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The centre of mass of a system of massive bodies. The system orbits around its barycentre. In a system of two bodies of equal mass, the barycentre lies midway between them. If the masses are unequal, the barycentre will lie closer to the greater mass. If one mass is much greater than the other, the barycentre may actually lie within the body of the larger mass, as in the case of the barycentre of the Earth–Moon system which lies about 1600 km below the surface of the Earth. The barycentre of the Solar System lies just outside the Sun's surface, so that the Sun actually performs a complicated orbit about that point.

Subjects: Astronomy and Astrophysics.

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