Among other things, Darren New saw fit to write:

>> That's it. The solid earth can be considered rigid, while the water in
>> oceans obviously isn't.
> 
> Except you get tides even when a planet is entirely made of liquid. So
> that's not really it.  It took people a long time to explain why there
> are two tides per day instead of one (given that such was noticed
> thousands of years before Newton was alive), and many of the old
> incorrect explanations still float around as "lies to children".

Yes, but we "notice" tides in relation to the solid earth, so the liquid
sphere being deformed while the solid one remains rigid is what we call
tide, isn't it (in a broad sense)?

Anyway, I agree my explanation may not be the best one, and the differential
rotation speed could be a "more true" one. I recall this was beautifully
explained by Isaac Asimov (back to science fiction) in one of his writings.
Also, it is the tidal force (wich in truth also works on solid bodies),
which has made the rotation periods of the Moon around itself and around
the Earth to match, so that we see always the same face of the Moon. Which
could be a bit harder to explain is the marvelous chance of the Moon not
only having almost the same apparent size as the Sun from Earth, but being
sometimes a bit larger and sometimes a bit smaller, so that we can see
total as well as annular eclipses. We are so lucky!

-- 
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9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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Jellby wrote:
> Yes, but we "notice" tides in relation to the solid earth, so the liquid
> sphere being deformed while the solid one remains rigid is what we call
> tide, isn't it (in a broad sense)?

Cetainly, speaking informally, the tide cannot "go out" if there's no 
solid beach from it to go out from.

But if you measure the shape of a world made entirely of liquid, you 
still get two bulges. And if you have a world entirely of solids, you're 
still have less apparent gravity at two places instead of one. And if 
you're tidally locked (like the moon is with the earth) but with liquid, 
you get "tides" that don't move.

That's generally the sort of thing people mean by "tides" when they're 
talking about the astrophysics of things.

> Also, it is the tidal force (wich in truth also works on solid bodies),
> which has made the rotation periods of the Moon around itself and around
> the Earth to match, so that we see always the same face of the Moon. 

It's also what makes the gaps in saturn's rings, makes the Lagrange 
points stable, and lots of other effects as well.

-- 
   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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Darren New <dne### [at] sanrrcom> wrote:
> Jellby wrote:
> > Yes, but we "notice" tides in relation to the solid earth, so the liquid
> > sphere being deformed while the solid one remains rigid is what we call
> > tide, isn't it (in a broad sense)?
>
> Cetainly, speaking informally, the tide cannot "go out" if there's no
> solid beach from it to go out from.
>
> But if you measure the shape of a world made entirely of liquid, you
> still get two bulges. And if you have a world entirely of solids, you're
> still have less apparent gravity at two places instead of one. And if
> you're tidally locked (like the moon is with the earth) but with liquid,
> you get "tides" that don't move.

And here we go back to Larry Niven's work, specifically, Jinx...  where it's
tidally locked, entirely solid, and pretty damn close to the Roche limit,
so you've got the two "tides" poking out of the atmosphere...

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