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From: Michael Andre-Driussi <mantis@sirius.com>
Subject: (whorl) out of plane and kidney bean orbits
Date: Wed, 5 Apr 2000 13:48:31 

Another problem with using a highly eliptical orbit (better known as
orbital eccentricity) on a scale using significant fractions of an
Astronomical Unit is that the world will have gross "seasonal" effects as
it moves closer and further from the primary.  (Seasonal in quotes because
our seasons are all just a result of axial tilt rather than the Earth
getting closer and further from the sun.)

Out of plane orbit.  This avoids the problem of gross seasonal effects, but
there is still the familiar problem that the orbits will intersect at two
points rather than just one, and it is hard to see how "six local years"
could figure in any immediately helpful way: both worlds will have the same
orbit, and thus the same orbital speed, more or less (as I understand it).

(I think) one visual effect would be that (from the northern hemisphere)
the out-of-plane planet would look like a star in the north above the
primary (in fact, would it be visible?  Even "daystar" and "evening star"
Venus/Mercury are within the plane: at right angles, an out-of-plane planet
would compete directly with the primary, and that is no contest!), then it
would loom large over a few months, then it would sink below the horizon
and vanish for half a year; then it would come back up, like a giant
playing peek-a-boo, and begin shrinking as it climbed up to that point
above the primary.

Oh, wait!  It would be a crescent near the primary, then
half-light/half-dark at the mid-way point, then full (or dark) at encounter
(depends on if it is coming long or short; if dead on, then half/half is
maximum): its time as a visual disk would only be at or near the encounter,
if at all.

Kidney-bean orbits.  These are weird, all right.  Easier for me to see in
cases of big gravity wells, like multiple star systems.  Or a similar thing
with smaller scale: asteroids doing the gravity dance between Jupiter, Sun,
and Earth, etc. But Blue and Green are similar Earth-like planets.

All this reminds me of Jack Vance's THE DRAGON MASTERS, where the alien
"star" Coralyne comes close to planet Aerlith every once in a while, and
each time waves of grephs come over to mess with the humans.

(But of course, there was an sf magazine in the '30s or '40s that had a
story about devil-like aliens swimming down from a moon or a world.  Was it
a cover story?  Well, there was an illustration--probably was a cover.  Has
anybody found that magazine; know the name of that story?  It was a major
inspiration for the Short Sun, but that's another story for another time.)


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