system
Given the following post, respond as if you're commenting on Reddit:
human
Why is earth's orbit elliptical and not round like that of a revolving ball attached to a string?

I know the Kepler's law of planetary motion but still whats the reason its not perfectly circular orbit?
gpt
No real-life orbit is *perfectly* circular. Circular orbits are a special case of elliptical orbits where the two foci of the orbit coincide. 

There are many ways to make an unstable orbit, and there are many ways to make a stable but elliptical orbit. There's only one way to make a perfectly circular orbit, and that's by setting up the initial conditions such that the orbit velocity always balances the force due to gravity (or by adjusting an elliptical orbit to be circular, which is just as hard). This means you need to start your orbit at *exactly* the right speed -- if you're a single meter per second away from that number you'll end up in a stable but elliptical orbit.