Medical astrology is among the oldest practical applications of astrological knowledge. The assumption underlying it — that the celestial bodies which govern the great cycles of the natural world also govern the cycles of the human body — was not considered mystical but scientific by practitioners from Babylon to the Renaissance. Hippocrates himself reportedly wrote that a physician ignorant of astrology had no business calling himself a physician.
The tradition reached its most systematic form in the work of Claudius Ptolemy (2nd century CE), whose Tetrabiblos codified the relationships between planets, signs, body parts and diseases. This Ptolemaic system, refined by the Arabic physicians al-Kindi and Ibn Sina (Avicenna), entered medieval European medicine through Latin translations and remained standard practice in European universities until the 17th century.
The physician-astrologers of the Renaissance — Marsilio Ficino, Girolamo Cardano, Richard Forster — did not see themselves as practitioners of two separate disciplines. Diagnosis, prognosis and treatment were all informed by the patient's natal chart, the chart of the onset of illness, and the transits and progressions active at the time of treatment. Medical astrology was not an alternative to medicine; it was medicine.