Center of Mass, The Barycenter & Orbital Motion

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  • 0:02 What Orbits What?
  • 0:51 Barycenter: Center of Mass
  • 2:35 Wobbling of Our Sun and Stars
  • 5:03 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will tell you what orbits around what: the sun around the Earth or the Earth around the sun. The same goes for the moon and Earth. Does the moon really orbit around the Earth?

What Orbits What?

Want to become a trivia champ? Sure you do. Everyone does. I'll give you two questions, right now, which you might need to know for some high caliber trivia. Here they are:

Question #1: What does the moon revolve or orbit around?

Question #2: What does the Earth orbit around?

If you're like most people, the answers to these questions will be the Earth and sun, respectively. We tend to think of a smaller celestial body orbiting a larger one. The moon orbits the Earth and the Earth orbits around the sun.

For simplicity's sake, that's kind of what we say, and that's it.

The detailed truth of the matter is different, however. The Earth doesn't technically orbit around the sun, and the moon doesn't orbit around the Earth either.

Barycenter: Center of Mass

Instead, two celestial objects orbit each other around their common center of mass, or balance point of a system. In astronomy, the center of mass of two or more celestial bodies is sometimes called the barycenter.

This kind of interaction happens because the force of gravity is mutual. As one celestial body pulls on the other, the other responds in kind until a balance point is found.

Let's demonstrate the concept of a center of mass with a couple of familiar examples. Grab a ruler from somewhere. Try and balance it on your finger. You'll need to place the finger smack in the middle of the ruler to balance it. That's because its center of mass is at its center since it's a uniform ruler. Neither end is heavier than the other end.

Contrast this with a hammer. If you put your finger in the middle of the hammer to balance it, what'll happen? The hammer will tip and fall over. Make sure it doesn't hit your foot on the way down by the way!

This happens because the hammer's center of mass is somewhere else because one end is made of heavy metal and the other is of light wood. You'll need to place your finger really close to the heavier, more massive metal end, in order to balance the system, so to speak. Thus, the common center of mass is very close to the metal end in this example.

In some cases, like a box or ball, the center of mass may actually be inside the object itself.

What these two examples should've taught you is:

  • Two celestial bodies of different masses will balance at their common center of mass
  • The center of mass will be located closer to the more massive object

The Wobbling of Our Sun and Stars

In the case of the moon and Earth, where the Earth is more massive than the moon, the common center of mass is actually inside the Earth. So as the moon orbits this center of mass on one side, the Earth actually moves around this same center of mass on the other side.

Conversely, because the sun is so much more massive than Earth, the center of mass of the sun-Earth system is deep inside the sun, almost at its center. And so, that's why we just stick to saying the Earth orbits around the sun for simplicity's sake.

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