<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Correct me if I am wrong, but I believe that a perfectly elliptic orbit applies to the two-body central force problem with the potential arising from a force obeying the inverse square law. If the central force varies slightly from the inverse square or if orbits are perturbed by action of a third body I think you may find orbits that are approximately elliptic but that exhibit phenomena like precession, or, in the case of a perturbation from a passing third body, a shift. If you get significant interactions from several bodies, all bets are off.This may be implied in your post, since if an orbit is perfectly elliptic it cannot change shape and orientation with time. A truly elliptic orbit is closed. To change with time you need, in the two-body case, you need a periodic or and open orbit. <br />Posted by DrRocket</DIV><br /><br />Yeah, I thought the concept of osculating orbits would be a bit much until I found out what was being said.</p><p>I should have stated "approximately" but didn't want to get too complex too fast <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-foot-in-mouth.gif" border="0" alt="Foot in mouth" title="Foot in mouth" /></p><p>However, I believe even with a 2 body solution, there is precession (and possibly change in shape) due to the sun's gravity (that pesky Einstein fellow again). IIRC, Mercury's orbit would precess even if it was only the sun and that one planet. It would therefore imply that the same would happen with any other distance, though the effects would be MUCH smaller.</p><p>In addition, non gravitational forces (Need not be a third body) such as the solar wind, radiation absorbtion/reradiation effects, etc would cause a very small shift in the shape of the ellipse as well.</p><p>Granted they are very small for a planet sized object, but the forces are not zero.</p><p>Edited for clarity (6 words) and typos (more than one word)</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>