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:
BARYCENTER
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.
WANT TO LEARN MORE?
Here are a few articles that go into a little more detail about barycenters:
- What is a Barycenter? from NASA Space Place.
- Our Wobbly Solar System, from Medium.
- Astronomers Have Located the Centre of the Solar System to Within 100 Metres, from Science Alert.
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.”
Planet Pailly 
Excellent example of how common-usage can embed the wrong idea in my head – the Moon orbits the Earth, the Earth orbits the Sun. Close enough for most of my thinking, but not quite right.
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I think it’s fine to say it that way in most contexts, even if it’s a little inexact. Although when it comes to a system like Pluto and Charon, or Alpha Centauri A and Alpha Centauri B, it’s a lot harder to ignore the importance of the barycenter.
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Maybe instead of a planet and moon, we should say double planet… or double dwarf planet. Asteroids too, I suppose.
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As I recall, the I.A.U. considered reclassifying Pluto (and Charon) as a binary planet before settling on dwarf planet. I kind of wish they’d done that. That would have been cool.
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Interesting! I didn’t realize our sun moved so much.
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It’s wild, isn’t it? The Sun’s movement seems pretty chaotic at first, too, but there are recurring cycles of motion that match up with the orbital periods of the four giant planets.
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Might be jumping ahead here, but it’s really cool how astronomers found a way to use this to detect exoplanets via doppler spectroscopy. Science is crucially dependent on empirical observation, but also logical reasoning.
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I agree. It is really cool. The part that kind of surprised me was that scientists knew about this as far back as Newton. Obviously telescopes we’re sensitive enough back then to detect the wobble of distant stars, but I kind of just assumed the whole wobble method was based on more recent discoveries.
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Wasn’t precision limited with equipment prior to the 1990s? I thought I read somewhere that it only became viable around that time, even though the theoretical possibility had been discussed earlier.
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Sounds right to me. I just assumed, prior to researching this post, that the idea of the Sun or any other star wobbling around like that must be a 19th or 20th century development. I didn’t realize it dated all the way back to Newton.
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There was a similar issue with stellar parallax, using movement of a star’s position in the sky over the course of a year due to the Earth’s orbit to measure the distance to that star. People in the 1500s cited the lack of measurable parallax as an issue with heliocentrism. Even though heliocentrism was widely accepted by the late 1600s, no one had instruments precise enough to measure a star’s distance until c. 1832.
Sometimes the tech takes a long time to catch up with theory. Something I try to remember when it’s difficult to imagine a particular experiment ever happening.
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The combination of the moon and the sun within the close proximity of Earth is the reason we’re still here. Just look at all the craters on the moon caused by asteroids.
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The Moon has definitely taken a few hits for us in the past. Though I’ve heard more and more skepticism about the idea that having a big moon like our Moon is necessary for life. Life on other worlds may get along just fine without that extra protection.
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