Heinrich Rantzau (1526–1598), the Danish king’s governor in the duchies of Schleswig and Holstein, was a multifaceted and fascinating personality. His diverse activities as a landowner, financier, entrepreneur, patron, builder, and politician have repeatedly occupied historians. As a leading representative of humanistic education and culture, he achieved European fame, and with his passion for books was considered a black swan among contemporary noblemen. Rantzau was well versed in astrology, aligned his entire life with the stars, corresponded with numerous important scholars of his time, and edited several astrological textbooks.The book examines Heinrich Rantzau’s relationship with astrology in detail for the first time and treats historical horoscopes as a special source genre in the history of science.