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It's commonly known that the center of gravity of our solar system is not the suns axis, and, that the same goes for other orbital systems like Pluto and Charon, and so on. When was it discovered that our sun has an orbit around a barycenter?

pol0
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  • This doesn't answer your question, but Kepler's Laws of Planetary Motion are the first instance I can think of (off the top of my head) where the concept of "a common center" exists. However, you could argue that the concept of epicycles also had the Sun revolving around an empty point. Just as a note, make sure you're not confusing revolution with rotation. The sun revolves around the barycenter (sort of -- it's more complicated than that though), but rotates on its own axis. –  Oct 17 '18 at 04:29
  • the question isn't when it was discovered or entered public consciousness that planets revolve around the sun, it is a bit of a leap to go from that to that the sun is revolving a barycenter of the solar system as a whole – pol0 Oct 17 '18 at 04:34
  • I've seen August Möbius Der barycentrische Calcul from 1827 attributed for the concept of a barycenter, but what I'm looking for is when it became known, or, contemplated about, that our sun orbited a barycenter of the solar system as a whole, when that theory had begun to be applied to our solar system and discussed and talked about in the scientific literature – pol0 Oct 17 '18 at 04:36
  • OK, my point was that Kepler thought (correctly) that Jupiter (for example) doesn't orbit the Sun, but rather the Sun and Jupiter both orbit a common center of mass. –  Oct 17 '18 at 04:40
  • Aristarchus did as well. When did it became widely considered, data collected to prove it, and so on, the diameter and shapes of the orbits mapped? I'm looking at it from a history of ideas perspective to correlate with other ideas that developed during the 19th century mainly. – pol0 Oct 17 '18 at 04:48
  • It's inherent in Newtonian gravitation, but it's not exactly easy to determine the motion of the barycenter relative to the Sun without precision instruments. – PM 2Ring Oct 17 '18 at 05:21
  • The Greeks had developed orbital models as well. When was it, empirically, discovered that our sun orbits a barycenter, in a way where it was not just theory but people knew that it was so? – pol0 Oct 17 '18 at 05:26
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    @barrycarter isn't Kepler first law incompatible with your statement? Or is "the Sun at one focus" not what Kepler actually wrote (genuine question)? – ProfRob Oct 17 '18 at 06:35
  • Please add what information you already have about this question. As far as I know (which may be little indeed) Aristarchus wrote about the Earth revolving around the Sun, not a barycentre. And to prevent further answers that you don't want, please clarify your question. It seems you are actually asking when observations became available that were capable of being interpreted as motion around the barycentre. – ProfRob Oct 17 '18 at 07:55
  • @barrycarter To clarify - the planets do not move in elliptical orbits with the Sun at one focus. The barycentre is at the focus. So if Kepler wrote that it was the Sun at the focus, then he did not propose that the Sun orbits a barycentre. – ProfRob Oct 17 '18 at 08:06
  • @RobJeffries I don't think this is correct. As https://spaceplace.nasa.gov/barycenter/en/ notes, "The sun, Earth, and all of the planets in the solar system orbit around this barycenter". To me, this means the barycenter is the center of the ellipse, ie, the midpoint of the two foci, and not one of the foci itself. The accepted answer to https://astronomy.stackexchange.com/questions/18169/what-point-does-earth-actually-orbit/26698#26698 (which may itself be wrong) suggests the Earth orbits the Sun, not the solar system barycenter. –  Oct 17 '18 at 18:25
  • @barrycarter I don't see how you extrapolate that from the statement you quote. The solar system barycentre is nowhere near the midpoint of the two foci of Mercury's elliptical orbit and the disproof of your version is easily seen by considering two similar mass stars in an eccentric orbit (e.g. https://en.wikipedia.org/wiki/Binary_star#/media/File:Orbit5.gif ) . Kepler's first law is approximately true if $m \ll M$. It is exactly true if you replace "Sun" with "barycentre". – ProfRob Oct 17 '18 at 18:52
  • @RobJeffries OK, I assume you're NOT saying Mercury's orbit is a perfect ellipse around the solar system barycenter (SSB)? Rather, you're saying using the SSB as one focus is more accurate than using the Sun as one focus? –  Oct 17 '18 at 19:10
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    @barrycarter Thinking... The best way to think about a small planet's orbit is as an orbit with the Sun at the focus with a secondary (smaller amplitude) orbit with a 12-year period around the Sun-Jupiter barycentre superimposed. I am withdrawing my suggestion that if you modelled Mercury's orbit as an approximate ellipse that the SSB would be the best focus. This would be exactly true for 2-bodies, is approximately true for Jupiter, but is not true for >2 bodies where a third body is much more massive. – ProfRob Oct 17 '18 at 19:57
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    @uhoh I think you have misinterpreted. It is quite the opposite. Mercury has an 88 day orbit essentially with the centre of the Sun at one focus (actually the Sun-Mercury barycentre). If you look on longer timescales the Sun then orbits the solar system barycentre. – ProfRob Oct 19 '18 at 12:02
  • @RobJeffries I got excited and somehow missed the word "withdrawing" in the sentence. I'll now do some withdrawing myself ;-) – uhoh Oct 19 '18 at 12:08

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Newton explained the motions of planets in the solar system in Principia in 1687.

This amongst many other things "defines the very slow motion of the Sun relative to the solar-system barycenter;" (https://en.m.wikipedia.org/wiki/Philosophiæ_Naturalis_Principia_Mathematica ).

Since Newton came up with the ideas of gravity and barycentres, surely this is the answer? (Though, I note there is some debate that the inverse square law may have been around and discussed during the previous decade).

Ideas that the Sun moved were of course commonplace before then.

ProfRob
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  • Not when the theory was discovered, asking more for when the actual motions were mapped. Aristarchus and so on had detailed the theory as well. Today, online, I can see detailed maps of the suns orbit around the barycenter, when did that begin to enter the scene? – pol0 Oct 17 '18 at 07:36
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    You should ask that then @pol0. Currently your question doesn't say that – Rory Alsop Oct 18 '18 at 22:05