Moon-phase display in watches

A moon-phase display is a watch complication representing the Moon's phase as seen from Earth. Distinguish it from the date and from a day/night indicator. A decorative Moon motif on a dial alone does not establish a functioning moon-phase display.

What does the display represent?

Moon phases result from seeing different portions of the Moon's sunlit half during its orbit. According to NASA, a new-Moon-to-new-Moon cycle lasts approximately 29.5 days. This differs from the daily alternation of day and night.

Patek Philippe describes phase displays through an aperture and also versions using a hand. As a specific example, reference 5320G-011 has both a moon-phase aperture and a separate day/night indicator. This illustrates why the functions should not be confused. It does not establish that every older watch from the brand has the same arrangement.

Checking an older watch

Identify the correct reference and instructions before adjustment. Setting procedures, permitted operating times and correction requirements must be investigated for the model. Do not treat a maker's stated long-term accuracy for one construction as a guarantee for all moon-phase watches.

As a practical example, a Moon disk may look correct at one moment without its mechanism's advance having been tested. Record what has actually been examined. Document the dial, operation and known repairs in a condition report, and retain service papers.

Next step: Browse OBA's second-hand wristwatches and read each example's functional description. When offering a watch for sale, its reference and clear dial photographs provide a useful starting point.

Sources: NASA on Moon phases, Patek Philippe on moon-phase displays and the maker's documentation for 5320G-011.