From: jrs@merlyn.demon.co.uk   
      
   JRS:  In article <1143395632.386123.16380@i39g2000cwa.googlegroups.com>,   
   dated Sun, 26 Mar 2006 09:53:52 remote, seen in news:sci.space.tech,   
   Mark Adler  posted :   
   >   
   >The low orbit periods for the moon and Mars are both about two hours.   
   >   
   >Now isn't that odd.  Why should they be the same since their sizes and   
   >surface gravity are so different?  Well, if you look at the equations,   
   >you'll notice something interesting.  The "low" orbit period around a   
   >spherical body (where the "low" altitude is defined to be some fixed   
   >proportion of the radius, say 10%) turns out depend only on the average   
   >density of the body.  The size of the body does not matter!  This is   
   >left as an exercise for the reader.   
      
      
   In fact, the period of *any* circular orbit depends only on the average   
   density within the sphere that the orbit circumscribes, assuming the   
   obvious symmetry.  T = Root ( 3 Pi / G D ) .   
      
   For a homogeneous sphere, the time to slide from rest along any   
   frictionless chord and back is equal to low orbit time.   
      
   See sig for those.   
      
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