TNB Night Owl – The Barycenter

A montage of Saturn and Jupiter.
A montage of Saturn and Jupiter. Image captured by the News Blender.

When you were a small child (most likely of preschool age) did an adult or teen hold you by your hands and spin you around in circles? It was sort of like flying like Superman or Superwoman, only sideways! After you became an adult (or at least a teenager) you might have spun a toddler around like this. As you spun around, centrifugal force kept the child’s feet up in the air and wanted to fling the child away from you. To compensate and balance out (or cancel out, if you prefer) the centrifugal force, not only do you have to hold on tight to the child’s hands, you also must lean back a little, so your shoulders are no longer directly above your feet (as they would be if you were not spinning a small child around in circles).

When you spin around by yourself, your center of mass is directly above your feet, otherwise you’d lose your balance and fall down like some kind of lonely clown. (Hey, I’m feeling lyrical tonight). When you’re leaning back and spinning around with a child extending outwards from your arms, the center of balance is between you and the child. The heavier the child, the further you must lean back. You and the child balance each other out. The two of you spinning around one another share a combined center of balance or center of mass.

So it is with the Sun and planets. For a moment let’s consider only Jupiter, the most massive planet in our Solar System. The Sun is holding on to Jupiter with gravitational force (the equivelant of an adult holding on to a child’s hands as the child spins around). But Jupiter has alot of mass and gravitational pull too, so Jupiter is pulling on the Sun as well. The balance point is where the combined center of mass is (or center of gravity, if we aren’t sticklers for accurate terminology). Effectively, the Sun and Jupiter both revolve around this point, which is called the barycenter.

Strictly speaking, Jupiter does not revolve around the Sun, it revolves around the barcenter it shares with the Sun. Likewise, the Sun revolves around that barycenter as well. Get that? The Sun is not stationary! The Sun wobbles!

The barycenter is much closer to the Sun than Jupiter is, because Sol has approximately 1,000 times more mass than the gas giant. (In point of fact, the barycenter is often located within the Sun, but not always – the Sun is constantly moving in relation to the barycenter). The following two short videos may help you visualize it.

“Everything in the Solar System orbits the center of mass (it’s rarely in the center of the Sun!)” (0:56):

“Jupiter does not orbit the sun” (1:23):

Saturn is not as massive as Jupiter, and it’s much further away from the Sun, but it still exerts a gravitational pull on old Sol. In fact, every planet and moon in the Solar System exert a pull on the Sun and the Sun pulls on them. Combined, the sum of the Sun, planets, and smaller bodies, create a single center of mass called the Solar System barycenter. Our Sun wobbles while it revolves around the Solar System barycenter. This is why astronomers know that a distant wobbling star means it has at least one massive planet in its orbit, which has led directly to the discovery of many exoplanets.

This next (and last) great little video puts it all together.

“SkyMarvels™ SOLAR SYSTEM BARYCENTER (celestia celestia4all)” (4:09):

Question of the Night: What was the most fun thing you did when you were a child, that you’re too big to play at now?

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About Richard Doud 622 Articles
Learning is a life-long endeavor. Never stop learning. No one is right all the time. No one is wrong all the time. No exceptions to these rules.