shedding light on moonphase complications vs day / night indicators
The other evening, I was able to impart some knowledge to a fellow watch collector, through one of the watch forum boards that I frequent. This particular collector was still in the early stages of the hobby, and was attempting to gather pricing information on moonphase or lunar phase timepieces, for an upcoming purchase. What inevitably followed, were a flood of well-meaning watch examples, provided by forum members, but I immediately recognized that the majority of watch examples given, were not moonphase watches, but rather, were watches with day / night, or sun & moon indicators. The original poster was able to gain a solid understanding of the difference in complications, through a very productive conversation exchange, so I thought I would share the information here on my blog also, as the misrepresentation is a fairly common one in watch circles.
The term "Moonphase" is widely used in retail, to describe most any timepiece which has a sun / moon, or day / night wheel complication. But these timepieces do not have the ability to display each lunar phase of the moon. The moon / sun wheel, advances in 12-hour increments, much like a date wheel, as the time advances on the dial; think of it as a 24-hour display, in graphic form. As you see in the picture of the Lucien Piccard timepiece (above), the time is 8:39 AM, as indicated by the analog time and 24-hour register, and the day / night wheel at the 12 o'clock position, has the sun starting to appear. The sun display will be dead-center of the register window, at high-Noon.
In a true Moonphase, or lunar phase complication, the various phases of the Moon, will be shown as a graphic representation on the dial. The level of accuracy of the Moon's phases will vary, but should reflect the current lunar phase of the moon. In the example shown above, the Renato Master Horologe, utilizes a photo-realistic disk of the moon surface, with a second shadow disk passing over, to create the various lunar Moon phases.
Hopefully this sheds a little bit of clarity on the difference between a day / night complication, and true lunar, Moonphase complication. Lunar Moonphase complications require much more engineering, and therefore a timepiece with a lunar Moonphase complication, will be more costly in price than a piece with a Sun / Moon display.