Eerst nog wat informatie over excentriciteit in het algemeen:
http://www.mijnwoordenboek.nl/encyclopedie...28astronomie%29
Om op je vraag terug te komen:
hier vind je een Engelstalige site waar alles wordt uitgelegd.
It has always been a puzzle to astronomers why most of the planets and moons within the solar system have nearly circular orbits (low eccentricity). If they were objects captured from outer space by the suns gravitational force, or if they were formed from material ejected from the surface of the sun, the most likely orbit would be a highly elliptical one such as seen with most comets. Conventional wisdom has it that these orbits would remain essentially unchanged for the objects lifetime, except for perturbations caused by close encounters with other objects. Yet the orbits of most planets and moons within the solar system have quite low eccentricity (nearly circular). What causes low eccentricity when high eccentricity is to be expected?
Enter gravity drag. This is the mechanism by which high-eccentricity orbits could gradually (over billions of years) lose their eccentricitythat is, to change their orbits from very elliptical to nearly circular, with the sun at the center.
In a highly elliptical orbit, an object passes quite close to the suns surface during perihelion. At this time in the orbit two of the factors which influence gravity dragvelocity and gravitational forceare at their greatest. As a result, the planet would experience the maximum gravity drag effect. It would tend to slow slightly as it passed near the sun. As it returned to outer space, its orbit would be slightly less eccentric, causing it to reach a little less distance on the return to aphelion (the furthest point of its orbit from the center of gravity). With each orbit, the planets orbital path would become just a tiny bit more circular. Over millions or billions of years the orbit would eventually reach its equilibrium with gravity draga perfectly circular orbit (less the effects of perturbations of nearby planets and/or moons). Thus the oldest planets, those which have been orbit around the sun the longest, would have the most circular orbits, while those with more elliptical orbits would be relatively younger (i.e. acquired by the sun more recently). In a similar but more complicated way, orbits which are originally at a large angle relative to the suns equator will gradually shift until they are nearly perpendicular to the rotational pole of the sun, as it is for Earth, Mars, Venus and Mercury. That is, the equilibrium orbit caused by gravity drag is circular, and around the equator of the sun or gravitational object. All objects, if they are in orbit long enough, will have such an orbit, barring significant perturbations from other nearby objects.
Alhoewel we dus een zeer elliptische omloopbaan verwachten hebben we toch een lage excentriciteit.