Anything Else

A large rock, about the size of Mars, is doomed. It's heading toward Earth, destined for a slightly off-center impact that will set everything that isn't already rotating into a frenzy of spin.

Upon impact, material from the incoming object and from the new Earth is cast into space. A ring of debris orbits the planet, and in an amazingly short amount of time -- about one day -- it begins to coalesce into a satellite. It takes somewhere between 1 and 100 years for the Moon to gather most of the stuff into a ball.

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The Moon does not just go around the Earth. In reality, the two objects orbit about a common gravitational midpoint, called a barycenter. The mass of each object and the distance between them dictates that this barycenter is inside Earth, about three-fourths of the way out from the center.

So picture this: The center of the Earth actually orbits around this barycenter, once a month. The effect of this is very important. Think, for a second, of a spacecraft orbiting Earth. Its astronauts experience zero gravity. That's not because there's no gravity up there. It's because the ship and its occupants are constantly falling toward Earth while also moving sideways around the planet. This sets up a perpetual freefall, or zero-g.

Like the orbiting spaceship, the center of the Earth is in free-fall around the barycenter of the Earth-Moon system.

Here's the kicker: On the side of Earth opposite the Moon, the force of the Moon's gravity is less than at the center of the Earth, because of the greater distance. It can actually be thought of as a negative force, in essence, pulling water away from the Moon and away from Earth's surface -- a second high tide.

Read the rest. Also how to save the earth.