distance

Jerry Friedman · 11 Apr 2002, 19:56 · from urth-v0204.mbox · original

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From: Jerry Friedman <jerry_friedman at yahoo.com>
Subject: RE: (urth) distance
Date: Thu, 11 Apr 2002 12:56:04 -0700 (PDT)
Message-ID: <urth-v0204-586 at digest.urth.net>
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--- Dan'l Danehy-Oakes <ddanehy at siebel.com> 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,=20
>=20
> 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=20
> short period.

I quite agree.

> > Thanks!  You made everything so much easier! =20
>=20
> Oh. Good.
>=20
> > So we know that the moon was moved from about 80 to 50 thousand
> > leagues in Severians day=20
>=20
> 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)=20
> time.

Not that I've ever heard.  See for example
<http://www.astronomynotes.com/angmom/s2.htm>,
<http://www.ridgenet.net/~do_while/sage/v2i2f.htm>, and with the
least detail but the greatest appearance of reliability,
<http://helios.gsfc.nasa.gov/qa_earth.html#earthslow>.  I think it's
extremely unlikely that our moon turned into Severian's Lune on its
own.

> > ... 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.
>=20
> 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
>=20
> mistaken, not actually inside the Earth.)=20

Not quite.  The formula for the position of the center of mass is
x_cm =3D (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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--=20

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