In this book, we walk the path of medieval science with a real-life guide, a fourteenth-century monk named John of Westwyk—inventor, astrologer, crusader—who was educated in England's grandest monastery and exiled to a clifftop priory. Following the traces of his life, we learn to see the natural world through Brother John's eyes: navigating by the stars, multiplying Roman numerals, curing disease and telling the time with an astrolabe. We travel the length and breadth of England, from Saint Albans to Tynemouth, and venture far beyond the shores of Britain. On our way, we encounter a remarkable cast of characters: the clock-building English abbot with leprosy, the French craftsman-turned-spy and the Persian polymath who founded the world's most advanced observatory. An enthralling story of the struggles and successes of an ordinary man and an extraordinary time.