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Date: Wed, 13 Mar 2002 08:50:29 -0800 (PST)
From: Jerry Friedman 
Subject: Re: (urth) Green's diameter, angular and otherwise

--- Michael Andre-Driussi  wrote:
> Another way to approach the Blue/Green conjunction is to determine the
> size
> of Green in the sky of Blue, from its "usual" size to its greatest size
> at
> closest approach.
> For simplicity sake, let us assume that both Blue and Green have surface
> gravities of 1 g.  More or less "twins" in separate orbits (and Green
> having an orbital eccentricity that carries it out to a close approach
> with
> Blue every x number of local years).

I don't think the eccentricity has much to do with it.  The two
planets will come into conjunction every x number of local years.  If
Green's orbit is eccentric, then sometimes the distance at conjunction
will be greater and sometimes less.

> I was watching for details when I read before and I don't recall finding
> any, other than "baneful eye."
> OTOH, we have the happy case of an eyewitness who also sees Lune
> (maybe?)
> and the Old Sun (definitely). The Old Sun (assumed before to have an
> angular diamter of around 0.83 degrees) is "swollen" to his eyes, but
> iirc
> he never says anything in comparison to Green's biggest size.
> Using the Moon as a model.
> MOON    240,000    0.52 degrees
> LUNE    150,000    0.83 degrees
> GREEN   105,000    1.18 degrees
> So even a puny world like the Moon (diameter 2160 miles) would be more
> than
> twice the size of the Moon in our sky when located at a distance
> attributed
> to Green.
> A twin planet would naturally be much bigger.
> Based upon this, the Bluvian astronomer is quite likely to be waaaay off
> in
> his estimate of the "closest approach" distance as being 35,000 leagues.

How are you thinking the Bluvian astronomer would make his estimate?
I can't think of a method that would be thrown off by Green's large
size.  The only way I know of at the Green level of technology would be
parallax, which doesn't depend on Green's angular size at all.  (Though
the parallax method would require a fair amount of work, notably
measuring the distances between widely separated cities, as well as
astronomical observations at conjunction, which is presumably the
worst possible time.  Seems unlikely during the first twenty years
of settlement.)

Maybe Gagliardo used observations made from the lander and
information from the monitors, who could at least have told him
the cruising speed of the landers.  Or maybe he's doing the same
thing we're doing and guessing that the Short Sun is the same size
and brightness as the Old Sun, which makes the radius of Blue's
orbit the same as Urth's (if he knows it); then from the time
between conjunctions he can get the radius of Green's orbit.

Anyway, the higher the true distance, the harder the problem of how
the inhumi cross.

Jerry Friedman

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