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If we want to be really pedantic, the earth doesn't even orbit the sun. Rather, both bodies orbit a point that's (roughly) their combined center of mass (which

If we want to be really pedantic, the earth doesn't even orbit the sun. Rather, both bodies orbit a point that's (roughly) their combined center of mass (which in our case just happens to be somewhere inside the sun). And of course that "roughly" is standing in for a lot of other variables, too.

pdonis 1y ago
> the earth doesn't even orbit the sun. Rather, both bodies orbit a point that's (roughly) their combined center of mass Yes, that's true. Indeed, when we take into account other planets (and the Moon, and the asteroids, etc., etc.), all of these objects orbit a common barycenter. However, that doesn't change the fact that the Moon does not orbit the Earth in this sense; its net acceleration (in a barycentric inertial frame) is never towards the Earth, or towards a common barycenter of the Earth-Moon system. Only if we ignore the motion around the solar system barycenter do we get the approximation in which the Moon and Earth orbit a common center of mass.
- chrisweekly 1y ago
  Ok, but this (the whole subthread, not just this msg I'm replying to) gets so deep into pedantry that it becomes borderline meaningless. The moon circles the earth approx every 28 days. When people -- even reasonably well-educated ones like me, with a couple semesters of astronomy from a world-class university -- say "orbit", that is what they mean. I'm deliberately omitting caveats about sidereal month vs synodic period, to avoid the same slippery slope I'm gently criticizing. Given there's no absolute spatial frame of reference, I'd argue that piling such caveats atop each other doesn't "move" us closer to a truer understanding. All that said, it is kind of fun to consider.
  - pdonis 1y ago
    > The moon circles the earth approx every 28 days. In what sense? You state this like it's an obvious fact. It isn't. It contains a number of hidden assumptions, not all of which stand up to close scrutiny. > Given there's no absolute spatial frame of reference, I'd argue that piling such caveats atop each other doesn't "move" us closer to a truer understanding. I disagree, because "there's no absolute frame of reference" is not the same as "all frames of reference are equally useful". The former is true. The latter is not. Once you face up to the fact that different frames are useful for different purposes, you should realize why you can't just state things like "the moon circles the earth" as obvious facts. At the very least, you have to explain why you've picked the frame of reference (centered on the Earth) in which that's (approximately) true. Basically, that would be because you're ignoring the rest of the universe and you only care about the Earth and the Moon. Which is fine for many purposes (it worked for sending humans to the Moon and back), but not for others (like saying "the moon orbits the earth" as opposed to "the moon orbits the sun"--which is simply not true, once we bring the Sun into the picture, you can no longer ignore the Sun's effect on the moon).
NiloCK 1y ago
Can you go further and say that the Earth and sun "really" are orbiting the center-of-mass of the milky way? And does it end there? I guess that galaxies are far enough apart from one another to escape this logic with their relative escape velocity.
- pdonis 1y ago
  > Can you go further and say that the Earth and sun "really" are orbiting the center-of-mass of the milky way? Not according to the definition I gave. Calculate the acceleration of the Earth and Sun towards the center of the Milky Way and compare it with the Earth's acceleration towards the Sun.

pdonis 1y ago

> the earth doesn't even orbit the sun. Rather, both bodies orbit a point that's (roughly) their combined center of mass Yes, that's true. Indeed, when we take into account other planets (and the Moon, and the asteroids, etc., etc.), all of these objects orbit a common barycenter. However, that doesn't change the fact that the Moon does not orbit the Earth in this sense; its net acceleration (in a barycentric inertial frame) is never towards the Earth, or towards a common barycenter of the Earth-Moon system. Only if we ignore the motion around the solar system barycenter do we get the approximation in which the Moon and Earth orbit a common center of mass.

chrisweekly 1y ago
Ok, but this (the whole subthread, not just this msg I'm replying to) gets so deep into pedantry that it becomes borderline meaningless. The moon circles the earth approx every 28 days. When people -- even reasonably well-educated ones like me, with a couple semesters of astronomy from a world-class university -- say "orbit", that is what they mean. I'm deliberately omitting caveats about sidereal month vs synodic period, to avoid the same slippery slope I'm gently criticizing. Given there's no absolute spatial frame of reference, I'd argue that piling such caveats atop each other doesn't "move" us closer to a truer understanding. All that said, it is kind of fun to consider.
- pdonis 1y ago
  > The moon circles the earth approx every 28 days. In what sense? You state this like it's an obvious fact. It isn't. It contains a number of hidden assumptions, not all of which stand up to close scrutiny. > Given there's no absolute spatial frame of reference, I'd argue that piling such caveats atop each other doesn't "move" us closer to a truer understanding. I disagree, because "there's no absolute frame of reference" is not the same as "all frames of reference are equally useful". The former is true. The latter is not. Once you face up to the fact that different frames are useful for different purposes, you should realize why you can't just state things like "the moon circles the earth" as obvious facts. At the very least, you have to explain why you've picked the frame of reference (centered on the Earth) in which that's (approximately) true. Basically, that would be because you're ignoring the rest of the universe and you only care about the Earth and the Moon. Which is fine for many purposes (it worked for sending humans to the Moon and back), but not for others (like saying "the moon orbits the earth" as opposed to "the moon orbits the sun"--which is simply not true, once we bring the Sun into the picture, you can no longer ignore the Sun's effect on the moon).

NiloCK 1y ago

Can you go further and say that the Earth and sun "really" are orbiting the center-of-mass of the milky way? And does it end there? I guess that galaxies are far enough apart from one another to escape this logic with their relative escape velocity.

pdonis 1y ago
> Can you go further and say that the Earth and sun "really" are orbiting the center-of-mass of the milky way? Not according to the definition I gave. Calculate the acceleration of the Earth and Sun towards the center of the Milky Way and compare it with the Earth's acceleration towards the Sun.