Eclipses and astrology
The purpose of the volvelle on the back of this unusual astrolabe is not entirely clear, but the ornament of the the head and tail of a dragon probably refers to the nodes of the moon (caput and cauda draconis, used in the computation of eclipses) which suggests that the volvelle had an astrological purpose.
Astrolabe with Volvelle, 1613? (Inv. 47367)