People keep coming up with more interesting thought experiments to test the proposed definition of planet. This one, the changing orbit of the Moon. The Earth-Moon barycenter (center of mass) is currently within the Earth, however, the Moon is drifting away slowly, and in a few billion years, the Moon will be far enough that the barycenter will be outside the Earth's surface. Should that happen before the red giant Sun engulfs us, we'll have to have a Moon-Planet birthday party. Though, when the exact moment that happens is may depend upon things like if it's over Mt Everest or an ocean trench, and do we count the oceans and air and such as part of the Earth?
And what if there's a binary planet system w/ really eccentric orbits, or a mutliple planet system, then we might have the barycenter sometimes within a planet, and sometimes not, so the others would sometimes be moons and sometimes planets. Oy!
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