Reduced mass

Reduced mass is an idea from physics. The two-body problem is a problem of two bodies influencing each other. It can be difficult to solve. With reduced mass, the two-body problem is changed to a one-body problem, which is much simpler to solve. The idea is to reduce the mass of one of the two bodies, and to increase the mass of the other.

An example where reduced mass was used was the development of Heliocentrism. All the planets move around the sun. When calculating, people usually don't take into account that the planets also influence each other, or that a given planet and the sun influence each other.