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:
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?