Two sciences with one name
A modern reader separates astronomy, the study of how heavenly bodies move, from astrology, the claim that those movements affect life on earth. Medieval scholars did not draw the line so neatly. Both subjects belonged to the quadrivium, the group of four mathematical arts taught in schools, and the same words were often used for both. A person who calculated where Mars would stand next month was an astronomer in one sense and an astrologer in another.
The ideas rested on a picture of the universe inherited from ancient Greek thinkers and passed on through Arabic texts. Earth sat at the center, surrounded by the moon, the sun, the known planets and the fixed stars. The heavens were thought to be orderly and, in some way, to influence the changeable world below. Observing the heavens was therefore both a mathematical task and a practical one, since it promised to explain what happened on earth.
Where astrology came from
Much of the technical content came from the Islamic world. Texts on casting horoscopes, predicting seasons and choosing good moments for action were translated into Latin, especially in the twelfth century, and spread through universities and courts. These works added detailed rules, tables and the idea that each planet had its own character. The instruments and tables behind them also helped astronomy, because predicting planetary positions required real mathematics.
Medicine and the body
Medicine was probably the most respectable use of astrology. A physician trained in the university tradition often believed that the moon and planets affected the body’s fluids, which is why humoral theory and star-lore were so easily combined. Practical aids reflect this:
- Calendars and almanacs marked days when the moon’s position made certain treatments more or less advisable.
- Bloodletting, a common treatment, was often timed to avoid unfavorable positions of the moon.
- Pictures of a “zodiac man” linked each sign of the zodiac to a part of the body, so a physician could see which area was thought to be influenced by which sign.
- Doctors were sometimes expected to be able to read a patient’s chart as part of deciding whether to treat.
None of this means medieval doctors relied only on stars. They also observed symptoms, urine and pulse, and prescribed diet and medicines. Astrology was one layer in a broader theory, not a replacement for it.
Kings, courts and the wider public
Rulers and nobles employed astrologers to advise on timing: when to begin a campaign, hold a coronation, lay a foundation stone or sign a treaty. Courts in several regions had such advisers, and ambitious scholars sometimes gained patronage by offering them. Predictions also carried political risk. A forecast of a king’s death or a rival’s rise could be dangerous to make, and rulers varied in how much they trusted the practice.
Ordinary people encountered astrology in simpler forms. Almanacs, calendars and popular prognostications offered advice on the best days to plant, travel or marry, and a good deal of folk custom ran alongside the learned versions. How much any individual believed is not easy to establish, since most surviving texts come from educated writers.
The church’s position
Church thinkers had to deal with a real problem. If the stars determined everything, there seemed to be no room for human free will or for God’s providence. Early Christian writers were suspicious of star-reading for this reason, and attitudes remained mixed in the Middle Ages.
A common compromise, associated with scholars such as Thomas Aquinas, held that the heavens could influence bodies and, through bodies, inclinations, but could not compel the will. Predicting natural matters such as weather or harvests was acceptable to many. Predicting a person’s fate, or claiming certainty about future events that depended on free choices, was condemned. Using astrology for demon-assisted magic was considered still worse. Church authorities disagreed with each other, and the line between permitted and forbidden was blurry in practice. Some universities and church figures in the thirteenth century restricted certain teachings, and critics in the later Middle Ages questioned how much the practice could really show.
Instruments and learning
Astrology also encouraged the making of instruments. The astrolabe, a portable device for measuring the height of the sun or stars and solving time-related problems, was valuable to astronomers and astrologers alike. Tables of planetary positions were revised over the centuries, and observers sometimes found them inaccurate, which fed the slow improvement of astronomical practice. The same devices also helped travelers and sailors fix time and direction.
How to read it today
It is easy to dismiss medieval astrology as superstition, but that misses what it did. It was a formal discipline, tied into mathematics, medicine and philosophy, and its practitioners thought they were studying natural causes. Its decline came slowly, with changes in how scholars understood the heavens and testing of predictions. For the Middle Ages, the useful question is not whether it worked, but how people used it to make sense of an uncertain world, and why even skeptical writers took it seriously enough to argue about it.