[Gorjus]

I define a planet as anything Pluto or bigger that directly orbits a star.

Actually, if you look at the orbit, "our" moon actually orbits the sun, not Earth. The moon's orbit is always concave toward the sun. Sometimes (once each month) the curvature of the moon's orbit becomes more than the average curvature of the Earth-moon system, and the moon moves a bit further from the sun, but the moon's orbit never curves away from the sun, nor does it ever 'back up'. The same thing can be said of the Earth itself. In effect, we have a double-planet system.

Calling Luna a planet also meets the quoted definition of a planet, where the gravity overcomes the strength of material (round). Of course, there are a lot of 'moons' of Saturn and Jupiter that are round. In those cases the moons' orbits do curve away from the sun.

Maybe you need to expand your definition and say that it's a bigger than Pluto and is the largest object within a million miles.

[Jedi Master Pikachu]

in English, at least American English, Earth's moon is the Moon. Also, the Solar System is the Solar System (not the Sol system) and the Sun is the Sun (not Sol).

[beezdotcom]

In effect, we have a double-planet system.

Yep. But YOU try getting everyone to stop calling it "The Moon".

In fact, I would argue that all other moons are incorrectly defined and named - our sister planet was FIRST to get the name "Moon", so we should probably use a different term other than "moon" to describe natural satellites.

[GretchenM]

Maybe you need to expand your definition and say that it's a bigger than Pluto and is the largest object within a million miles.

I was hoping we could keep Hillary's posterior out of this discussion.

[UnbelievingScumOnTheOtherSide]

Actually, if you look at the orbit, "our" moon actually orbits the sun, not Earth. The moon's orbit is always concave toward the sun.

It depends on what is meant by "directly orbiting". This geometric definition is not valid. The wave formed by the motion of the moon around the earth being carried around the sun is an accident of the location of the earth. If you moved the earth far enough from the sun so that the moon's orbital velocity around the earth were faster than the earth's orbital velocity around the sun, then the moon would no longer fit that definition. The definition of a planet should not depend on its distance from the sun.

If the sun were gone, earth would still be a planet but the moon would not because it would remain in orbit around the earth (and earth is not a star). The moon could only be a planet then if the earth were also gone. Therefore, the moon is in direct orbit around the earth, not the sun.

Almost every moon is bound in orbit to the sun in the sense that if the planet suddenly disappeared, that moon would remain in the solar system. The only exceptions might be moons very close to Jupiter whose velocity around Jupiter combined with Jupiter's orbital velocity would exceed escape velocity from the sun at certain points in its orbit.

But I submit that any smaller object which would remain gravitationally bound to a primary object other than the sun if the sun were gone has an intermediary and is not in "direct orbit" around the sun and is not eligible to be designated a planet. (Certain objects that are merely in orbital resonance with another body around the sun, which would escape that primary body if not for the sun's gravity, such as certain Apollo asteroids which come around one side of the earth and then the other forming an extremely elongated and unstable temporary "orbit" but would fly away if not for the sun bringing it back are in "direct orbit" around the sun and not the earth.)