This is a volvelle, a medieval device that allowed you to calculate the phases of the moon and the latter’s position in relation to the sun. The dials, with their charming depictions of moon and sun, tell you what you need to know. What’s most remarkable about the device is not so much its crafty nature - it consists of complex layers of rotating disks - but that it is usually fitted inside a medieval book. Some are so bulky that they pierce the adjacent pages. What a surprise it must have been for the medieval reader who thumbed through such a book for the first time. Turning a page, he or she was confronted with an ingenious piece of machinery. A medieval computer.
Pic (BL): London, British Library, Egerton MS 848 (15th century). More information about the manuscript here.