Quote:
Originally Posted by Brett
Proof looks good to me. Do you know why it is so often written that the result only holds if the third mass is light? Is it that the configuration was discovered (by Lagrange) by considering a test mass?
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L was supposedly looking for places in the rotating reference frame where the attraction of the two bodies and the centripetal acceleration summed to zero. The amount of mass of a third body or its absence would not alter the solution, but enough mass would invalidate the assumption that the second body is in a stable, near circular orbit around the first.