Middle English medical astrology: finding health in the heavens. From the article: 

Chaucer returned to the topic of celestial edification a few years later in Treatise on the Astrolabe (ca. 1391), which he wrote for his ten-year-old son, Lewis. Although “Little Lewis” (Lytyl Lewys), as he is affectionately known in the text, is identified as the intended audience, the boy also serves as a stand-in for audiences of all ages, for “every person . . . that readith [or] hearith this little treatise.”

The 33 still-extant copies of the Treatise attest to the breadth of its readership; its numbers are second only to Chaucer’s most famous text, The Canterbury Tales…. Today, only 11 volumes preserve the diagrams that are believed to have accompanied Chaucer’s original text. Altogether, these illustrations define a distinctive device that historian Catherine Eagleton has termed the “Chaucerian” astrolabe….

In sixteenth-century England, medical astrology was by no means fringe. It was a curricular requirement for postgraduate study in medicine, and it figured prominently in the training of most physicians, barber surgeons, and local healers.

Bound up with theories of humoral medicine and celestial-terrestrial correspondence, medical astrology was, in the eyes of its remaining early-modern practitioners, built upon a “rational” substrate: the precisely calculable and predictable movements of the heavens. These movements, in turn, were thought to move “humors” or fluids—e.g., blood, phlegm, etc.—within the human body. When the body’s humoral fluids were in balance, a person was considered healthy. When they were out of balance, illness allegedly followed.

Accordingly, a key consideration in medical astrology was timing. While sundials and mechanical clocks satisfied most basic time-telling needs, some physicians turned instead to the astrolabe. They used this instrument to quickly figure the time of day or night at any time of year, to assess when curative treatments should be applied—or avoided altogether. Such physicians, scholars claim, served persons of high nobility or even royalty.