More than 300 years ago Isaac Newton pointed out that a powerful cannon on a hilltop could shoot a ball right around the world and back to its starting point - if only there were no air to resist it. At every moment, the ball falls towards the ground but it gets no closer because the Earth is round. The speed of the ball to accomplish this feat is 7.9 kilometres per second (more than 28,000 km/h) and 84.4 minutes elapse before an unwary gunner is hit on the back of his head.