Medieval Medicine and Astrology: When the Stars Determined Your Treatment
The next time you use the word lunatic, consider where it actually comes from. It derives from luna, the Moon, and reflects a time when lunar cycles were considered a genuine clinical factor in a patient's mental state. The same is true of melancholy, rooted in the Greek for "black bile," one of four bodily substances whose balance was thought to be governed, in part, by the planets. These are not curiosities of fringe belief. They are remnants of a medical worldview that dominated European practice for well over a thousand years, one in which the heavens and the human body were understood as two expressions of the same underlying order.
Medieval medical astrology, sometimes called iatromathematics, was not superstition dressed up as science. Within its own framework, it was a coherent, internally consistent system, taught at universities, applied by licensed physicians, and trusted by kings. Understanding it properly requires setting aside modern assumptions and entering a world where the boundaries between cosmos and body were genuinely considered porous.
Roots in Greece and Rome: The Framework That Medieval Europe Inherited
The practice did not originate in the Middle Ages. Its roots reach back to Babylon, where cuneiform tablets from the mid-first millennium BCE already linked specific planets to specific organs, Jupiter to the spleen, Mars to the kidneys. What medieval Europe inherited, however, was largely the Greek and Roman synthesis of these older traditions.
Hippocrates, regarded as the father of Western medicine, wrote on the relationship between celestial cycles and human health, and his name was later attached to various astrological medical treatises, including works on the influence of the Moon according to a patient's zodiac sign. The second-century Roman astrologer Claudius Ptolemy codified the theoretical basis in his Tetrabiblos, arguing that the movements of heavenly bodies exert powerful and measurable influences on terrestrial life, including the human body. This text remained authoritative deep into the early modern period.
The physician Galen added another layer. His elaboration of the four humours, blood, phlegm, yellow bile, and black bile, became the dominant medical paradigm of the medieval world. Each humour carried its own elemental qualities: hot, cold, wet, or dry. Health was a matter of keeping these in balance, and the movement of the planets was believed to constantly shift that balance, imparting varying quantities of heat, cold, dryness, and moisture to the body. The heavens were not a backdrop to life; they were an active participant in it.
The Zodiac Man: Mapping the Sky onto the Body
Perhaps the most striking expression of medieval medical astrology is the figure known as the Homo Signorum, the Man of Signs, or Zodiac Man. First documented in the eleventh century, this diagram became one of the most widely reproduced images in medieval medical manuscripts, appearing in physician's handbooks, folding almanacs, Books of Hours, and surgical manuals.
The figure depicts a man, usually nude, arms outstretched, with the twelve zodiac signs mapped directly onto the body regions they were thought to govern. The schema ran from head to foot in a deliberate sequence: Aries ruled the head, Taurus the neck and throat, Gemini the arms and shoulders, Cancer the chest, Leo the heart and upper back, Virgo the digestive system, Libra the kidneys, Scorpio the reproductive organs, Sagittarius the hips and thighs, Capricorn the knees, Aquarius the lower legs, and Pisces the feet. The body, in other words, was a microcosm of the zodiac, a smaller version of the celestial order laid out in flesh and bone.
This was not merely decorative. The Zodiac Man was a practical clinical tool. Physicians, barber-surgeons, and other practitioners consulted it to determine when it was safe, and when it was dangerous, to bleed, operate on, or apply remedies to a particular part of the body. The key variable was the Moon. If the Moon occupied the zodiac sign governing a specific body part, it was considered unsafe to treat that area. The logic followed from observation: since the Moon was understood to influence tides and the movement of fluids, operating on a body part "ruled" by the Moon's current sign risked an uncontrollable surge of blood. It was a precautionary principle, not arbitrary superstition, within the framework that physicians at the time accepted as true.
One vivid description of a surviving English example, a fifteenth-century folding almanac now in the Edinburgh National Library, shows a Christ-like figure with zodiac symbols attached to his body, in some cases literally connected by ropes, the Gemini twins weighing down his arms, Pisces at his feet, the goat and bull of Capricorn and Taurus floating above his head. These images were striking precisely because they were meant to be. A physician needed to read them quickly and accurately.
Bat Books and Bloodletting: Astrology in Clinical Practice
So how did a medieval doctor actually use astrology in practice? The process was more structured than it might initially sound.
