Previously on rec.arts.sf.tv.babylon5.moderated, Vince M Hudd said:

> > 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:

Five?  I had only previously heard of two, the ones that don't seem to
make sense leading and trailing the smaller body (Moon, in this case).

> 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. 

Ok, that's an obvious.  It's a "midway" point, being mid-way on the
gravitational inclines.

> 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.

That makes sense too, essentially a geo-stationary orbit in-line around
the pair.

> 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. 

Ok.

> 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.

Which men in the white coats?  I can think of two types... :)

-- 
Jeffrey Kaplan                                         www.gordol.org
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