Two Kinds of "Coincidence"

Two kinds of "coincidence" that at first glance, might seem kind of similar.

1. The moon and sun are about the same size when viewed up in the sky. This makes solar eclipses really neat to look at.
2. The time it takes for the moon to rotate and go around the earth are the same. From this, the same side of the moon always faces the earth.

There is a huge (HUGE) difference between the nature of these two coincidences. To find out, all you have to do for each example is ask "Why?".

For the 1st example, it's just dumb luck it worked out that way. The moon can orbit the earth at any distance; there's no reason why it should appear the same size as the sun. For the 2nd example, tidal forces lock the moon's period of rotation to be the same as it's period of revolution. If you're interested in learning more about this you can watch my video here: http://www.youtube.com/watch?v=Spz_IHYeoLY

The point I want to make is the 2nd example has a much more satisfying answer than the first. "It just is" is an answer that has never satisfied anybody, but it turns out most good questions have answers that have a lot more substance. I didn't understand the significance of this difference until late in high school. Coming to understand this difference was life-changing for me at the time; it made me care about science in completely new ways.

There are real reasons for say, why a ball's trajectory traces the path of a parabola, or that a plant gets four times as much light when it's half the distance from a bulb, or that exponential functions perfectly model the decay of radioactive atoms, or a cue ball stays still when it smacks another ball straight on... none of these are dumb-luck coincidences. There are perfectly satisfying answers for "Why?" in all these cases.

I was lucky to find satisfying answers before I stopped asking "Why?". Tragically, I think most people don't. There's a lot to be gained in re-invoking people's curiosity.