Earth's Moon is larger in relation to Earth than any other moon to its planet in the Solar System. Maybe the Earth-Moon system, which an astronomer described to me as a double planet, is unusual or even unique?
This matters because I have read or heard several reasons why the Moon is necessary for life, or at least for our kind of life, on Earth:
its gravity thinned our atmosphere, making Earth unlike Venus;
tides facilitated evolution by enabling fish to become amphibians, then land animals;
the Moon's gravity stabilizes the Terrestrial core or the magnetic field (?) (or something);
it stabilizes the Terrestrial rotation axis.
This last reason is given in The Stars Are Also Fire where it is speculated that sentience is unique to Earth. However, Andrea the Knowledgeable tells me that several extrasolar double planets have by now been discovered. Let us hope.
1 comment:
Kaor, Paul!
And Anderson wrote about a double planetary system in his short story "Among Thieves." One planet was terrestroid and was settled fairly easily but the other planet needed some terraforming before it could be colonized. So it's COOL that real double planets orbiting other stars has been found.
Sean
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