Subject: (whorl) Double planets

From: mary whalen <marewhalen at yahoo.com>
Date: Thu, 09 Jul 1998 22:09:43 +0000

This is Sean Whalen (prion).

As I looked over the Whorl archives, it seemed to me that not everyone
agreed that the planets Blue and Green were part of a double planet
system, but were instead both in separate orbits around the sun.  I
think their are many reasons to think that they're a double-planet
system.

Hyacinth is described in the text as pointing towards Blue and Green
and asking if Auk is going to one of those.  She points just once, and
it seems unlikely that two separate planets would happen to be close
enough together for this at just that time.

Also, the inhumi fly between the two when they're in conjugation. 
It's unlikely that they can fly in a vacuum, they probably have to
breathe.  They also would have no way to fly in a vacuum with nothing
to beat their arms against.  If they used technology to get their,
they should be able to do this at other times as well.  If they are
together, at conjugation they could have an airbridge between them at
a certain time and not others if they have an elliptical orbit about a
common center.

The fact that they're close in an elliptical orbit also would show why
they cause tidal waves and earthquakes in conjugation.  If they are in
separate orbits, they would not be anywhere near enough to cause
significant or noticeable seismic activity (not even the very-close
Moon does anything to Earth, so it would almost have to be a very
close double system to have more of an effect than we experience with
our large and near neighbor).

The fact that Green rises as seen on Blue and appears almost as bright
as a second sun also shows how big and recognizable it must be,
indicating nearness.  Green would have to be impossibly close if it
had a different orbit to look that bright, not to mention that it's
unlikely to rise on the particular night that Horn finishes his book;
but a double-planet would have a much greater chance of rising on any
random night.

Also, it's probable that the Whorl is parked in one of the Lagrange
points of Blue and Green, where their mutual gravitational attraction
on it cancel each other out, and it remains at rest with no power
being needed to maintain orbit.  Otherwise, it's unlikely Horn would
be able to pick it out among all the other stars unless it was always
visible and near.  It could be always visible because it's orbiting
Blue, but it wouldn't be, because it took three days on the course
toward Blue for it to become obvious that that was their destination. 
If the Whorld were in orbit, it would be apparent immediately.  If it
weren't in a Lagrange point, it would be very unlikely for an inhumu
to pass in front of it while flying, as Horn seems to think occurs.

prion
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From: Peter Westlake <peter at harlequin.co.uk>
Date: Mon, 13 Jul 1998 12:37:46 +0000

>to pass in front of it while flying, as Horn seems to think occurs.

Unfortunately, I don't think Blue and Green *have* a Lagrange point.
Hal Clement has a short story called "A Little Knowledge", in which
a fleeing criminal parks his spaceship what he fondly imagines is
a Lagrangian point of a double star. But the two stars are too closely
matched in mass for it to work: one mass has to be considerably bigger
than the other. (If anyone has the story, and the ratio is 25.8 : 1,
then I shall appear very clever; if it isn't, well, how can I be expected
to remember a detail like that?) The story ends with the fugitive
finally getting round to looking out of the window, to find himself
plunging into one of the stars as it orbits towards him.

Whatever the masses of Blue and Green may be, it isn't very likely
that they could differ enough to have Lagrangian points while both
being reasonable places to go and live.

SBear.

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From: Alastair Reynolds <areynold at estsa2.estec.esa.nl>
Date: Mon, 13 Jul 1998 13:00:38 +0000

> plunging into one of the stars as it orbits towards him.

actually, any two orbiting point masses will always have five lagrangian 
points around them; the one between them (exactly in the middle for equal 
masses) is L1. L1 isn't stable - something placed at L1 only needs a 
tiny nudge in either direction and off it goes. 

The point HC might have been making is that L4 or L5 (the "usual" Lagrange 
points, off to either side) might only be stable if the ratio is as above?

blimey - the first thing on the list I've responded to in ages!

al