Check the calendar on your computer or your phone. Until what year does it allow you to reach? It might not go on for 100 years, it might not even reach 2100. And that's given since we probably won't live for that long. How about this: try going as far back as you can. You probably won't be able to go beyond 1900. What if there were a calendar that can track all days, months, and years from the year 1 to 4000? You might think such a thing is too tedious to build or even unnecessary. Who would want to know on which day a certain date a thousand years from now fall? Well, the astronomer and mathematician Giovanni Antonio Amedeo Plana built the Perpetual Calendar that does exactly that. It can tell you on which day of the week, say, January 1, 3000 falls.
The device, which resides in the Chapel of Bankers and Merchants, operates via a simple wooden crank under the adorned golden frame, a crank that hides a stunningly accurate universal mechanical calculator spanning the years 1 to 4,000. Want to know the day of the week that the Western Roman Empire fell to the barbarians, on September 4, 476? The calendar will tell you that it was a Monday. Or maybe the phase of the moon on the day you were born? Or the date of Easter a thousand years from now (April 18, 3019)? All of this information can be accessed by a pre-internet machine made of fragile wood and paper, and communicated through 46,000 little numbers carefully arranged around nine cylinders. Each of these is linked to a central one—the only adjustable part of the device—where the user can input the year. That cylinder synchronously regulates all the others through gears and chains.
(Image credit: Ranna Utida/Atlas Obscura)