You could say, something's a planet if it's big and the gravitational force of the sun on it is stronger than the force due to any other body. That's more precise than "orbits a star directly".
That 'correctly' categorizes all the moons and planets, except for Earth's moon! The sun pulls on the Moon slightly more strongly than Earth does, IIRC from an essay by Asimov. We could think of Luna and Earth as planets with similar orbits, so close that they perturb each other strongly..
Slashdot Mirror
That 'correctly' categorizes all the moons and planets, except for Earth's moon! The sun pulls on the Moon slightly more strongly than Earth does, IIRC from an essay by Asimov. We could think of Luna and Earth as planets with similar orbits, so close that they perturb each other strongly..