Thank you very much, Wilmer, for the excellent points you made showing that the dynamics of Island in the Sky present an astronomical predicament, and not just an astronomical impossibility. [...]
On a related note, I think the golden moon in The 24-Carat Moon presents a much better candidate for an astronomical impossibility. I really like this story, but I ve never been able to understand how the golden moon could have possibly remained hidden from the view of Earth behind the Earth s moon. Earth s moon is already tidally locked with Earth; it s 28 day rotation is the same as a single orbit about the Earth, and the Earth always appears in the same part of the lunar sky. If the 24-carat moon were tidally locked with Earth s moon it would essentially be part of the Earth-Moon system. As described in the story, it would always have to be opposite the Earth as seen from Earth s moon. To do this the golden moon would have to orbit Earth s moon once every 28 days. This means it would have to be a long ways out from the moon while always remaining opposite from the Earth. Its orbital radius would in fact be so long that at times the golden moon would be visible from Earth, and would appear to peek around the moon s limb.
The golden moon could not just hover over the far side of the moon, so if it were not tidally locked with Earth s moon, it would have to be either in another orbit of the Earth-Moon system, or just passing through. In either case, it could be seen from Earth.
Regardless, The 24-Carat Moon could also be a great source for a sequel.
About the 24-carat moon:
Actually, it's quite possible for something to orbit behind the
Moon, and never be visible from Earth. There are in the Earth-Moon
system (as in similar systems of orbiting bodies) five points (called
Libration Points and designated L1 through L5) where a third body
can orbit without ever changing its position relative to the first two. Two
of these points (L4 and L5) are in the orbit of the Moon, in the
positions that form an equilaterial triangle with Earth and Moon (you
may have heard of the L5 Society, which proposes colonizing these
points); the other three are in the line that runs between the centers of
the two. I don't recall offhand which number is where, but one is
between Earth and Moon, one "behind" Earth as seen from the Moon,
and one (the one in question) "behind" the Moon as seen from Earth.
It is in the latter position, obviously, that the 24-carat moon was
located.
Truly is it said that all knowledge resides in this mailing list.
Quack, Don
Don, I completely agree. The 24-carat moon was located at the L2 libration point in the Earth/Moon system. You make some excellent observations about the five libration (or Lagrange) points, that describe when the motions of bodies in a three body system, gravitational and centrifugal forces, are in equilibrium.
For true stability, two of the masses would have to be relatively small, and the third mass extremely large in comparison. In the Jovian system the L4 and L5 points are very stable because the Sun is much larger than Jupiter, and Jupiter is much larger than its Trojan asteroids. With the Earth/Moon/24-Carat Moon system this requirement is only marginally met (Earth diameter of 7000 miles; Moon diameter of 2100 miles; 24-carat moon diameter of 500 miles). Regardless, the L2 point is real in the Earth/Moon system, although much further out and beyond the moon than was shown in the story. And that was why I felt it would realistically be visible from Earth. The 24-Carat moon was so large in comparison to the moon, and would have to have been located so far back of the lunar disk as seen from Earth, that I feel it would have occasionally been visible, peeking around the rim, as it were, as the moon sped up and slowed down along its Earth orbit.
I never had problems with this story when I was young. ;-)
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