Explorations — Interactive Astronomicum Caesareum
A book becomes an instrument — an instrument comes to life.
Published in 1540, Peter Apian’s Astronomicum Caesareum is a richly illustrated book of astronomy with working instruments built into its pages. Layered paper discs, known as volvelles, turn around their centers, while threads carry readings between scales. Together, these moving parts let the reader calculate the positions of celestial bodies.
I’m drawn to the way the Astronomicum brings beauty, calculation, and discovery together. Its influence runs through the Lunation Calendar: the circles, the threads, and the invitation to learn by exploring. It is an analog computer made of paper. Its geometry encodes a mathematical model, and the reader works through a calculation by turning discs and positioning threads. Understanding grows through the act of using it.
I wanted to get my hands on the book and try it for myself, but that wasn’t possible. I found videos showing the instruments in motion, and wanted to go deeper: to choose a date, move the parts, and follow a calculation all the way through. I looked for an online version that would let me do that and couldn’t find one. Eventually, I realized I could build it myself.
This is the result: one working volvelle for calculating the lunar nodes, accompanied by its supporting text and table. I’ve distilled Apian’s telegraphic instructions into simple steps in an explanatory notebook alongside the original pages. You can explore freely, turning the disc and moving the thread, or choose a date and work through the algorithm. Make each adjustment manually, or press Play to watch the steps unfold.