|MadSci Network: Astronomy|
Even though the moon *does* move further from the Earth every year, the distance it moves is actually minimal. So it would not have been shielded the earth from impacts much better than it does today. Also, a lot of things have hit the earth in past. In fact, if the earth had no oceans to hide impact craters, and if weather and geological processes (volcanoes, earthquakes, shifting of the continental plates) didn't erase the signs of impacts, the surface of our planet would look just like the surface of the moon. However, the moon does shield the Earth from impacts to some extent and it is probably why the far side is more heavily cratered than the near side. Finally, one point to clear up: meteor showers (even violent ones) are not what produced the big craters we see on the moon. Meteor showers come about when the earth's orbit crosses the path of a bygone comet. Comets leave little chunks of rock behind as the sun's heat melts or sublimates (turns ice directly into steam) the comet's surface. When these grains hit the earth's atmostphere, they heat up, causing streaks of light to appear in the sky. The main point is that these chunks of rocks are *small*. They start out about the size of your fist. Most of them burn up completely before they hit the ground. Even though the moon has no atmosphere, these little space rocks wouldn't make a big crater. Those are caused by asteroids, or planetesimals -- chunks of rock that are meters to kilometers in size.
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