-> Our moon's orbit is always concave towards the sun.  It is for this reason t 
-> some assign the double-planet status to the Earth-Luna system.  I believe th 
-> same is true of the Pluto-Charon system.  It is also probably true of many 
-> pairs of astereoids. 
  
I doubt very much that Charon's path is always concave toward the sun. 
Technically, Charon's motion is retrograde, tilted at more than 90 
degrees to the plane of Pluto's orbit. And it zips around the planet in 
about 6 days, compared with the 250 years or so that Pluto takes to go 
around the sun. So in Charon's "month", Pluto hardly moves at all, so 
there is almost no curvature toward the sun. 
  
Some of the outermost satellites of Jupiter have paths that are always 
concave to the sun. But they are all very small satellites. Surely, we 
shouldn't consider Jupiter to be a double (or multiple) planet because 
of them. 
  
I have seen it suggested that the Earth-Moon sustem would be considered 
a double planet if its centre of gravity were outside the Earth. But it 
isn't. 
  
                              dow 
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