The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.And it's the most beautiful bit of science I think I've ever read. NASA has a chart explaining what's going on:
The cool part can be found by comparing the last two columns - they're more or less the same. It's so simple, it's beautiful.
Carol Bayer Sager was also born today. If you don't know her name, perhaps you'll know what she had a hand in writing:
Ok, so YOU might not know this stuff, but it's the soundtrack of my life from when I was a yute.