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Date: Thu, 11 Apr 2002 12:56:04 -0700 (PDT)
From: Jerry Friedman 
Subject: RE: (urth) distance

--- Dan'l Danehy-Oakes  wrote:
> > Well, Blattid, since you put it so succinctly, the choice is easy.  Of
> > course the 30,000 league figure is incorrect!  Which means that Green
> > might only fluctuate a tiny amount from its 80,000 league distance, 
> I'm no orbit mechanic, but that seems unlikely with a six-year period,
> and _very_ unlikely to cause the inhumi to be able to cross only for a 
> short period.

I quite agree.

> > Thanks!  You made everything so much easier!  
> Oh. Good.
> > So we know that the moon was moved from about 80 to 50 thousand
> > leagues in Severians day 
> Ahem. We do not know that it "was moved." I believe that most models
> suggest that the Moon _will_ in fact move closer over (geological) 
> time.

Not that I've ever heard.  See for example
, and with the
least detail but the greatest appearance of reliability,
.  I think it's
extremely unlikely that our moon turned into Severian's Lune on its

> > ... the moon is proportionally very large for a satellite when
> compared
> > with the earth's mass.  It is huge compared to the satellites of most
> > of the gas giants.
> Correct; we are actually best regarded as a double-planet system. (The
> center about which the Earth-Moon system mutually orbits is, if I am not
> mistaken, not actually inside the Earth.) 

Not quite.  The formula for the position of the center of mass is
x_cm = (m_moon x_moon + m_earth x_earth) / (m_moon + m_earth), where
x's are positions and m's are masses.  If we let x_earth be 0 and
use the radius of the moon's orbit for x_moon, we find the center
of mass to be about 4620 km from the center of the earth, less than
the radius of the earth, which is about 6380 km.

Jerry Friedman

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