## Planetary orbital foci

Planetary orbits are not perfectly circular; in fact, they are ellipses. An ellipse is a mathematical shape approximately equivalent to what is typically called an oval. An ellipse, though, meets some very specific criteria. One is that, unlike a circle, it has two foci instead of a single center. Where a circle is defined as the set of points whose distance to the center are some constant distance (the radius), an ellipse is the set of points whose distances to the two foci add to a constant. This allows you to construct an ellipse with some pins, some string, and a pencil.

So what are the two foci of planetary orbits? Well, one is the Sun. The other one? Just some random spot in space. And because each planet has a different size orbit with a different eccentricity (a measurement of how non-circular the orbit is), each planet has a non-Sun focus in a different place. Here is a Google doc spreadsheet with information on each planet’s orbit. A visualization after the jump.

Venus and Earth have very round orbits. Mercury’s is surprisingly un-round – nearly as eccentric as (although much smaller than) Pluto’s.