William Lilly was a prominent 17th-century English astrologer, and author of Christian Astrology (1647), the first widely circulated astrological textbook available in the English language and  the most comprehensive and important historical textbook on horary. At the height of his career, Lilly is reputed to have judged over two thousand charts a year, and his annual almanac sold over 30,000 copies annually. Lilly's astrology peaked during the period of the English Civil War, during which he advised both King and Parliament. He was also credited with predicting the Great Fire of London (1666) fifteen years before the event. 

Things were not always peaceful for Lilly, and he was thrice tried in a court of law for his astrological prognostications. In later life, he retired with a reasonable fortune to the countryside of Hersham, Surrey, where he practiced medicine for local townsfolk by donation only. His autobiography, published shortly before his death, relays Lilly's close associations with many other leading astrologers of his time – most notably, Elias Ashmole, his patron and friend; Henry Coley, his adopted son; and, John Gadbury, his student turned adversary.