Jeffrey Kaplan  wrote:   
   > Previously on rec.arts.sf.tv.babylon5.moderated, Andrew Swallow said:   
      
   > > Is a spacestation at L1 a good idea? - Yes.  EML1 is the natural   
   > > crossroads in space.  This makes the EML1 spacestation a good place to   
   > > change spacecraft.   
       
   > Why is a Lagrange Point a "natural crossroads"? What is it about one of   
   > these points that make it better for such use than merely someplace in   
   > orbit?  For that matter, what IS a Lagrange Point? I've heard the name,   
   > but never understood what it is or why it's a good thing.   
      
   A Lagrange Point is a point in space relative to an orbital pair - such as   
   Earth and Moon, or Sun and Earth - where the various mutual forces cancel   
   each other out such that a much smaller object can remain there unaffected.   
      
   I think there are five for each pair:   
      
   One sits between the two (closer to the smaller of the pair than the   
   larger). This one is probably the easiest to understand, because the biggest   
   factor is the two gravitational pulls. This is the one Andrew was referring   
   to as EML1 - Earth/Moon Lagrange Point 1; being directly between the two   
   bodies (obviously closer to the Moon than Earth) it's a logical place to put   
   a 'stepping stone' between Earth and Moon.    
      
   Another sits *beyond* the smaller of the two bodies. Here, anything in the   
   Langrange point is orbiting the larger object on the same orbital period as   
   the smaller one - and the gravitational pull acting on it is the combined   
   pull of both of them, which is counter-acting the centripetal force brought   
   about by its orbital speed.   
      
   Another sits on the same line, but on the other side of the larger object,   
   and the same principal applies as with the second one.    
      
   The other two sit at points along the orbital path of the smaller object,   
   one ahead of it, and one following it. IIRC, their positions effectively   
   form two equalateral triangles with the centres of the two bodies, and I'm   
   not quite sure I understand the physics of these two. If I try to make sense   
   of one, the other makes my head explode because it just seems wrong, so I   
   just trust that the men in the white coats know what they're talking about.   
      
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