When a patient fell ill, the physician's first task was to establish the position of the Moon at the onset of the illness. This moment, when the patient first took to bed, carried enormous prognostic weight. The constellation the Moon occupied at that point helped determine the nature of the disease, its likely course, and crucially, when treatment would be most effective. This technique of casting a chart for the moment of falling ill would later be formalised as the decumbiture chart, but its principles were already embedded in medieval practice centuries before the term was coined.
For bloodletting, the most common medical intervention in the medieval period, used to treat conditions as varied as headaches, fevers, and skin complaints, physicians consulted elaborate hand-drawn charts to time the procedure astronomically. About sixty surviving English manuscripts known as "bat books", named for their distinctive folded shape, contain precisely this kind of astrological and medical material. The oldest was produced at Glastonbury Abbey around 1265; the youngest dates to the 1470s. Most were almanacs compiled for practical clinical use, containing calendars, eclipse tables, and the charts physicians needed to calculate auspicious and dangerous times for phlebotomy.
Two men, Nicholas Lynn, who worked for John of Gaunt, and John Somer, a Franciscan friar who worked for Joan of Kent, originally computed the astronomical tables contained in many of these manuscripts in the 1380s. Working in Oxford, they calculated the future movements of celestial bodies across four nineteen-year Metonic cycles, providing physicians with the data they needed to practise astrological medicine for decades into the future. That two such prominent figures invested this level of scholarship into medical astrology says something important about how seriously it was taken.
Diagnosis, too, had its astrological dimensions. When a physician wished to prognosticate the course of an illness, he needed the exact time the patient became ill, the position of the planets at that moment, and the patient's birth chart if available. With these three pieces of information, the physician could apply the principles of medical astrology with what was considered mathematical precision. For a world without the germ theory of disease, without laboratory testing, without imaging, this was a serious attempt to bring rational order to the chaos of illness. The universe, after all, ran on predictable cycles. Why should the body be any different?
The Planets as Physicians: Rulerships, Temperaments, and Treatments
Medieval physicians recognised seven planets: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn, the seven celestial bodies visible to the naked eye. Each governed not only a part of the body but also a temperament, a set of diseases, a range of herbal remedies, and even specific metals used in treatment.
Saturn, cold and dry by nature, was associated with melancholy (an excess of black bile), chronic diseases, the bones, the spleen, and conditions of age and decay. Jupiter, warm and moist, ruled the liver, arterial blood, and conditions linked to excess and abundance. Mars, hot and dry, governed the gallbladder, inflammation, fevers, and acute diseases. The Sun ruled the heart and vital spirits. Venus governed the kidneys and reproductive functions. Mercury was associated with the nervous system and the lungs. The Moon, perhaps the most clinically significant in everyday practice, ruled bodily fluids, the brain, and the left eye in men, the right in women.
A patient born under a strong Saturn might be constitutionally predisposed to cold, slow-moving conditions. Someone with Mars prominent in their birth chart might be more vulnerable to fevers and inflammations. This was not fatalism, it was constitutional medicine, an attempt to understand why different people responded differently to the same environment and the same illnesses. The personalised nature of astrological diagnosis was, in this sense, one of its appeals.
For herbal treatment, the planetary framework was equally central. Plants were themselves assigned to planetary rulerships, and a physician selecting a remedy would choose herbs governed by the planet associated either with the affected body part or with the planet opposing the disease's governing influence. Crucially, herbs had to be gathered at the correct astrological moment, when their ruling planet was visible and well-placed in the sky. To gather a plant out of its proper astrological season was believed to render it medicinally inert.
The Black Death and the Stars: When Astrology Met Epidemic
The limits of astrological medicine were tested most dramatically by the catastrophe of the Black Death. When bubonic plague swept through Europe from 1347 onwards, killing somewhere between a third and half of the continent's population, physicians and astrologers scrambled to explain it within their existing framework. The outbreak was widely attributed to a malign conjunction of Saturn, Jupiter, and Mars in 1345, a celestial event that had, in the astrological view, poisoned the atmosphere and set the conditions for mass sickness.
The University of Paris's medical faculty, consulted by the King of France, issued an official report in 1348 laying this conjunction at the root of the epidemic. The meeting of three such powerful planets, the faculty argued, had drawn up corrupt vapours from the earth, creating pestilential air. It was a serious institutional response to a catastrophic event, reasoned within the most authoritative scientific framework available. That it was wrong about the mechanism does not make it less historically significant, it shows medical astrology at full institutional strength, deployed in a genuine public health crisis.
