If this ellipse described an orbit of a planet, then the Sun would be located
at one of the *foci* and the path of the planet would be along the
ellipse. the *eccentricity* of the ellipse is given by:

*e* = **c/a** .

As shown in the table below, the orbits of the planets in our solar
system are nearly circular, with *eccentricities* that are very small.
Within the inner solar system, Mercury has the highest orbital eccentricity,
0.2. In fact, only Pluto, the outermost known planet, has a higher
*eccentricity*. All other orbital *eccentricities* within
our solar system are less that 0.1, with most having *eccentricities*
of only a few percent. This means that the orbits are nearly circular, with
the two *foci* of their orbits being only very slightly separated.
In fact, a circle is a degenerate forn of an ellipse, with

** a = b = r **

and an *eccentricity* of 0. [For comparison,
the *eccentricity* of the ellipse shown above is 0.79.]

Planet | Orbit Radius | Eccentricity |
---|---|---|

(km) | ||

Mercury | 5.79 × 10^{7} | 0.206 |

Venus | 1.08 × 10^{8} | 0.007 |

Earth | 1.50 × 10^{8} | 0.017 |

Mars | 2.28 × 10^{8} | 0.093 |

kstrom@hanksville.phast.umass.edu