What is a moonphase on a watch?

A moonphase on a watch shows the current phase of the moon as you see it in the sky. Having an accurate moon phase in a watch is a little more difficult than it would seem because a lunar cycle lasts 29 days, 12 hours and 44 minutes meaning that a 135-tooth gear is necessary to drive the moon disc optimally for the exacting standards of watchmakers. Shop a collection of vintage and new moonphase watches from some of the world’s top boutiques on 1stDibs.