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Fine Example of a Perpetual Calendar

Circular figure showing a perpetual calendar, which was used to calculate future dates and events.

The concentric circles of the diagram contain important information for figuring time, including zodiac signs, months, prominent saints’ days, and phases of the moon. At the bottom is an astronomical table containing a calculation of the dates of the equinoxes and other celestial events.

Perpetual calendars are calendars that are valid for multiple years. They were usually used to calculate dates in the future and were intended to simplify date-finding, which would otherwise involve logarithms and complex calculations. This appears to be a proposed perpetual calendar system. The calendars have been in use since Medieval times and have taken various forms including tables and paper, wood, and metal instruments.