Re: Why does the moon revolve around the earth?

Maurya:

That is a really good question. I often thought about stuff like that myself. Why don't the Earth and Moon just sort of run into each other, for example? If you think about your question, we could make it really big by asking why doesn't everything just sort of orbit around the biggest thing out there? An interesting idea, but we won't answer that directly, because as you will see, it is all sort of relative.

In order to answer your question, we need to know two major things, first a little bit about gravity, and second, a little bit about what it means to orbit. Gravity between two objects is determined by two things: 1.) How big they are, and 2.) how far apart they are. The strength of gravity is given by the product of the combined masses of the objects over the square of the distance (that is, the distance multiplied by itself). Thus, there is a small gravitational attraction between, say, you and your computer or me and a friend living across town, but the Earth is so much larger that it is essentially the only gravity we feel. However, as you get further away from something, gravity has less and less of an effect very quickly.

Second, is a little bit about orbits. Orbits represent an equilbrium of sorts between flying in one direction and falling back towards whatever you are orbiting. Think about it like throwing a baseball straight; eventually it curves back to the ground. If you could throw it really fast, its forward speed is equal to the falling rate back to the Earth. It is perpetually falling toward the Earth, but moving fast enough forward that it never hits the Earth.

The Moon is not a lot different. The Moon orbits the Earth because it is the nearest really big thing around. [It is affected by the Sun's gravitational pull, but the Sun is farther away so, even though it is much more massive than the Earth, its gravitational force on the Moon ends up being less than that of Earth.]

The Moon is also big enough to affect Earth. The Moon orbiting the Earth has enough of its own gravity to pull our oceans out several feet, and this significantly contributes to the tides. Secondly, remember that as the Earth orbits the Sun, the Moon is riding along as well, so it too, is orbiting the Sun, we just don't perceive it as a distinct orbit like The Earth, or Mars, or even an asteroid or comet, which orbit the Sun also.

Thanks for the question, Maurya.