QUESTIONS:
1) Why do planets have more than one moon?
2) Why isn't our moon a planet?

ANSWER from Ed Erickson on May 19:
Not all planets have more than one moon.  The earth has only one, for example.
Jupiter, though, has four moons that are easily seen through binoculars,
and a bunch of others which can be seen with telescopes.

Our moon is considered a moon because it orbits the Earth, not the sun.
Planets are bodies that orbit (go around) the sun.  Moons are bodies that
orbit planets.

The planets orbit the sun because of the gravity force attracting them to
the sun; the planets' motion keeps them from falling onto the sun. The
planets are much smaller than the sun, so the sun moves very little while
the planets go around it. Moons orbit planets in a similar way for the same
reasons; the moons are typically a lot smaller than the planets.

An exception is Pluto's moon, Charon, which is not a lot smaller than
Pluto, so that Pluto actually moves some because Charon is orbiting.

Moons may have been small bodies that were wandering in the solar system
and  then were "captured" by the gravitational attraction of a planet when
they got too close.  They might also be a piece of the planet which got
knocked off when an asteroid hit the planet.  Perhaps our moon was formed
this way, but scientists argue about it, and no one is sure.