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Name: Erin
Status: educator
Age: 30s 
Location: N/A
Country: N/A
Date: 1999 - 2000

I recently had a student ask me what held the sun in the posistion that it is in. So my question is... What does keep the sun where it is? Or does it actually move in relation with the earth and all of the planets around it.

The sun is doing several things at once. It's orbiting the center of mass of the galaxy, along with all the other stars in our galaxy. This is so slow that we can't see it.

Also, the sun is moving very slightly in response to the motion of the planets. This is easy to understand in a system that includes only a star and one planet, but very complicated when more than two bodies are involved.

In the two-body problem, both bodies orbit the center of mass of the system. If the two bodies have the same mass, the center of mass is right in the middle, between them. If one body is more massive, the center of mass is closer to the more massive body, so that guy moves in a smaller orbit. In the extreme case of a star and a planet, the star is so much more massive than the planet that it moves only very slightly. Nevertheless, this motion is measurable -- astronomers use it to detect planets around distant stars. What they actually measure is the varying Doppler shift of the light from the star, as the star moves toward and away from us. Clearly, this technique works only if we're in or near the plane of the orbit.

Tim Mooney

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