Moon Phase

Why it matters

A moon-phase complication shows the current phase of the moon (new, waxing crescent, first quarter, full, etc.) on the watch dial, usually through a small aperture with a printed disc that rotates beneath. Standard moon-phase mechanisms drift by about one day every 2.5 years; precision moon-phase complications, like the one in the A. Lange & Söhne 1815 Moon Phase, drift by about one day every 122 years.

For buyers, moon phase is usually a price-add at retail and a price-deduct at resale. The complication is decorative more than functional, and most secondary-market buyers prioritize sport models, chronographs, and date complications over moon-phase displays. Patek Philippe is the consistent exception: the Calatrava 5396 and the perpetual calendar references with moon phase hold value better than most moon-only models. When evaluating a vintage moon phase, check that the disc rotates smoothly when the watch is set forward through several months. Sticky moon-phase mechanisms are a common service issue.

Common questions

What is a moon-phase watch?

A moon-phase watch displays the current lunar phase on the dial through a small aperture, usually with a rotating disc that shows the moon waxing and waning. Standard moon-phase complications need a one-day adjustment every 2.5 years; precision versions can run for over 100 years before drifting a day.

Do moon-phase watches hold their value?

Usually less well than the same brand's chronograph, GMT, or sport references. Moon phase is a decorative complication rather than a functional one for most buyers, so it tends to be a price-add at retail and a price-deduct at resale. Patek Philippe moon-phase references are the consistent exception.

Related terms

• Complication

  Any function beyond basic timekeeping: chronograph, moon phase, perpetual calendar, tourbillon, GMT, etc.

• Perpetual Calendar

  A complication that automatically accounts for different month lengths and leap years, requiring no manual date adjustment (until 2100).

• Calatrava

  Patek Philippe's quintessential round dress watch. The benchmark for elegant simplicity. The name every collector knows.

• In-House Movement

  A movement designed and manufactured by the watch brand itself, not sourced from a third party. Signals technical independence.