Sciency Words: Barycenter

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about those super weird (but super cool) words scientists like to use.  Today’s Sciency Word is:


Tell me if you’ve heard this one: every action has an equal and opposite reaction.  This is true even for moons orbiting planets, or planets orbiting stars.  Whenever a star exerts gravitational force on a planet, that planet exerts an equal and opposite gravitational force on the star.  As a result of this ongoing gravitational tug-of-war, we end up with a planet and a star spinning round and round their common center of mass, a point which scientists call a barycenter.

Definition of barycenter: In astronomy, a barycenter is the center of mass of two or more objects in space that are gravitationally bound together.  

Etymology of barycenter: The word barycenter traces back to a Greek word meaning “weighty” or “heavy.”  The word barometer has a related etymology (barometers measure atmospheric pressure—the “weight” of the atmosphere, in other words).

Sometimes a barycenter will be located deep inside the more massive of two celestial bodies, in which case the more massive body will appear to wobble in place.  This is the case for the Earth and the Moon.  The Earth-Moon barycenter is approximately 1700 km beneath Earth’s surface.  Other times, the barycenter will be somewhere in the empty space between objects.  For an example, look at Pluto and its largest moon, Charon.  The Pluto-Charon barycenter is more than 900 km above the surface of Pluto.

The concept of a barycenter dates back to Isaac Newton (though I can’t find any sources saying he coined the word, nor could I find any evidence that he ever used the word himself).  Newton’s Principia Mathematica, originally published in 1687, briefly discusses the Sun-Jupiter barycenter, saying, “[…] the common centre of gravity of Jupiter and the sun will fall upon a point a little without the surface of the sun.”  Newton also discusses the Sun-Saturn barycenter, which he describes as “[…] a point a little within the surface of the sun.”

And then there’s the barycenter of the Solar System as a whole: the “common centre of gravity of all the planets,” as Newton calls it.  Due to the combined gravitational forces of all the planets (most especially that of the giant planets: Jupiter, Saturn, Uranus, and Neptune), the Sun is constantly being pulled in multiple directions at once.

As a result, the Sun does not sit still in the middle of our Solar System.  It is “agitated by perpetual motion,” to quote Newton one last time.  Sometimes, as the Sun moves about, it happens to pass through the Solar System’s barycenter. Other times, it loops and spirals around the barycenter, as if performing an elaborate dance.


Here are a few articles that go into a little more detail about barycenters:

And here’s a link to the translation of Newton’s Principia Mathematica that I quoted in this post.  The relevant section is titled “Proposition XII.  Theorem XII.”