Joannes de Sacro Bosco, also known as John Holywood, was a 13th century mathematician and astronomer associated with the University of Paris. Originally composed in 1220, Sphaera mundi deals with the shape and apparent movement of the earth and other heavenly bodies and today is accounted the single most influential astronomy text ever published. This text and works subsequently informed by it would have been an essential tool for any medical astrologer, as an exceptional knowledge of the movement of planets and stars was a prerequisite for such a practice. In this image, a hand holds an armillary sphere, a model depicting the essential reference circles used for mapping celestial phenomena (and are still used today in astronomy and navigation).The armillary sphere presumes that the sky is a sphere with all celestial objects located on it. The sphere shows the earth at the center and projecting outward are the celestial equator (a projection of the earth's equator onto the celestial sphere), celestial north and south poles, and even the Tropics of Cancer and Capricorn. The band stretching from the upper left to the lower right, containing symbols of the zodiac, is the ecliptic, or the apparent pathway of the sun, moon, and planets against the background of stars. Hand-held armillary spheres were used by medieval scholars to visualize the celestial sphere and compute movements. The standard Renaissance university curriculum was known as the quadrivium of four subjects, including astronomy, arithmetic, music, and geometry. Astrological calculation employed at least three of these subjects.