Re: Why doesn't the Moon crash into the Earth?

You do know that the Moon orbits the Earth because of Earth’s gravitational pull, but the question still is, why doesn’t the Sun help pull the Moon into the Earth at certain alignments? The Sun’s influence here is less than you might think.

The gravitational influence of a body depends on two things: the object’s mass, ( how much “stuff” it’s made of) which increases the pull, and the square of the distance to the object, which decreases the pull. While it is true that the Sun is 335,000 times more massive than the Earth, it is also 400 times farther away. Let’s see 400 squared is 160,000. So the Sun’s great distance makes its huge mass less important. The Sun ends up only pulling on the Moon twice as hard as the Earth. But still the Moon is in no danger of crashing into the Earth.

First of all, you must realize that the Moon is traveling fairly fast in its orbit, over one thousand meters per second. A jet airliner travels less than 300 meters per second. But the Moon also ends up traveling in the right direction at this speed so that it never gets any closer, over time, to the Earth. If you choose a fixed point in the solar system somewhere, you would notice that the Moon does, indeed, have to go faster and slower at different places in its orbit relative to that fixed point in order to complete what we see as an elliptical path around the Earth.

By the way, the Moon is very, very slowly moving away from the Earth, but that’s an entirely other matter worthy of another question.

If this doesn’t yet explain it all, send me a note at fhiggins@ix.netcom.com