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The Encyclopedia of Horology

Component record · Dial & hands

Moonphase Disc

Partially verified

A rotating disc showing the moon's phase.

What it does

Displays lunar phase through an aperture, usually with two moon images.

How it works

A toothed disc is advanced once or twice daily, so that one of two painted moons appears progressively through a shaped aperture.

Evolution

Most domestic mechanisms approximate the lunation, so periodic correction is needed.

Known variations

  • 59-tooth two-moon disc
  • High-precision lunar trains

Identification for collectors

  • Aperture shape and moon decoration vary with maker and period.

Common failure modes

  • Worn drive teeth
  • Flaking decoration

Diagram

moon disc carrying two moonsaperture and shuttersadvance wheel
Schematic: moon phase drive. An outline schematic drawn to show the arrangement of the parts discussed here. It is not a dimensioned working drawing and does not depict a particular maker's calibre.

Sources & references

  1. de Carle, D., Practical Watch Repairing
  2. de Carle, D., Watch and Clock Encyclopedia
  3. Daniels, G., Watchmaking

Where sources disagree, the disagreement is stated rather than resolved silently.