Gravity is quite an easy concept to understand. If two bodies have mass they attract each other. They move together as a system. We understand very clearly how this works and have some simple equations to enable us to work out the forces involved. It’s simple enough for school children to understand.
So, two bodies, very simple. So how much harder can it be if three bodies are involved? It turns out that the answer is “very”. Working out the movements of three bodies (eg the Sun, the Earth and the Moon) as a gravitational system relies on approximations and numerical methods. Four body problems (which arise when adding a spaceship to the mix, say) rely on ignoring the presence of one or more of the bodies in order to create a set of solutions.
Solutions to multi-body problems rely on breaking them down into groups of parameters within which…
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