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Date: Tue, 12 Mar 2002 10:13:29 -0800 (PST)
From: Jerry Friedman 
Subject: Re: (urth) Leagues and gravity

Mantis: thanks for answering my distance-to-Lune question so

--- Grant Peacock  wrote:
[I wrote]
> > Anyway, you got me thinking about Green-Blue celestial mechanics. 
> Stop
> > me if this has all been done.
> Yeah I once posted some similar thoughts, although I didn't do any math.
> Have you considered the possibility that the narrator is accustomed to
> gravity
> less than 1 g?  I'm assuming the whorl was designed to prepare people
> for Blue. 
> The fliers in the whorl are perhaps more realistic in a .5g environment.

Reasonable, though we can hang-glide on Earth and the Fliers'
propulsion modules could easily be far in advance of our
technology.  Anyway, wouldn't the narrator or Hoof say something about
how heavy they felt on the Red Sun Whorl?  Maybe you don't feel the
local gravity in astral projection, but if so, they should notice that
the Urth folk walk as if they were nailed to the ground (if nothing

> Our moon exerts a stronger force on Earth than our Sun does.  I say this
> because
> tides are only a bit more extreme during a full moon.  (I forget the
> terminology. 
> What are neap tides??)  So I would guess there must be something wrong
> in your
> calculations. 

Tidal force decreases as the cube of the distance, not the square, so
the gravitational force of the sun on us is almost 200 times that of
the moon, even though the tidal force of the moon is something like
three or four times stronger than that of the sun.

(Spring tides, at the full and new moon, are the strongest; neap tides,
at the quarters, are the weakest.)

> I think you are surely right that the orbits are unstable, even if the
> force
> exerted by Green on Blue is a small percentage of that exerted on Earth
> by the
> moon.

It's got to be *more*--Green comes much closer to Blue than the moon
does to Earth, and it's surely more massive.  Anyway, we agree on the

> This means that the arrangement is a new one.  For those who
> believe the
> Blue=Ushas theory, what must have happened was that the white fountain
> grabbed
> Lune out of orbit as it came lumbering in and made into a planet.

Rostrum (I think) answered that point.

> > Let's also believe Incanto quoting one Gagliardo when he says the

Oops.  I meant Inclito, of course.

Jerry Friedman

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