All planets move in ellipses. A planet that moves in a perfectly circular orbit is actually an ellipse with its eccentricity (e) = 0, a parabola has e = 1 and a hyperbola the e > 1. So the closer to zero the planets eccentricity, the more circular its orbit.
For the planets, the furthest point from the sun in its orbit is called aphelion and the closest is called perihelion.
All of the planetary distances from the Sun are measured in Astronomical Units (AUs). One AU is the average distance from the Earth to the Sun, which is approximately 93,000,000 miles.
**Mercury: **e = 0.2056 and its AU = .39
**Venus: **e = 0.0068 and its AU = .72
**Earth: **e = 0.0167 and its AU = 1
**Mars: **e = 0.0934 and its AU = 1.52
**Jupiter: **e = 0.0483 and its AU = 5.20
**Saturn: **e = 0.0560 and its AU = 9.54
**Uranus: **e = 0.0461 and its AU = 19.18
**Neptune: **e = 0.0097 and its AU = 30.06
**Pluto: **e = 0.2482 and its AU = 39.44
If you notice only two planets have a high eccentricity; Mercury and Pluto. Only one of them cross the mean distance of another planet from the Sun and that is Pluto and Neptune. Briefly Pluto is closer to the Sun than Neptune when its orbit is at perihelion.
The eccentricity of our planet's orbit is mild; aphelion and perihelion differ from the mean Sun-Earth distance by less than 2%. In fact, if you drew Earth's orbit on a sheet of paper it would be difficult to distinguish from a perfect circle and that is with e = 0.0167. . As for the perfect circle, There never will be a perfect circle with the orbital elements. Remember the other planets are also "tugging" on each other. I brought up the perfect circle to show that a circle is a very special type of ellipse. The reason for that was that when we see ellipses in our mind, we see really elongated structures. Also when you look at a "map" of the solar system, they usually put it in a somewhat side perspective which exaggerates the appearance of the ellipse.
Most of the planets are so close to circles that on a piece of paper they would look just that. Again, the only two that would be even readily noticeable would be Mercury and Pluto.
For satellites orbiting the Earth, we have an added component of not only the atmospheric drag but the solar wind as well. To even further the complication our Earth is not a perfect sphere and has natural gravity wells due to the distribution of the landmasses and that it is an oblate spheroid instead of a perfect sphere (the difference is only about 15 miles between the equator and the poles). One more rub is that with long term measurements taken using a satellite in orbit (the LAGEOS), the Earth is very very slowly re-rounding itself out over time.
The other thing that is not readily apparent from most solar system maps is just how far apart the planets really are and also how tiny they are with reference to the solar system. |