Main Question or Discussion Point
I read that a non rotating planet like Lubricon VI does not have equator because it does not rotate. So why only rotating planets have equator?
Why must it rotate? It must orbit its star, but there's not requirement that it must rotate about its own axis. (Though it's extremely unlikely to have absolutely zero rotation)If it is not tidally locked, then it also must rotate.
That's interesting. The moon is rotating at the same rate that it is orbiting the earth. Since it is rotating, then it must have an axis of rotation. This axis of rotation is located inside the earth (earth moon barycenter). So does the moon not have an equator, even though it is rotating? Is it a requirement for the axis of rotation of a body to be located within the body itself in order for it to qualify as having an equator?I am having trouble getting my head around the notion of a planet that does not rotate. If it is tidally locked, like the moon, then it certainly rotates. If it is not tidally locked, then it also must rotate. What am I missing? Or am I still diverting blood flow from my brain to digest Christmas Dinner?
Exactly. The Moon rotates around its own axis of rotation in addition to orbiting the Earth-Moon barycenter.So the axis of rotation for the earth moon system is the earth moon barycenter, but the axis of rotation for the moon is inside the moon? So does it have an equator?
The same definition was implicit in the OP as well. Planets, by definition, are quite massive. The only way for a planet to have zero angular momentum would be to start with one with near-zero angular momentum and attempt to bring its rotation to a stop. Even that would be difficult since there are probably other gravitational bodies in the area - so you would need to compensate for tidal forces as well.You mean the definition Drakkith posted? I'm not seeing how that conclusion logically follows from the definition...