Expert: Steve Nelson - 3/10/2009

Question
sir..why mostly all the plannets in solar system revolve eleptically around the sun but not circularly...but as per the laws of force of attraction if centripetal and centrifugal is equal then that body will remain constant if it so the earth will be constant only when it revolves in circular path otherwise there would be variation in gravitational force between earth and sun...but in real the earth is constant...why..?

Answer
To move in a perfectly circular orbit a planet would have to form with exactly the right velocity for the distance that it formed from the Sun.  If it forms with any other velocity, or gains mass (space dust) or loses mass (some molecules in the atmosphere escaping into space) while in a circular orbit then the orbit will become elliptical.  

Most orbits are elliptical.  Orbital movement is very much like a marble in a bowl.  You can try it yourself with a marble and a bowl, when you roll a marble around in a heavy bowl there's almost always at least some elliptical nature to its path.

The Earth's orbit is nearly circular, but it isn't perfect.  There is a variation in the gravitational force between the Earth and the Sun as the distance varies, it's just small.  Wikipedia has an excellent article with good science on this subject (people often underestimate the quality of the science in wikipedia science articles) http://en.wikipedia.org/wiki/Orbital_eccentricity