-> As I understand it, the discovery of Pluto was more luck than arithmetic. It 
-> just happened to be very near the predicted spot, but the calculations were  
-> off. 
  
Yes. And it wasn't really very close to the predicted position. 
  
-> Well, Mercury is considered a planet and it isn't very much bigger than Plut 
-> Pluto is about the same size as the moon. Some argue that our moon is much t 
-> big too classify as a proper moon and that earth-moon system should be 
-> classified as a double planet. 
  
This is just playing with words. But there's an argument that there is 
a kind of "squatter's right" in astronomical classification. If an 
object is called a planet for long enough, then it is a planet. Mercury 
has been called a planet for many millennia, Pluto for a few decades. 
Even if Mercury were smaller than Pluto, its status as a planet would 
be assured. 
  
-> Plus that Pluto has a moon of its own, which is a pretty strong point for 
-> classifying it as a planet I'd say. 
  
Not unless you're prepared to include the very large number of 
asteroids that have satellites. 
  
-> Why? Does it really matter? 
  
Not much, but if we're going to have different names for various 
categories of objects in orbit around the sun, then we should try to be 
clear what is what. 
  
                          dow 
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