There is a missing chapter in the history of astronomy--between the work of Galileo and Newton--and it is a chapter that belongs to England. In the period before the English Civil War, Horrocks was the greatest astronomer in the kingdom. He knew the positions and motions of the planets more accurately than any person of his time, and was the first to appreciate the true scale of the solar system and formulate a valid theory for the wanderings of the moon. Yet he was not an elderly grey-bearded sage, but a young man living in provincial obscurity, who on his death had barely come of age but who left a great scientific legacy.