Why do the planets move in elliptical orbits ? Why can't they move in circular orbits ?

Asked by Siddharth Awasthi | 17th Oct, 2010, 12:00: AM

Expert Answer:

Dear student
Planets move along elliptical orbits because planets move in a gravitational force field which falls off as one over the square of the distance between the sun and the planet ( Force = GMm/r^2 from Newton's Law of Gravity). The planet is also subject to the conservation of momentum and energy, and the only 'solution' to this combination of requirements is an elliptical orbit. Intuitively, if a planet were in a circular orbit, but the orientation of this orbit were perturbed very slightly, because the strength of the gravitational force depends on separation, there will be some parts of the new orbit where the planet would feel a slightly stronger gravitational pull than at other parts. The speed of the planet would speed up slightly, and so the instantaneous shape of the new orbit would change. In a circular orbit, a planet would have the same orbital speed everywhere. This is not possible in a gravitational field unless, like a pencil balanced on its edge, the planet were orbiting at exactly the right distance from the Sun so that the centrifugal force caused by its speed was exactly balanced by the Sun's gravitational force.
we hope this clarifies your doubt.

Answered by  | 18th Oct, 2010, 12:14: PM

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