Short answer

A fake Da Vinci moon phase often has no moon phase. Most copies use a printed disc that turns once every 24 hours, or a disc that does not move at all. A real moon phase tracks the 29.530588-day synodic month. The IWC Da Vinci Perpetual Calendar holds one day of error in 122 years.

How the complication works

The moon returns to the same spot against the sun in 29.530588 days. A standard moon phase wheel has 59 teeth and steps twice per day. That yields a 29.5-day cycle. The gap is 0.030588 days per lunation, about 44 minutes. The error reaches one day in about 32 months.

A 135-tooth wheel cuts the error. That design holds one day of drift in 122 years. IWC used it in the Da Vinci Perpetual Calendar, reference 3750, released in 1985. Kurt Klaus designed the calendar module. On ref. 3750 the crown sets every calendar function. The case side carries no corrector pushers.

What copies get wrong

  • Disc speed. The disc steps once per 24 hours instead of once per 29.5 days.
  • Fixed art. The moon and stars are printed on the dial and never move.
  • No mask. A genuine aperture uses a metal mask over the disc. The mask edge casts a shadow line. A flat print has none.
  • Shallow build. A real disc sits below the dial surface. A print sits on it.
  • Year window. Copies print a fixed year, or two digits where the original shows four.

Checks before you pay

  1. Advance the time 24 hours with the crown. The moon should move about 3 percent of the aperture travel, near 12 degrees of disc rotation. A full jump or no motion points to a day/night disc or a static print.
  2. Advance 29 days. The moon should return to the start position.
  3. View the aperture at 10x. Look for a mask edge and for depth between the disc and the dial.
  4. Check the year display if the watch has one. Ref. 3750 shows four digits.
  5. Test the calendar at the end of February. A working perpetual calendar moves from 28 February to 29 February in a leap year, and to 1 March in other years.
  6. Ask for a video. A still photo cannot show disc speed.

Limits

No photo proves a movement. Case back engraving and dial printing can match while the module does not. Movement bases in replicas vary, and a moon module can run on a base that differs from the original. Weight, rotor noise, and power reserve are poor tests on their own. A watch that passes the 29-day moon test can still fail the leap-year test.

Reference points

  • Model: IWC Da Vinci Perpetual Calendar, ref. 3750.
  • Release: 1985.
  • Moon phase accuracy: one day in 122 years.
  • Synodic month: 29.530588 days.
  • Standard 59-tooth moon phase drift: one day in about 32 months.