Astronomy

Long before 16th-century astronomers began to suggest changes in the concept of the universe, theologians such as Nicholas Cusanus (1401-64) described an endless, infinite universe consisting of many inhabited worlds. Cusanus also believed that the Earth rotates and is not immovable. The radical Dominican friar Giordano Bruno (1548-1600) went so far as to teach that the Sun was only a star among many such stars, in multiple solar systems expanding into the universe. His belief in innumerable planets similar to the Earth revolving around many Suns was one of the heretical notions for which Bruno was burned at the stake. CounterReformation policies and the power of the Inquisition threatened astronomical progress for more than a century.

The science of astronomy, assiduously studied by ancient Greeks, treats the location, movement, and size of heavenly bodies as physical facts. Astrology (see later discussion) studies the possible influence of celestial bodies on earthly events. A Renaissance astronomer might predict the precise date of a planetary conjunction; an astrologer would interpret what the conjunction might mean for a specific individual or town. Astronomy had its origins in ancient cosmology; medieval cosmographic illustrations to Ptolemy’s Almagest and other works showed an immovable Earth at the center of the universe. The days, years, and seasons of our world were explained as movements of the Sun, stars, and planets. Because Copernicus based his discoveries of the heliocentric (Sun-centered) universe on arguments against the Ptolemaic system, Epitome in Almagestum Ptolemaei (Summary of the Almagest of Ptolemy), published in 1496, was an important starting point. That work was partially edited by Georg von Peurbach (1423-61), whose own research on planetary movement was printed several times during the 16th century. The Epitome was completed by Peurbach’s disciple Regiomontanus (1436-76).

Copernicus began formulating his theories more than two decades before they were published. Anomalies in planetary movements and other celestial events persuaded the astronomer that something was wrong with the current understanding of the cosmos. When he positioned the Sun at the center of our planetary system, several of the inconsistencies and errors were removed. This correction, in Copernicus’s opinion, restored celestial harmony: “We find, therefore, under this orderly arrangement, a wonderful symmetry in the universe, and a definite harmony in the motion and magnitude of the orbs, of a kind it is not possible to obtain any other way” (Ross and McLaughlin 1968, p. 596). By defining the heliocentric system, Copernicus described the Earth as a planet that rotated daily on its axis and revolved around the Sun each year. His drive for perfect harmony, however, held him to the concept of concentric spheres, with circular orbits for the planets. Elliptical orbits would be revealed in Kepler’s reforms of the Copernican system in the early 17th century.

Kepler was a student of the brilliant Danish astronomer Tycho Brahe (1546-1601), who built an observatory on an island given to him by the king of Denmark, designing his own instruments. Remarkably, Renaissance astronomers accomplished their discoveries without the aid of a telescope, which was not invented until the 17th century.