> 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.

Darren New wrote:
> Um, not really.

I don't see why not.

> 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.  [snip orbital mechanics]

This does not contradict Larry's version.

> 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.

So if the Moon were a point mass Earth would not experience tides?
Bzzt, try again.

Newton showed that a spherically symmetric body - i.e. one whose density 
varies only with radius - produces the same g-field (outside its 
surface) as a point mass.  I guess what you mean by this "inside a 
system" jazz is that the g-field of a less regular body, or group of 
bodies, can be considered spherically symmetric from far enough away but 
not up close.  (I wonder how far away a space-probe gets before its 
navigation can safely treat the Earth-Moon system as a single body.)

-- 
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine

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