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That goes down the road of saying that every circular path is an orbit. I'm reminded of the concept that the sun "orbits" around in the milky way. I don't rea

That goes down the road of saying that every circular path is an orbit. I'm reminded of the concept that the sun "orbits" around in the milky way. I don't really consider that an orbit either because the dynamics are totally different. I try to use "orbit" in relation to objects moving around a larger mass and/or a barycenter, which is conceptually a location of average mass. A third tiny object is rather in an orbit around one of the the bodies but that which is constrained by the gravity of the second.

mannykannot 5y ago
> That goes down the road of saying that every circular path is an orbit. That is a route I explicitly did not take, because arguments from dictionary definitions, if the issue is not specifically lexical, are usually just pedantic ways of avoiding the issue. >...the dynamics are totally different. It was a very important scientific discovery that they are, in fact, very similar, explicable from a small set of universal premises. This is a pattern that is observed widely throughout the sciences. A universe in which just the things you have mentioned were totally (or just largely) different would be a universe with entirely different physics than ours! >...and/or a barycenter... You seem to be inconsistent here, as, just before, you rejected the notion that the sun orbits around the milky way. > I don't really consider that... I try to use "orbit" in relation to... Claiming that it is wrong to use 'orbit' for the motion of the JWST around Earth's L2 on the grounds that this usage is not in your personal dictionary is not much of an argument - it is very similar to, but even weaker than, the sort of argument that you wrongly insinuated I was making! (And you cannot say I am doing the same, as I am defending the common usage among those who work with this issue - most specifically, those who are controlling the JWST.) Even the metastability of the JWST orbit is not much of a basis for your position, given that the orbits of the planets in the solar system, and of satellites around the earth, are also ultimately unstable.
spookthesunset 5y ago
On the “sun orbits the galaxy”… I’ve read that the galaxy isn’t bound together by some massive black hole but instead by dark matter. The black hole in the center doesn’t have nearly enough mass for things like the sun to orbit it. So what is the sun and all its neighbors actually orbiting around?
- sandworm101 5y ago
  They orbit each other. It isnt a bunch of stuff in orbit around something, It is a mass of stuff spining in a big homegeneous blob. Our sun feels far more pull from neighbouring stars than it does anything near the center of our galaxy.
- mannykannot 5y ago
  As big as that black hole is, its gravitational field is nowhere near large enough to hold the galaxy together. What everything is orbiting around is the center of mass of the whole system. [1] 'Everything', here, includes that black hole, all the planets of all the stars (which orbit along with their suns as a solar system), and also the dark matter, if there is any. Dark matter has been hypothesized to explain why galaxies generally seem to be rotating too fast to hold together, based on the mass of the matter that we have observed, and it does so by proposing there is more mass than we thought. There is another hypothesis (MOND) that posits that the anomaly is on account of gravity being slightly different at long ranges. In this view, too, everything is still orbiting around the center of mass - what's different is the size of the gravitational force each body is subject to. Note that, in general, you cannot calculate the velocities within a galaxy simply by assuming a point mass, equal to that of the whole galaxy, at its center. In a spherically-symmetric situation, only the mass inside of a body's radius counts, as that outside of it pulls in all directions in a way that cancels out (AFAIK, this is so for symmetric disks as well. [2]) In practice, the motion of any body close to our galaxy's center will be dominated by the central black hole, as it is located close to the galactic center. [1] https://en.wikipedia.org/wiki/Barycenter https://en.wikipedia.org/wiki/Barycenter [2] https://physics.stackexchange.com/questions/277820/could-a-disc-have-a-gravitational-pull#547568 https://physics.stackexchange.com/questions/277820/could-a-d...

mannykannot 5y ago

> That goes down the road of saying that every circular path is an orbit. That is a route I explicitly did not take, because arguments from dictionary definitions, if the issue is not specifically lexical, are usually just pedantic ways of avoiding the issue. >...the dynamics are totally different. It was a very important scientific discovery that they are, in fact, very similar, explicable from a small set of universal premises. This is a pattern that is observed widely throughout the sciences. A universe in which just the things you have mentioned were totally (or just largely) different would be a universe with entirely different physics than ours! >...and/or a barycenter... You seem to be inconsistent here, as, just before, you rejected the notion that the sun orbits around the milky way. > I don't really consider that... I try to use "orbit" in relation to... Claiming that it is wrong to use 'orbit' for the motion of the JWST around Earth's L2 on the grounds that this usage is not in your personal dictionary is not much of an argument - it is very similar to, but even weaker than, the sort of argument that you wrongly insinuated I was making! (And you cannot say I am doing the same, as I am defending the common usage among those who work with this issue - most specifically, those who are controlling the JWST.) Even the metastability of the JWST orbit is not much of a basis for your position, given that the orbits of the planets in the solar system, and of satellites around the earth, are also ultimately unstable.

spookthesunset 5y ago

On the “sun orbits the galaxy”… I’ve read that the galaxy isn’t bound together by some massive black hole but instead by dark matter. The black hole in the center doesn’t have nearly enough mass for things like the sun to orbit it. So what is the sun and all its neighbors actually orbiting around?

sandworm101 5y ago
They orbit each other. It isnt a bunch of stuff in orbit around something, It is a mass of stuff spining in a big homegeneous blob. Our sun feels far more pull from neighbouring stars than it does anything near the center of our galaxy.
mannykannot 5y ago
As big as that black hole is, its gravitational field is nowhere near large enough to hold the galaxy together. What everything is orbiting around is the center of mass of the whole system. [1] 'Everything', here, includes that black hole, all the planets of all the stars (which orbit along with their suns as a solar system), and also the dark matter, if there is any. Dark matter has been hypothesized to explain why galaxies generally seem to be rotating too fast to hold together, based on the mass of the matter that we have observed, and it does so by proposing there is more mass than we thought. There is another hypothesis (MOND) that posits that the anomaly is on account of gravity being slightly different at long ranges. In this view, too, everything is still orbiting around the center of mass - what's different is the size of the gravitational force each body is subject to. Note that, in general, you cannot calculate the velocities within a galaxy simply by assuming a point mass, equal to that of the whole galaxy, at its center. In a spherically-symmetric situation, only the mass inside of a body's radius counts, as that outside of it pulls in all directions in a way that cancels out (AFAIK, this is so for symmetric disks as well. [2]) In practice, the motion of any body close to our galaxy's center will be dominated by the central black hole, as it is located close to the galactic center. [1] https://en.wikipedia.org/wiki/Barycenter https://en.wikipedia.org/wiki/Barycenter [2] https://physics.stackexchange.com/questions/277820/could-a-disc-have-a-gravitational-pull#547568 https://physics.stackexchange.com/questions/277820/could-a-d...