14-billion-years-later: Elliptical Orbits
Just about every popular depiction I’ve ever seen of the solar system has the planets either in a straight line or in circular paths. In fact one humorous (but not satirical) book I own entitled LIFE HOW DID IT GET HERE? By evolution or by creation? goes so far as to compare the orbits of planets to the “orbit” of electrons around around a nucleus (the Bohr model no less) as evidence of God’s existence and apparent intelligence. In truth the planets do not follow these nice circular orbits instead they’re drawn out into ellipses. While on Earth this isn’t noticeable (our orbit only has an eccentricity of about 0.02) with comets and protoplanets such as Pluto this is hugely noticeable.
The idea of an elliptical orbit was first put forward by Johannes Kepler and was a major shock to astronomy at the time as it was believed that circular orbits were “perfect” and infallible. Within the orbit itself the larger of the bodies is situated on one of the ellipse’s foci. The other foci is empty space, but is the center of gravity determined by other objects such as planets or even distant galaxies, in fact by everything in the universe. The elliptical orbit also has the interesting affect of causing the orbiting body to change speed depending on where it is in its orbit. This means that when the planet or comet is closest to the sun (a point known as the perihelion) it is traveling faster than it is at the point farthest away (the aphelion).
