Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about those big, complicated words scientists use.  Today’s Sciency Word is:

## BARYCENTER

Excuse me, but I’m going to do that “um, actually” thing that people who think they’re really smart like to do.  Now you may think the Earth orbits around the Sun.  Um, actually… the Earth and Sun both orbit something called the barycenter.

The word barycenter comes from two Greek words meaning “heavy” and “center,” and it refers to the common center of mass for two or more celestial bodies.  Based on sources I found via Google Ngrams, the term started appearing frequently in astronomical journals during the early 20th Century, and it may have been in use as early as the 1880’s.

Let’s say you have two celestial bodies.  One is really massive, the other is much less massive.  In that case, the barycenter will probably be located somewhere inside the more massive object.  This is the case for the Earth and her Moon.  Based on numbers I got from Wikipedia, the Earth-Moon barycenter is about 1000 miles (1700 km) beneath Earth’s surface.  Or to measure that a different way, the barycenter is about 3000 miles (4600 km) away from the center of the Earth.

Now let’s say you have two celestial bodies of roughly equal mass.  In that case, the barycenter will be located somewhere between them.  Something like this has happened with Pluto and his giant moon, Charon.  Once more using numbers from Wikipedia, it looks like the Pluto-Charon barycenter is about 500 miles (960 km) ABOVE the surface of Pluto.

As for the Earth-Sun barycenter, it’s located deep inside the Sun.  So if you say Earth orbits the Sun, you’re not too far from the truth.  But of course Earth is not the only planet in the Solar System, and when you consider the Solar System as a whole, you’ll find the Sun wibbles and wobbles about in weird, loopy patterns.  As you can see in the highly technical diagram below, the Sun wibbles and wobbles so much it can wobble into a totally new position in just a few years.

As explained in this paper, this is mainly due to the gravitational influences of Jupiter and Saturn. Over longer time scales (centuries rather than decades), the subtler influences of Uranus and Neptune also have a noticeable effect.

So the next time someone tells you the Earth orbits the Sun, you can do the “um, actually” thing and explain what a barycenter is.  Trust me, it’s a great way to sound smart and make lots of new friends!

Next time on Planet Pailly, what did people in 1962 think we’d find on other planets?

### 9 responses »

1. Scott Levine says:

Great post. I sometimes like to try to break my brain and imagine the Sun wobbling and twisting as it deals with all of the zillions of barycenters it shares with everything in the solar system. Mercury pulling it a little this way, Earth pulling it a little that way, while the Moon pulls Earth a little in a different direction.

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• J.S. Pailly says:

It’s a lot to think about! There’s a good reason why the three body problem is so famously difficult!

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2. Those wibbles and wobbles turn out to be handy, particularly with other stars. It’s how we know about many of the exoplanets.

Liked by 3 people

• J.S. Pailly says:

That’s true. And if any aliens are watching, they might notice that Jupiter and Saturn are here, which is an interesting thing to think about.

Liked by 1 person

3. Kate Rauner says:

Ah ha! You’ve given me the phrase I need: Um, actually… all frames of reference are valid, but I’d hate to do the math!

Liked by 2 people

• J.S. Pailly says:

Glad to help, friend! And yes, all frames of reference are valid. Even a geocentric frame of reference is still useful for some things.

Liked by 1 person

4. Alien Resort says:

They should have told us this in school. One question–do you mean that the location of the Earth-Sun barycenter fluctuates due to the influence of the other planets?

Liked by 1 person

• J.S. Pailly says:

I’m glad you asked, because I was a little confused about that myself. I believe the Earth-Sun barycenter is stationary relative to the Earth or the Sun. Also, the Jupiter-Sun barycenter would be stationary relative Jupiter or the Sun. Same for the Saturn-Sun barycenter, or the Pluto-Sun barycenter, etc. If you’re calculating the barycenter for the Sun and just one planet, the barycenter should be stable. But once you start adding more planets to your calculations, the barycenter starts moving around. Does that make sense?

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• Alien Resort says:

Or that calculations that are based on your two body barycenter will need to be adjusted.

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