Short answer
A fake Portofino moon phase is a copy of the IWC Portofino Automatic Moon Phase. The moon disc decides the price. Most cheap copies use a 24-hour day/night disc. Better copies use a disc that turns once per 29.53 days, the length of a lunar month. IWC's own calibre 35800 moon phase needs one correction about every 122 years.
The genuine watch
IWC sells the Portofino Automatic Moon Phase in two sizes. The 37 mm version arrived in 2016. The 40 mm version followed in 2022. Both put the moon aperture at 12 o'clock. The case is stainless steel or gold. The crystal is sapphire. Steel models ship on alligator leather.
IWC lists calibre 35800 for the 37 mm watch. Power reserve is 42 hours. Retail for the steel 37 mm sits near $6,000. Gold and diamond versions run past $15,000.
How a moon phase movement works
A real moon phase runs off a wheel with 59 teeth. The wheel turns once per 59 days and drives a disc that carries two moons. The aperture shows one moon, then the other, then repeats. The synodic month is 29 days, 12 hours, 44 minutes. A 59-tooth wheel tracks that figure with a small error.
A day/night disc looks the same on the wrist. It turns once per 24 hours. Half the disc is dark and half holds a moon or a sun. It never tracks the lunar month.
How to test one in the hand
- Set the moon to the center of the aperture with the crown.
- Note the time and leave the watch running.
- Check the disc after 24 hours.
- Check again after 7 days.
A 24-hour disc moves half a turn in a day. A 29.53-day disc moves about 1/30 of a turn. Photos at day 1 and day 7 settle the question.
What else differs
- Moon disc art. IWC prints a moon face with craters and stars. Copies show flat circles or glossy stickers.
- Case thickness. A moon module adds height. Many copies measure 1 mm to 2 mm thicker than the IWC figure.
- Dial print. "Portofino" and "Automatic" sit close to the moon aperture. Copies shift spacing and font weight.
- Caseback. IWC engraves a reference number and a movement number. Copies reuse one number across a batch.
- Strap. Genuine alligator has a square scale pattern. Copies use embossed calf.
Price tiers
Copies run from about $60 to $400. Under $100 buys a quartz watch with a printed moon. $150 to $250 buys an automatic with a day/night disc. $300 and up buys a 29.5-day moon disc, sapphire crystal, and a Miyota or Seagull base movement.
Limits
IWC will not service a copy. Parts do not interchange. Water resistance on copies is not tested to the number printed on the caseback. Resale value is near zero. A moon phase module adds a gear train that a local watchmaker may not have parts for.