What followed was not entirely useless either. Recommendations to avoid certain foods, to move away from areas of stagnant air, and to time quarantine measures according to celestial cycles were wrong in their theoretical basis but occasionally right in their practical effect. The emphasis on air quality, for instance, turned out to be partially useful even if the reasoning behind it was flawed.
Nicholas Culpeper: The Last Great Medical Astrologer
By the seventeenth century, the scientific revolution was beginning to dismantle the theoretical foundations of humoral medicine and medical astrology alike. But it did not happen overnight, and one figure in particular stands out as a brilliant, combative, and deeply committed practitioner of the tradition in its final flourishing.
Nicholas Culpeper (1616-1654) was an English botanist, herbalist, physician, and astrologer whose work represents the fullest English-language expression of the medical astrological tradition. His English Physitian, published in 1652 and later expanded as the Complete Herbal, catalogued over three hundred herbs with their medicinal uses and, crucially, their planetary rulers. His posthumously published Astrological Judgement of Diseases from the Decumbiture of the Sick (1655) remains one of the most detailed practical works on medical astrology produced in early modern Europe.
Culpeper's approach to the decumbiture chart, the horoscope cast for the exact moment a patient fell ill, was methodical and sophisticated. By reading the planetary positions at that moment, he could assess the nature of the illness, its likely duration, and which remedies, governed by which planets, would be most effective in counteracting it. He was also a political radical who deliberately published in plain English rather than Latin, wanting ordinary people to have access to medical knowledge previously gatekept by the College of Physicians. His books were priced cheaply and spread widely, they have been in print almost continuously since the seventeenth century and remain a reference point for herbalists today.
Culpeper knew he was writing at a hinge point in history. The world was changing around him, and medicine was pulling away from the celestial framework. He died at thirty-seven, having packed an extraordinary amount of work into a short life. The tradition he represented did not die with him, but it did begin its long retreat from mainstream medicine not long after.
The Decline and What It Left Behind
The seventeenth century brought anatomical discoveries that simply could not be reconciled with humoral theory. William Harvey's demonstration of blood circulation in 1628 was particularly significant, if blood moved continuously around the body in a single system, the idea that it could accumulate as a stagnant excess in one location, requiring to be drained off, made far less sense. The microscope brought bacteria into view. Chemistry began to replace plant-lore as the basis of pharmacology. By the end of the seventeenth century, astrology had largely been displaced from mainstream medical practice. By the nineteenth century, it had been dismissed from institutional medicine entirely.
Yet the traces remain, in language, in symbol, and in the cultural memory of what healing once looked like. The word disaster comes from dis-aster, meaning "ill-starred." Influenza takes its name from the Latin for "influence," specifically the influence of the stars on human health, the name was given to epidemic illness in the belief that it spread through astrological conditions. Consider contains sidus, a Latin word for star. We still speak of someone as having a "mercurial" temperament, being "jovial," or being "saturnine", every one of these adjectives is a direct inheritance from the planetary psychology of medieval and early modern medicine.
The Zodiac Man himself, that strange, spread-armed figure covered in astrological symbols, continued to appear in almanacs and popular publications long after bloodletting had been medically discredited. He even featured in Poor Richard's Almanac by Benjamin Franklin and has never entirely vanished from popular astrological culture. Some images resonate because they express something true about how people have always felt, that the body is not a sealed system, that we are participants in something larger than ourselves, that health is more than mechanical.
Why This History Matters Today
It would be easy to read the history of medieval medical astrology simply as the story of how pre-scientific minds coped before better answers arrived. But that reading misses something important. The medieval physician's attempt to understand the individual within a larger cosmic context, to ask not just "what is wrong?" but "who is this person, what is their constitutional nature, and what cycle of life are they moving through?", is a question that never became obsolete.
The integration of astrology and psychology that Martyn J. Shrewsbury practises at the Wilfred Hazelwood Clinic works from a different theoretical basis than medieval humoral medicine, but it shares something with that older tradition: the conviction that a person is not simply a collection of symptoms to be corrected, but a whole being whose inner life, character, and relationship to time and circumstance all matter to their wellbeing. Jungian psychology, with its archetypes and its attention to the symbolic dimensions of experience, has more in common with the medieval macrocosm-microcosm view than either tradition might initially admit.
The question the medieval physician asked when he consulted the Zodiac Man was, at its core, a question about timing, context, and the nature of the individual. Those questions have not gone away. They have simply found new language.
