Perpetual calendar, German, 1765.
© Science Museum / Science & Society Picture Library
This single-sheet printed card calendar is constructed from a series of volvelles, or rotatable discs, that can be moved to provide a variety of information about any particular day of the year. The top volvelle shows the name of the month, the most important feast days, the zodiacal sign, and the relative lengths of light and dark each day. At the bottom is a Moon dial showing the phase of the moon day by day, as well as the length of time it is visible in the sky. To the left is a weekly calendar.