The three-body problem also happens in astronomy. The problem consists in calculating the course of three bodies, that influence each other with gravitation. The first to state the problem was Isaac Newton, in "Principia". Usually, two of the bodies are large, and the third is small. In the case where the two bodies have the same gravitational force, and that the bodies all have the same mass can be solved exactly. If this is not the case, the problem is solved through iteration and approximation. Many different patterns of motion can occur.