Larry Hudson wrote:
> No, it's not a resonnance.  The way I've heard the two tides explained 
> is that the ocean is raised by the moon's gravity on that side of the 
> earth, but it also pulls the _earth_ away from the water on the far 
> side.

Um, not really.

Tides are caused when any large body orbits a point. Consider two rocks 
on the moon, one on the ground very close to the Earth, one on the 
ground on the side we never see. The one on the ground close to the 
Earth is going slower than it would if it were all by itself in the same 
orbit without the moon. A lower orbit is a faster orbit, so the rock 
there is going too slow, so it should fall down towards the earth. A 
higher orbit is a slower orbit, but the rock on the far side is actually 
travelling faster than the rock on the near side instead of slower, so 
it would normally be "flung away" from the center. The smaller the 
radius of orbit compared to the size of the orbiting body, the more 
evident the effect. The stronger the gravity, of course, the more 
evident the effect.

It hasn't anything to do with pulling the centers of planets towards or 
away from anything. It has to do with the fact that from outside a 
system, gravity can be calculated as a point source, but inside a system 
you have to account for distances.

When a body gets close enough that the orbital pull overcomes its 
gravity, it breaks up. That limit (for a fluid, where only gravity is 
holding it together) is called the Roche limit.

http://en.wikipedia.org/wiki/Roche_limit

See the pictures for an explanation of tides that is actually correct. :-)



-- 
   Darren New / San Diego, CA, USA (PST)
    Neither rocks nor slush nor salted rims
    shall keep us from our appointed rounds.

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