The Moon phase dial was never created as a mere decoration for watches, but had a practical reason as well. In the 16th century the need for a moon phase dial arose when travellers were interested to know about full moon days so that they can plan their journey accordingly. The travellers wanted to avoid nights when the moon would be least visible as travelling those nights would be considered unsafe. Another theory suggests that the moon’s phases were directly linked to tide movements. Keeping this theory in mind, people were interested to know whether one can cross a coastal stream without any harm. One can usually find this feature in perpetual calendar watches as the moon’s movement and the calendar are intricately linked.
The moon travels across the night sky in 29.5 days. In these days it passed 8 specific phase, namely:
- New moon (dark moon)
- Waxing crescent
- First quarter
- Waxing gibbous
- Full moon
- Waning gibbous
- Last quarter
- Waning crescent
These positions are seen discretely on the moon phase sub dial through the intricate mechanism. The typical moonphase mechanism consists of a 30-toothed disc. The disc has two gold-colored moons opposite each other on the disc, surrounded by gold-colored stars on a blue background. The disc is partially covered by the dial, with a window cut out to view the current phase of the moon. When no part of the moon appears in the window, this indicates a new moon. As the days pass, the moon will appear from the left side of the window. When the image of the moon in centered in the window, this indicates a full moon and the disc will continue to travel clockwise or counter-clockwise (depending on the hemisphere) until the image of the moon disappears at the right side of the window, again signaling a new moon. At this point, the second image of the moon will begin to appear from the left side of the window, thus starting the entire cycle again.
To get the moon phase function working on your time piece, simply set your pointer towards the correct half circle, which is around the arch. One can simply refer to a lunar calendar to make sure one is entering the right details. A sophisticated feature, it involves 135 tooth gears to drive the moon disc to give an accurate prediction of the moon’s phases.