the sun again.

That's physics for you; and to make it more complex they don't travel at the same speed. As the planets approach the sun they go faster, as they leave it they slow down. When they are at the furthest point in their orbit they are moving at their slowest speed. The acceleration and velocity formulas that you learn in high school are only good for a certain point in time and space. To figure out the acceleration over the entire orbit you need to use calculus. That's why Isaac Newton had to invent calculus to describe physics.

The ancient Greeks thought the planet and stars were hung inside of perfect crystal spheres, but the observational data didn’t agree. In fact it drove the Church and astronomers nuts trying to figure it out. Finally it was shown that if you let the planets, including the earth, orbit the sun the observational data finally fit. This took out the unexplained complex loops and whirls that just about drove Copernicus crazy. It also is what proved that the Bible is not the word of God, but the word of MAN INSPIRED by God, which was a huge change for society.

Answer 4

Velocity is slightly different at different parts of the orbit cycle. The initial capture velocity (or spin off if you think the sun ejected all the planets) and the pull of other planets may cause the velocity variations on the ellipse..

Maintaining conservation of momentum amongst all the planets (with various orbit and submoon cycles) may also have some effect on the velocities of orbit cycles:

http://www.a2dvoices.com/realitycheck/Coriolis/

Answer 5

It's because they were travelling when they were captured by a vastly superior gravitational force, they get away as far as they can but the gravity finally slows them up and they turn and come back around. The gravity pulls them faster and faster and they "sling shot" past the source of the gravitational force and speed away in the opposite direction and then the same thing happens. If they are far enough away not to get pulled into the source of the force then they go into an ellipse.

Answer 6

Because there is a central force acting upon them.

Answer 7

Just Uranus. I'm not being silly its actually true