Was Newton the first to mention the orbital barycenter? A barycenter is the common center of mass of an orbiting system. Here is a illustrative gif from wikipedia:

The first mention of something like a barycenter that I could find is in a translation of Newton’s Principia, published in 1687, proposition 12, theorem 12:
https://en.wikisource.org/wiki/The_Mathematical_Principles_of_Natural_Philosophy_(1846)/BookIII-Prop2.
This includes the quote that: “the common centre of gravity of Jupiter and the sun will fall upon a point a little without the surface of the sun” which to me demonstrates a clear understanding and articulation of the idea of a barycenter. Does anyone know of an earlier clear reference to a barycenter?
Related: What are Kepler&#39;s laws (as he wrote them)?
I'm pretty sure Newton was the first to apply the notion of the barycentre to celestial motion, although the concept of a centre of mass may predate him.
Before Newton, there was a very strong demarcation between celestial motion and terrestrial motion, inherited from the Greek philosophers like Aristotle. (Similar philosophies applied outside the Western world). Celestial bodies had a celestial nature, and moved according to celestial principles, eg, they were perpetually in motion, and that motion was composed of perfect circles. Bodies on the Earth obeyed a different set of laws appropriate to their earthly nature. The essential quality of the earth element is to move towards the centre of the universe and to not otherwise move, unless something else interferes. So terrestrial objects didn't move unless they fell, or were pushed in some way, and would soon stop moving.
Newton did a lot of great scientific and mathematical work, but his outstanding insight was that a single scheme could describe both terrestrial motion and celestial motion. And that is the &quot;core&quot; point of the anecdote about Newton seeing the falling apple. Prior to Newton, the very idea of attempting to apply mundane mechanics to the heavens would have seemed strange to most natural philosophers, if not downright blasphemous.