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#21
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A small, polar-orbiting moon
In article , Jake
McGuire wrote: With close flybys in 3-body systems you can either eject or capture something, but that's not the situation you're describing. If Cynthia originally had a sister, a double asteroid like Hermes was just found to be, then you have a three-body system that might allow capture. It would take the same hand-of-god that placed Luna at just the right size and distance to give us nice solar eclipses, but it's possible. Especially with an aerobrake to bleed off some energy, then the second component circularizes Cynthia's orbit before being flung into the utterdark. (A single asteroid aerobraking gives an orbit that passes inside the atmosphere on subsequent passes, which quickly leads to lithocapture and a nice iridium layer for the next intelligent species to find.) Of course, the protagonist realizes this after single-handedly recapitulating the works of Galileo, Newton, Halley etc. from our time line to develop orbital mechanics, Percival Lowell to find the cast off sister, Shoemaker to discover that it will hit Earth in 3 years, Goddard, Tsiolkovskii and Korolev to build a rocket, Oppenheimer and the gang for the payload, and Bruce Willis to get the box office. Bogen: Luna will give off more light in total because it's larger but Cynthia is much closer so each solid angle measure (steradian?) should be brighter. I think Cynthia will lokk like a brighter, fast moving Venus. No, brightness per steradian depends on illumination (how far from the Sun it is--the same as the Moon is, plus a bit of Earthshine) and how reflective it is, but not on how far away it is. (The inverse square law, an approximation in this case, comes entirely from the solid angle shrinking with distance.) That's why a tree nearby doesn't burn your eyes out while a tree-covered distant mountain is other than black. -- David M. Palmer (formerly @clark.net, @ematic.com) |
#23
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A small, polar-orbiting moon
(Gordon D. Pusch) wrote in message ...
(Bill Bogen) writes: Joseph Hertzlinger m wrote in message link.net... On 17 Oct 2003 04:29:24 -0700, Bill Bogen wrote: The ancients would probably deduce that Cynthia was brighter (relative to size) than Luna because it's closer to Earth. I thought the brighness is proportional to the solid angle. Luna will give off more light in total because it's larger but Cynthia is much closer so each solid angle measure (steradian?) should be brighter. I think Cynthia will lokk like a brighter, fast moving Venus. You have that exactly backwards. Both Luna an "Cynthia" are receiving the same intensity of sunlight, so unless "Cynthia's" surface has a much higher albedo than Luna's, they will reflect light with roughly equal intensity. Hence, their surfaces will look about equally bright per unit solid angle. How much _total_ light the reflect will depend on how large they are, but not the amount of light per unit solid angle. "Cynthia" will only appear "brighter" if it is highly reflective, or if it is so close that it appears to subtend a larger solid angle than the moon. (Note that the latter would be a Very Bad Thing, as that would imply it would be raising Huge Tides.) Yes, I was partially wrong: Luna and Cynthia are equally bright per unit of solid angle. Since Luna would appear about 12.6 times wider than Cynthia, or 158.5 times the area, then Cynthia would only have a magnitude (at best) of -7 as compared to Luna's -12.5. But Venus has a magnitude of only -4.9 (at best) so Cynthia, as I said, would like like a brighter, fast moving Venus. It should often be visible in daylight. |
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