| You've no doubt heard that Earth revolves around the sun. Well, actually, that's not quite true! |
| Have you heard the term "center of gravity"? It's a technical-sounding term for something pretty simple. It's the exact center of all the material (that is, mass) that makes up the object. For example, if you have a straight stick, like a ruler or yardstick, there's a place at the middle where you can balance it on your finger. That's its center of gravity. |
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| But the center of gravity may or may not be the point that is exactly in the middle, distance-wise, of the object. Some parts of the object may be heavier (denser) than others. If you have something like a sledge hammer that is heavier on one end than the other, the center of gravity will be much closer to the heavy end than the lighter end. |
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| To get an idea of where the center of gravity is, rest the ends of any object like the ruler or a pencil on one finger from each hand. Slowly move your fingers together without dropping the object. Your fingers will meet underneath the object's center of gravity. You can balance the object on one finger at that special place. |
| The actual center of gravity could be close to the surface or deep inside, depending on whether the object is flat like a ruler or a dinner plate, or "three-dimensional," like a box or a ball. And if you let the object spin (like when you throw it), it will try to spin about that point. |
| In the case of the Earth and the sun, both bodies actually revolve, or spin, around the very center of the mass (similar to center of gravity) between them. This point is called the "barycenter." Earth and the sun are "connected" by the gravity pulling them together. It's just like the light end and heavy end of the sledge hammer. Compared to the size of the sun, Earth is about like a flea on a cat! So the center of mass between the Earth and the sun is almost--but not quite--the very center of the sun. |
| In the case of a planet the size of Jupiter, which is 318 times as massive as Earth, the barycenter of Jupiter and the sun is a bit further from the sun's center. So, as Jupiter revolves around the sun, the sun itself is actually revolving around this slightly off-center point, located just outside its surface. Thus, a planet the size of Jupiter will make the sun (or any star) appear to wobble a tiny bit. This picture shows you that the center of mass and barycenter can be slightly different points. It isn't meant to be very accurate! |
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| We can take advantage of this bit of knowledge and look for large planets in other solar systems by learning to detect this type of tiny wobble in the star's position. |
For more on how we can do this, see Dr. Marc's amazing fact about the StarLight mission. |
