Copernicus and the decline of astrology

by Salman Hameed

Astronomy and astrology got separated out a few centuries ago. Today, we - astronomers - only get a bit annoyed when someone calls us astrologers. Ugh. Many of the leading Muslim scholars in the medieval times also worked as astrologers (this was a respectable field). The foundations of Baghdad were laid by Abbasid Caliph, Al-Mansur, based on an auspicious day (July 30, 762 C.E.) based on Zoroastrian astrology. And yes, cities used to have horoscopes as well. London and Paris had horoscopes as well. The horoscopes would predict the weather and the fires - both highly unpredictable in the medieval times - in the large cities.

So it comes as no surprise that Copernicus was also interested in astrology. What is fascinating, however, is that some of his motivations for a heliocentric universe may also have been due to astrology. A new book by Robert Westmnan, The Copernican Question: Prognostication, Skepticism, and Celestial Order, addresses this question directly. Here are a couple of excerpts from a review in Science:

Robert S. Westman has now brought us a hefty and enormously erudite treatment of Copernicanism. While nonetheless wearing its learning lightly, The Copernican Question presents a historical picture that puts Copernicus where he belongs: in his own time and place. Copernicus was a 16th-century astronomer in a European world where astronomy and astrology were not really separate in either disciplinary identity or respectability. “The science of the stars,” as Westman compendiously dubs the endeavor in which Copernicus participated, sought an understanding of the physical universe that incorporated the effects of the heavens upon the Earth, whether in the form of regional prognostications concerning such things as the weather, famines, plagues, and wars or predictions concerning individual people (often called “judicial” astrology, although these categories and labels were endlessly variable). Astral effects on the Earth were taken as a given: the great ancient authority Ptolemy had not only written the geocentric masterpiece of mathematical astronomy, the Almagest, but also the astrological Tetrabiblos, the latter as fundamental in its way in this period as the former. Copernicus, Westman argues, had become immersed in the prognosticatory aspects of the science of the stars while a student in Bologna, and he brought this understanding of the basic astronomical problematic back with him to Polish Prussia. 

He also brought back knowledge of considerable difficulties. Giovanni Pico della Mirandola's Disputationes adversus astrologiam divinatricem (“Disputations against divinatory astrology”) had appeared in 1496, and the work's arguments against astrological prediction remained current for over a century. Part of Pico's criticism concerned the theoretical mathematical astronomy of the planets, on which astrological forecasts depended. Ptolemy's geocentric Almagest and Tetrabiblos, on which astronomers, Greek, Arabic, and Latin, had based their work, discussed important themes concerning the order of the planets: How were the sizes of the orbs that carried them to be determined? From a central Earth, one can only measure the direction of a planet, not its distance. Ptolemy had simply assumed that the longer a planet appeared to take in completing its celestial circuit again the backdrop of the stars, the further away it must be. But this rule ran aground in the cases of Mercury and Venus, because each takes, on the average, the same time for its circuit: one year, the same time as that needed by the Sun. Although the other planets took longer, how could any distinction of distance from the Earth be made for these two? Pico identified this as a problem for the prognosticators because knowledge of the ordering of the planets played a crucial role in determining the astral qualities possessed by each. As the Tetrabiblos showed, which bodies neighbored which others affected the properties of each and hence the effects of each on terrestrial affairs. If this knowledge was compromised by uncertainty, so would be the reliability of the astrological forecasts made on its basis. 

Westman thus makes a concern with the reliability of prognostications central to Copernicus's new approach, which claimed as one of its great advantages the ability to measure the relative distances of the planets from the new center of the universe, the Sun. This could now be done by using the Earth-Sun distance as a triangulatory baseline: now that the Earth moved, planetary directions could also yield distances. Hence, one of Pico's serious attacks on the divinatory potential of the science of the stars had been disarmed. Copernicus's close follower and promoter, Georg Joachim Rheticus, provides Westman with good evidence for Copernicus's concern with these questions despite the master's own silence on prognosticatory issues (itself convincingly explained in terms of the contemporary discourse of disciplinary subdivisions in astronomy).

And a few decades later, Kepler was also looking at the motion of the planets to detect the music of the spheres. But it was Newton that, in many ways, ended the effectiveness of astrology. This is particularly ironic, as Newton himself was also interested in all sorts of occult sciences, alchemy, and some unorthodox religious worldviews: 

Kepler's physico-theological ideas for explaining the motions and form of the universe included attempts to restructure the physical basis of astrological prognostication. But, ironically, The Copernican Question presents the decline of astrological forecast as the long-drawn-out consequence of the increasing success of Copernican astronomy in the decades following Kepler's Rudolfine Tables of 1627. The distinctions between astrological practice and its theoretical substructure became too problematic, too difficult for a Copernican science of the stars. Newton's work answered the Copernican question in a way that simply ignored astrology.

Read the full review here (you may need subscription to access the article).