| MadSci Network: Astronomy |
Elliptical orbits are a natural result of gravity. Left to itself, an elliptical orbit is also very stable. Yes, planets do travel at different speeds at different parts of their orbits. A planet moves more quickly when it is closer to the Sun.
The change in speed is due to angular momentum. This is just momentum that is associated with rotation. It is determined by how far the object in question is from the point it is rotating around, and by how quickly it is rotating. Like energy, angular momentum must be conserved. Conservation of angular momentum is why, for example, an ice skater spins more rapidly when she pulls her arms in. As her arms come closer to her body, she must spin more quickly in order to have the same angular momentum. The same is true for planets orbiting around the Sun.
Think of it as a competition. As a planet moves further from the Sun, it slows down in its orbit. Because it is orbiting more slowly, it begins to "fall" towards the Sun, because of the Sun's gravity. As the planet comes closer to the Sun, it speeds up in its orbit, in order to conserve angular momentum. As it speeds up, it can pull away from the Sun. As it moves further away, it begins to slow down, and the process repeats. The result of this constant back-and-forth is an elliptical orbit.
The elliptical orbits of the planets will change slowly. This is due the gravitational influences of the other planets. Because these are much weaker than the gravity we feel from the Sun, the changes in planetary orbits are very slow.