Expert: Jayendra Upadhye - 9/9/2004

Question
Exactly why do planets rotate around the sun in ellipses and not in circles.And why do they rotate on their own axes?

Answer
Hi Mark,
The planets go round the sun in elliptical orbits not due to any particular reason, but simply because the ellipse happens to be a class of orbits.
Circular orbits also are a particular class of the more general "ellipse".
The planets rotate around their axes because of the way they are formed.
Imagine a doughnut shaped cloud of debris orbiting a star.
Over time, gravitation collapses the doughnut and makes it slimmer.
There is a differential of speed between the inner regions of the doughnut and the outer peripheries.
As the planet coalesces out of this doughnut by accumulating material towards one random point on the doughnut that predominates gravitationally, these differing speeds of the inner and outer regions, compel the planet to "turn" on that axis.
Obviously, this axis happens to be perpendicular to the ecliptic.
This is also the reason why most planets tend  to turn in the same fashion.
Also that is the reason why planets in the same region (earth + mars) have days of similar length ..24 hours, and 10 hours for jupiter / saturn.
Hope that satisfies you.
Jayen