Why elliptical ?

Because Kepler's second law says so?

Nobody's perfect.

What would the odds be that a planet would happen to be in a perfect circlular orbit given all the other possibilities?

Add to that the interaction of other neighbors and odds get much worse. Statistics.

Add to that strikes by large asteroids and the tug of moons etc. and chaos follows.

Brad

This is what Newton's theory of gravitation is all about. He proved that if his theory that all bodies attract other bodies with a force proportional to the product of their masses and inversely as the square of the distances between them; then in general their relative motion would be along conics. The circle is of course a special critical case of a conic. But in general trajectories are likely to be ellipses or hyperbolas. The parabola is also a very special case. When an apple falls from a tree it falls in a straight line, which is really an ellipse with eccentricity one. Perfectly circular orbits could hardly happen naturally. Engineers have a very tough job keeping geostationary satellites in exact circular orbits above the equator.

There is only one velocity that is giving a circular orbit if you have a known distance between two objects.

Any other velocity below the escape velocity is resulting in an elliptical orbit.

A velocity equal to the escape velocity will give a parabolic trajectory,

any higher velocity a hyperbolic one.

The greater mystery is the nearly circular shape of orbits of massive objects around stars.

RHABE

The bigger question is, why are they so nearly circular?

The Discovery Channel just had discussion about this today, where our Earth is presently in a very close circular orbit around the sun, but alternately will migrate back to an elliptical orbit to max out some millions of years from now. Not something we should be looking forward to.

The Earth's orbit is defined by five facts:

• Earth's Rotation: The Earth rotates around its axis, taking 23 hours, 56 minutes, and 4.091 seconds to line up relative to the stars (Sidereal day), and 24 hours plus or minus 20 seconds to line up relative to the sun (Solar day). The solar day varies because the Earth is closer to the sun at some times of the year than others, which affects its rotation;
• Moon's orbit: The moon revolves around the Earth approximately every month. As the moon passes over a section of the Earth, its gravity raises the water in the seas, causing the tides.
• Earth's Revolution: The Earth revolves around the sun approximately 365.24 days, or one year. The Earth revolves in an elliptical path, so it is closer to the sun at some times of the year than others; the Earth moves faster when it is closer to the sun, reflecting Kepler's laws of planetary motion;
• Axial tilt: The Earth is tilted on its axis approximately 23.5 degrees relative to its orbit around the sun. Because of this tilt and the Earth's orbit around the sun, the Earth's northern hemisphere is tilted toward the sun from March 21 to September 21, and the southern hemisphere is tilted toward the sun from September 21 to March 21. This is what causes the seasons.
• Precession: It takes 365.2421 days for the Earth to revolve far enough to line up with the sun(the tropical year), and 365.2425 days for the Earth to revolve far enough to line up with the background stars (the sidereal year). This phenomenon, called equinox precession, causes the sun to appear to us to move a little bit relative to the stars every year in a 24,000 year cycle called the Great Year, which was the basis of the Zodiac.

Cause God Said So. Genesis 1:1

Cause God Said So. Genesis 1:1

Now there is the definitive answer. Goes back to troubleshooting 101: first read the manufacturer's instruction manual.

A circle is just a special case of an ellipse that has the major and minor axes equal. The fact that the orbit is elliptical can be derived from the principle of conservation of angular momentum.