Quote Originally Posted by a500lbgorilla View Post
An orbit is moving forward fast enough that you miss as you fall toward the Earth. If you're orbiting the earth at an altitude you would consider in space, you're in free fall like a skydiver; but you've got so much forward velocity and no air to hold you back that the Earth moves out of the way below you.

You fall to the Earth and perpetually miss. You just move forward so fast, the Earth seemingly moves out of the way.
That's all quite intuitive. What isn't however, is that wouldn't even a slight change in velocity, even the tiniest bit of acceleration or deceleration cause the planet to either get loose or crash? How come this doesn't happen? What creates this universal (ha!) tendency for all objects to just play nice and circle each other at exactly the right speed?

Well, it's a logical fallacy. The ones with the incorrect speed have indeed already left the orbits or will do so later. None of the orbits are stable, for example the moon is moving away from earth at 3.8cm/year, and will eventually break loose. More about it here: Curious About Astronomy: Is the Moon moving away from the Earth? When was this discovered?. Ok, some orbitses(? orbii? wtf) are quite stable, such as earth's orbit around the sun. There are tiny oscillations in the elliptical orbit, but none of them suggest we're moving neither towards nor away from the sun. There are, however, a couple other forces at play. One is the same tidal interaction as between the earth and the moon, just quite a bit weaker. It turns out that the yearly increase in the distance between the earth and the sun from this effect is about one micrometer (a millionth of a meter, or a ten thousandth of a centimeter). Another effect is that due to the sun being powered by nuclear fusion, it's continuously losing mass, causing earth's orbit to widen. However, over the lifetime of the sun, about 10 billion years, it will only lose about 0.1% of its mass, causing the earth to move out by a total of about 150,000km, that is, about 1.5cm/year. So, we're not breaking loose anytime soon.