The best model for the origin of earth's moon is
About 4520 million years ago, an object perhaps
the size of mars or
about 10-15 % the mass of current day Earth
collide with the
protoearth. This collision was not a direct head
on but instead a
glancing blow. This gigantic collision put about
1/80 of the total
mass of projectile plus embryo protoearth into a
orbit around the earth. Stuff in this debris ring
within about 100,000
to a million years coagulated to form the moon.
Since that time, tidal
forces between the moon and earth have acted to
increase the earth-
moon distance to its present radius of about
380,000 km. The moon is
receding from earth at rate of about 2 cm/year.
This is being measured
continuously since 1969 using a laser beam and a
mirror left on
lunar surface by astronauts.
I'm not sure we know. There was a theory
running around for a long time that a Mars-sized
planetoid hit the Earth about 4.4 billion years
ago and that the moon is just splash from that
impact that collected in orbit. This was supported
by the lower density of the moon (the moon does
not have a large iron core, unlike the Earth,
which would happen if it splashed off of the
Earth's surface), and by various computer models.
However, I believe this theory has come under some
debate, and I don't know what (if any) conclusion
that debate came to.
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