Galileo's observations of the moons of Jupiter made him realise that not everything orbits the Earth. Once Newton had laid down the principles of gravity which determine the orbits of the planets, Halley was able to extend them to comets and predict the return of the comet that bears his name. The telescope led to a search for new planets and the discovery of the asteroids and outer planets, a quest that is continuing today. Read by Timothy West, Annette Badland, Robin Sebastian, Julian Rhind-Tutt and John Palmer.