Medieval and Renaissance astrology

What it is

The long period — roughly the eighth to the seventeenth centuries — in which astrology was not a subculture but an institution: taught in universities, integral to medicine, employed by courts, and argued about by the best minds available.

Where it came from

The Hellenistic material reached the Islamic world through Syriac and Persian intermediaries and was transformed there. Baghdad in the eighth and ninth centuries is the pivot.

Māshāʾallāh ibn Atharī, a Jewish Persian astrologer, was among those who elected the foundation moment for Baghdad in 762. Abū Maʿshar (Albumasar, 787–886) wrote the Great Introduction, which became the standard theoretical text and did more than any other work to give astrology a philosophical justification in Aristotelian terms. Al-Kindī worked on the theory of celestial rays.

Al-Bīrūnī deserves a particular mention: he wrote an outstanding astrological manual, the Book of Instruction in the Elements of the Art of Astrology (1029), while being personally sceptical of the judicial parts. A first-rate scientist producing the definitive textbook for a practice he doubted is a combination worth noticing.

The material returned to Latin Europe through the twelfth-century translation movement centred on Toledo — Gerard of Cremona, John of Seville, Adelard of Bath — and entered the universities with the rest of the Aristotelian corpus.

How it is practised

Medicine was the main application, and this is the part most surprising to modern readers. Physicians were expected to know astrology. The Zodiac Man — melothesia — assigned each sign to a body part, Aries the head down to Pisces the feet, and dictated that you should not cut into a part while the moon occupied its sign. Bloodletting, purging and the administration of medicines were timed by lunar position; the course of a disease was judged by "critical days" reckoned astrologically.

The most consequential single instance came in 1348. Commissioned by the King of France to explain the plague, the Paris medical faculty produced a report attributing it to a conjunction of Saturn, Jupiter and Mars in Aquarius in March 1345, which had corrupted the air. It was the official scientific explanation of the Black Death, issued by the leading medical authority in Europe.

Politics was the other application: court astrologers, elections of moments for coronations and battles, and mundane astrology reading conjunctions for the fate of nations.

What we can and cannot say

We can say astrology in this period was mainstream and institutional. Anyone describing it as a fringe practice that science later dislodged has the history wrong — it was in the universities, in the medical faculties, and at the courts.

We can say the Church's position was more complicated than either "condemned" or "endorsed". The distinction drawn was between natural astrology — weather, agriculture, medicine, physical influence — which was generally acceptable, and judicial astrology, which predicted human choices and therefore threatened free will and divine providence. Sixtus V's bull Coeli et Terrae (1586) condemned divinatory astrology outright. Popes nonetheless employed astrologers, and the gap between doctrine and practice was wide.

And the most important thing in this article: the strongest pre-modern critique came from inside. Giovanni Pico della Mirandola's Disputationes adversus astrologiam divinatricem (1494) is a sustained demolition by a Renaissance Neoplatonist — a man deeply committed to occult philosophy, magic and hermetic thought, who nonetheless concluded that judicial astrology was baseless.

His arguments anticipate most of the modern ones. The constellation boundaries are arbitrary human conventions. Twins born moments apart have different fates. Thousands die together in a battle or a shipwreck with wildly different charts. Astrologers disagree with each other and cannot resolve their disagreements. Predictions are vague enough to fit any outcome. Where celestial influence is real — the sun's heat, the moon's tides — it is general and physical, not specific to persons.

Pico wrote this in 1494, before Copernicus, before the telescope, before the scientific revolution. The case against astrology did not require modern science. It required someone willing to press the questions.

We cannot say the tradition was intellectually careless. It was defended by serious people with sophisticated arguments, and the fact that Pico's objections did not settle the matter says something about how deeply embedded it was.

Further reading

• Pico della Mirandola, Disputationes adversus astrologiam divinatricem (1494) — the critique that came from inside the Renaissance occult tradition.
• Al-Bīrūnī, Book of Instruction in the Elements of the Art of Astrology (1029).
• Roger French and others on the Paris faculty's plague report, for how astrology functioned as official medicine.