Medieval Cosmology

circular diagram As both an amateur medievalist, and the product of many years of graduate school in the sciences, it seems only natural that I become interested in the idea and the practice of "science" in the Middle Ages and early Renaissance.

I find the interplay between "received wisdom" and actual observation fascinating. Not surprisingly, I'm most interested in the very early Renaissance, when it became apparent that received wisdom was not in fact adequate, despite the centuries of trust in those classical sources, and that in fact we didn't know nearly as much as we thought we did.

I'm particularly interested in the science of the late 15th and early 16th centuries. Some of the material included here had been treated in similar form for centuries (the geocentric cosmology), while other areas were in rapid development (cryptology). My timeperiod of choice is somewhat fuzzy at the beginning, but cuts off quite firmly at the end, particularly for the astronomical information, since I'm restricting it to a pre-telescopic and geocentric worldview, culminating in my favorite scientific work, the Astronomicum Caesareum of Peter Apian (1540).

I am also interested in the physical expression of the science of the time, in the form of scientific instruments and tables and calculation methods. In astronomy, developments were frequently driven by astrological purposes - see the astrology section of this site for more information. The astrolabe was one of the essential astronomical and astrological instruments of the time. I've also been working on an astrologer's notebook.

The geocentric perspective includes the Ptolemaic construction of epicycles, deferents and uniform circular motion. The drawing on the front page of this website is based on a 15th c. manuscript, and shows the order of things. The earth is in the center, surrounded by spheres (the onion model of the universe) for the other three elements (not shown on that diagram). The seven planets (wandering stars, those visible with the naked eye) are next, and finally the fixed stars.

From the outside in:

One of the challenges of astronomy was to explain the motions of the planets. Not only did they move through the constellations of the Zodiac, sometimes they reversed course. The idea of uniform circular motion was essential, circles being "perfect", but how to use circles to replicate the observed motions of the planets?

This is what the motion of Mars looks like from Earth (in the center, marked O for observer), as recorded over a 10-year period. The distance from Earth is based on the apparent size of the planet. The data used to construct the figure come from NASA JPS Horizons.

Motion of Mars

Not only does the planet periodically reverse its apparent motion, but the sizes of the retrograde "loops" vary.