| MadSci Network: Astronomy |
Alex,
We know that the orbits of the planets are elliptical (egg shaped), not circular. A parameter referred to as the eccentricity is used to define the deviation of a planet's orbit from circular. The values for these eccentricities range from 0.007 (Venus) to 0.056 (Saturn) to 0.206 (Mercury) to 0.248 (Pluto). These numbers reveal that Pluto has the least circular orbit of the planets. Additionally, the orbital inclination of the planets, relative to the plane defined by the Earth's orbit, range from 0 (Earth) to 3.39 (Venus) to 7 (Mercury) to 17.15 (Pluto).
The combination of these factors, the elongated orbit that is out of the "orbital plane" defined by the other 8 planets, yields the effect that for about 20 years during each orbit of Pluto (248 years), Pluto is closer to the sun than is Neptune. This has been true since 1979 and will remain so until 1999.
If you use your web browser to connect with www.jpl.nasa.gov you will find more detailed information in the "Welcome to the Planets" area and in the "Basics of Spaceflight" area.
John Haberman, Space Scientist NASA/Goddard Space Flight Center Greenbelt, MD john.haberman@gsfc.nasa.gov
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