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We often say that the planets orbit the sun. But, in reality, they orbit the solar system's collective center of mass. How far is that point from the center of the sun?

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  • $\begingroup$ This probably would be better-suited for Astronomy SE, a duplicate there: astronomy.stackexchange.com/q/14684/31410 $\endgroup$
    – WarpPrime
    May 10, 2022 at 12:34
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    $\begingroup$ Re But, in reality, they orbit the solar system's collective center of mass. No, they don't. While the equations of motion are in their most simple form in a barycentric frame, the planets do not orbit the barycenter. They orbit one another. There is no attraction toward the barycenter. You can see this in my answer to Do the planets really orbit the Sun? at physics.SE. $\endgroup$ May 10, 2022 at 14:32
  • $\begingroup$ Also see astronomy.stackexchange.com/a/28036/16685 & astronomy.stackexchange.com/q/44851/16685 $\endgroup$
    – PM 2Ring
    May 10, 2022 at 16:16
  • $\begingroup$ As I said on Astronomy.SE, the nominal radius of the Sun is 695,700 km, and the mean distance between the centre of the Sun and the Solar system barycentre is ~829,200 km, but the actual distance varies considerably. $\endgroup$
    – PM 2Ring
    May 10, 2022 at 16:24

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The solar system barycenter, as it's called, is a dynamic point that moves (or the Sun moves relative to, depending on how you think about it) depending on where the planets are in their orbits and relative to one another (Jupiter has the strongest effect). There is a nice image from the barycenter Wikipedia article depicting the location of the solar system barycenter for a ~50 year period:

solar system barycenter

You can see that it doesn't stray too far from the Sun, maybe about 2 solar radii or ~1 million kilometers (it is after all 99.86% of the solar system's mass).

You can find its position using something like JPL SSD's HORIZONS.

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