Quick answer

A fake Duomètre à Quantième Lunaire copies the dial, the case shape, and the moon phase window. It does not copy the movement. The genuine watch uses Jaeger-LeCoultre Calibre 381. Jaeger-LeCoultre states the moon phase needs one correction every 3,887 years. Most copies use a printed moon disc on a 59-tooth wheel or a 24-hour disc. Those drift by one day in about 2 years 8 months. Check three things: the moon disc surface, the barrels through the caseback, and what the crown moves.

Genuine specs

  • Maker: Jaeger-LeCoultre, Le Sentier, Switzerland.
  • Movement: Calibre 381, hand-wound.
  • Design: Dual-Wing. Two gear trains. Two barrels.
  • Work split: one barrel for hours, minutes, and seconds. One barrel for the date and moon phase.
  • Moon phase correction: one day per 3,887 years, per the maker.
  • Caseback: sapphire crystal. Movement visible.

The model name is Duomètre à Quantième Lunaire. The search spelling duomeetre is a misspelling, not a second model.

Moon phase math

One new moon to the next is 29.53059 days on average. Watchmakers round this to 29.5 days with a 59-tooth wheel. The error is 0.03059 days per cycle, so the display gains one day in about 2 years 8 months. A 135-tooth wheel cuts the error to about one day in 122 years. The Jaeger-LeCoultre figure of 3,887 years needs a different wheel count and a separate drive train.

Checks on a replica

  1. Moon disc. Look for a paper or vinyl decal under the aperture. Genuine discs are metal with painted detail.
  2. Disc movement. Watch the disc for 24 hours. One step per day is normal. A disc that slips back or stalls is not.
  3. Crown function. On a Dual-Wing watch the complication train runs on its own barrel. On most copies one crown drives all functions, so pulling the crown disturbs the moon disc.
  4. Barrel count. Two barrels sit side by side on Calibre 381. Copies show one barrel or a solid plate with engraving.
  5. Print and hands. Compare the aperture edge, the date scale, and hand length against the reference.
  6. Caseband. Corrector positions change between references. Compare with the reference number, not with a stock photo.

What a copy cannot do

The 3,887-year figure comes from a specific wheel count and a drive that is separate from the time train. That work costs machining time and assembly time. A low price copy does not include it. Two watches from the same replica factory can hold different moon modules, so one sample does not describe the next.

Ask the seller

  • Reference number and case diameter.
  • Photo of the movement through the caseback.
  • Photo of the moon disc through the aperture, in daylight.
  • Video of the disc over 24 hours or while the corrector is used.
  • Stated moon phase period in days, not a marketing phrase.

Not known: replica makers do not publish gear counts or module sources. If a seller gives no gear count and no period, treat the moon phase as decoration.