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Why is the Earth’s Orbit Around the Sun Elliptical?

-- | December 8, 2013

Question: Why is the Earth’s revolution around the sun elliptical rather than a perfect circle? I feel like if there is the same amount of inertia and gravity acting on the Earth at all times, the revolution would end up being a circle, or at least closer to a circle than what it actually is. (Plese answer in the simplest form; I’m only in middle school) — Cheri

Answer: In fact, a circular orbit is just a special case of an elliptical orbit. Elliptical orbits are stable, possessing the same amount of total energy over the orbit as circular orbits. This is due, for example, to the fact that when the Earth is closer to the Sun in its elliptical orbit it orbits faster, while when it is further away it orbits slower, averaging to a value equivalent to that of a circular orbit. Elliptical orbits are much more general and allow for a wider range of initial conditions which existed when a planet/star system forms, thus making them a more probable solution for the properties of a planet’s orbital characteristics. If you want to see a nice historical account of how Johannes Kepler discovered the existence of elliptical orbits in the motion of the planets, see the How Stuff Works video on elliptical orbits.

Jeff Mangum

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