This year Einstein's theory of general relativity celebrates its one hundredth birthday. It supersedes the non-relativistic Newtonian theory of gravity in two aspects: i) there is a limiting velocity, nothing can move quicker than the speed of light and ii) the theory is valid in arbitrary coordinate systems. While point i) is by definition the necessary difference between relativistic and non-relativistic theories, one might wonder if there exists a version of Newtonian gravity thatallows point ii), a theory of non-relativistic gravity that is invariant under general coordinate transformations. Indeed, such a theory was constructed a few years after Einstein's theory of general relativity and it is calledNewton-Cartan gravity. This theory ...