On the face of it, it seems inexplicable how Kant could apply Newton’s theory of the tides to the fate of Earth’s axial rotation. But in the Spin-Cycle essay (1754), Kant arrived at the right result for the right reasons. Newton showed that the primarily lunar gravity acts on ocean tides. This action, Kant argues, constitutes a retarding moment on the Earth’s surface (1:187); this retardation, he infers, slows down the Earth’s rotation (1:188); and the lunar brake only lets go, he concludes, when days are as long as lunar months (1:190). He found the solution despite multiple handicaps: minimal data, unimpressive formal skills, and no instruments.