John Westwyk and “The Equatorie of the Planetis”

If you think of the words “occult”, “arcane”, or “mystical” when you look at this Medieval text, you aren’t alone — the association of the Middle Ages with backwardness and the irrational has a long tradition. But it wouldn’t be a correct impression, at least not entirely. And this manuscript shows why. Written in 1393 by a Benedictine monk named John Westwyk, this is _The Equatorie of the Planetis_, and it describes an instrument that could correctly figure out the position of the planets.

An equatorium was a tool used by people who specialized in astronomy in the Middle Ages. It could tell the longitude of the sun, moon, and planets, as well as the latitude of the moon, for any given year. The cool thing is that these predictions were correct, *even though* they relied on a geocentric model of the solar system. By the late 1300s, Medieval astronomers had incorporated the most advanced mathematics and astronomical writings of their Islamic neighbors in Spain and the Middle East.

John Westwyk’s tract describes how to make an equatorium and how-to use it, but mostly presents tables of annual and daily values for mean longitudes and latitudes of the planets (as well as the sun and the moon). In his recent book, _The Light Ages_, historian Seb Falk puts the story of John Westwyk and _The Equatorie of the Planetis_ center stage to argue that that there was not a conflict between knowledge and religion in the Middle Ages, and that the period should be instead appreciated for the open-mindedness of its scholars.

The University of Cambridge has a digitized version of the manuscript and a simulation of an equatorium that you can manipulate for yourself — I will put the link in the comments. It’s really neat to try out, and certainly shows that those who designed such instruments were able to use observation and logic to make observable predictions of their skies. . . .

To view the simulation if the equatorium, see @cudl.lib.cam.ac.uk/models/equatorie/#