Answer first
A fake Duomètre moon phase is a date disc with a moon printed on it. The real Jaeger-LeCoultre Duomètre à Quantième Lunaire runs two separate gear trains and a 135 tooth moon wheel. No replica has copied that movement. Every check on this page comes back to that one fact.
What the real watch uses
Jaeger-LeCoultre released the Duomètre à Quantième Lunaire in 2010. The movement is Calibre 381. It carries two barrels and two gear trains on one mainplate. Barrel one drives the balance and the time display. Barrel two drives the date, the moon phase and a jumping seconds hand. Each barrel stores 50 hours of power. The case measures 40.5 mm across and comes in rose gold or white gold.
The moon display sits on a wheel with 135 teeth. A lunar month runs 29 days, 12 hours, 44 minutes and 2.9 seconds. The 135 tooth wheel matches that value. The display drifts by one day every 3,887 years.
What the replica does
Replica factories build one gear train. The moon disc stacks on top of the date mechanism. The disc steps once per day, so the cycle length equals the tooth count of the wheel under it. A 31 tooth date wheel gives a 31 day cycle. A 29 tooth wheel gives a 29 day cycle. The lunar month is 29.53 days. Neither replica matches it. The moon leaves position inside a year.
Checks you can run on a listing
- Cycle length: note the moon position, then check the same position 30 days later. The step from new moon to new moon measures 29.5 days on the real watch. A 31 day result means a date wheel drives the disc.
- Seconds hand: the real watch uses a foudroyante seconds hand with six jumps per second. Most replicas use a 4 Hz sweep or a 1 Hz tick.
- Case height: a date module adds 2 mm to 3 mm of stack height. Compare the case side profile against the reference.
- Moon in daylight: some replica discs print the moon across the full circle, so it shows at noon. On the real dial the moon sits on the night half.
- Spelling: listings write Duometre, Duo Metre or Duo-metre. The correct name is Duomètre.
- Price: real references trade in the tens of thousands of dollars. Replica listings sit in the hundreds.
What photos cannot show
A photo set shows the dial, not the gear train. A seller can photograph a correct moon aperture and still ship a 31 day disc. Ask for two photos of the moon window taken 30 days apart. If the seller declines, treat the moon phase as decoration and nothing more.
Short summary
The moon phase on a replica Duomètre is a printed disc on a date wheel. Count the cycle days before you pay. One barrel, one train, one date wheel: that is the tell.