if perpetual motion is impossible, why/how do planets orbit and spin continually?
In: Physics
This answer leaves out stuff like solar winds and other forces because, well, it does. Sorry.
As I understand it, planetary orbits ARE changing/decaying, just at a rate slow enough to be virtually imperceptible to us. In space, massive celestial bodies don’t have air resistance or friction to slow their momentum like on Earth – they are mainly affected by the gravitational forces pulling them back and forth between bigger things. Planets in a solar system orbit the densest thing around – a star – and that star, in turn, moves through space pulled around the galactic center, an even denser mass that holds the entire Milky Way in its pull. The gravity between these things is basically fighting against the outward explosive releas pf energy that created separate planets and whatnot to begin with.
The massive scale may give the illusion of permanence, but the universe is always expanding outward with tremendous speed, and the energy that propels us spreads out, too. Eventually the Sun’s hold on us will weaken, and the momentum that started our orbit will carry us out of the system, maybe even out of the galaxy.
Or maybe the galactic center will shift, and the whole system will spin out until eventually the energy of that initial push is expended, or spread so thin so as to prevent the further growth of space.
So what we’re really seeing is just a process of energy transfer that’s efficient enough, I guess, that it seems limitless. There IS a limit, it just doesn’t mean much in the context of a human lifetime.
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