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I see no reason why it should be impossible. A binary planet system should exist in a reasonably stable orbital configuration. As Javier pointed out, Pluto and Charon are similar in size and mass, like a small-scale version of what you describe. The important thing to note here is that the barycenter of the Pluto-Charon system is outside either planet. In ...


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There are some qualifications on the equivalence principle that often aren't made clear in casual summaries. One is that the equivalence principle only works in the limit as the size of the spacetime region where measurements are performed approaches zero (or to use another term, in an "infinitesimal" region of spacetime)--in other words, you'd have to ...


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Answer to your old question: Right now, the perihelion currently occurs around the same time as (2 weeks after) the northern winter solstice, but 10000 years from now it will occur around the northern summer solstice. So, like other people have said, it's just coincidence that right now they both happen at about the same time. Answer to your new question: ...


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Thank you both for your very considered replies. I've come up with a modified view based on both replies. It is natural to assume that: the train is falling freely. But if you think about it more closely, only the Centre of Gravity is in free fall (by definition). The CoG will also move toward the front (singularity end) as it increasingly ...


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I would agree that a circular orbit with full tidal lock is more "in equilibrium" (everything is time independent in the rotating frame) than one in which the orbiting object had zero rotation in an absolute Cartesian reference. There is however an important notion for determining whether something is rotating, and that is its angular momentum. An object ...



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