of orbital derangement from an ellipse to a parabola or a hyperbola. Sir Isaac Newton showed that a body controlled by the Sun moves in a conic section—that is, an ellipse, a parabola or a hyperbola. Because the latter two are open curves, a comet which pursued such a path would go off into space never to reappear. A derangement of orbit from closed to open curve has doubtless happened often. References Non-periodic comets 1264 13th century in science 126407 Great comets