The unexplained marginalization of astrology

As a retired physicist, I like to read about the history of physics and of science generally. Since one type of history depends on another, I also read about political, religious, and other types of history.

Scientists of the past had to live under conditions I find hard to imagine. Galileo was threatened with torture and death by the "Holy Office" in Rome for the unforgivable sin of teaching that the earth goes around the sun. Kepler had to work like hell to defend his mother from a charge of witchcraft. Kepler didn't doubt the existence of witches, he just denied that his mother was one of them. Thus history of science is mixed up with history of religion and superstition.

The history of astronomy has an even more intimate relationship with the history of astrology. Astronomers like Copernicus, Tycho Brahe, Galileo, and Kepler lived in an age when astronomy and astrology were seen as indivisible parts of a whole. Astronomers were expected to, and did, cast horoscopes. King Frederick II of Denmark supported Tycho Brahe so that Tycho would cast horoscopes for the king's children. One of those children was crowned in 1596 and promptly kicked Tycho off the island, Hven, where Tycho had built an observatory and compiled observations of unprecedented accuracy. Tycho bounced around Europe for a while and ended up at Prague, where he met Kepler. That meeting led to Kepler's first two laws of planetary motion.

The scientific revolution of the seventeenth century coincided roughly with a marked decline of scholarly interest in astrology. Astronomy was no longer tied to astrology at the time of Christiaan Huygens and Isaac Newton, neither of whom ever cast a horoscope. It's tempting to say that advances in astronomy made astrology seem ridiculous, but historians have not discovered any evidence for this interpretation. I would like to know how and why astrology had been marginalized by the about 1650.

A conference on 'The Marginalization of Astrology in Early Modern Science and Culture' was held in Utrecht in 2015. Despite the extensive scholarship made evident by this conference, the causes of this marginalization remain unclear.

One of the papers from this conference begins with a telling anecdote:

"In 1659, the famous astronomer and mathematician Christiaan Huygens was asked to cast a horoscope for one of the princesses of Orange. The princes of Orange were his family’s main patrons and Huygens was therefore hardly in a position to turn down the request. However, he had to admit that not only he did not believe in astrology, but even that he had never occupied himself with it. He knew nothing better to do than to pass on the request to his friend Ismael Boulliau.

"It is perhaps not very surprising that Huygens felt skeptical about astrology, but it is remarkable that he was ignorant about it. After all, Huygens had had a full training in mathematical and astronomical theories and techniques. He had been taught at The Hague by Stampioen, at the University of Leiden by Frans van Schooten Jr, and later at Breda he had been familiar with John Pell. Yet in spite of all this training, he was not capable of casting a simple horoscope. Apparently, by the middle of the 17th century it was possible to be a first-rate astronomer and mathematician and not have the haziest idea about astrology.

"Less than 50 years earlier, astrological techniques were a self-evident part of mathematical training at Dutch universities. When in 1607 Isaac Beeckman asked the professor of mathematics at Leiden, Rudolf Snellius, for advice on the study of mathematics, Snellius gave him a list of books to study that included not only works of “Astronomia” (“Ptolomaeus, Copernicus”), but also “Astrologia” (“Ptolomaeus, Hermes”). Snellius praised the history and dignity of astrology in the preface to his commentary on Cornelius Valerius’s book on spherics."

- Rienk Vermij, University of Oklahoma, USA
http://journals.sagepub.com/doi/abs/10.1177/0073275314529862