A moonphase watch for a formal watch is very appropriate.
This Piaget Moonphase watch has precision gearing and thus only renders a single day's inaccuracy in 122 years. This is thanks to having a greater number of teeth on the gears so that the watch runs with greater precision.
A less precise watch will have a 59 teeth gear (a lunar month is 29.5 days, 29.5 * 2=59 teeth for the gear) and will thus make the moonphase off by one day in about 30 months.
Lastly, notice the detail on the moon. It kind of reminds you of the botched surface of the moon with craters.
