The replica Fifty Fathoms Full Moon copies Blancpain's Bathyscaphe Quantième Complet Phases de Lune, a dive watch that pairs a complete calendar with a moon phase at 6 o'clock. Here is the short version: the moon phase is the tell. Cheap builds use a printed disc that turns once every 24 hours, so a sun and a moon swap places instead of tracking a lunar month. Better builds run a true 29.5-day moon disc, driven by a module stacked on a common automatic base. Everything else, the case, bezel, dial print, is fixable at the factory level. The moon phase is where the money goes.
How the moon phase should behave
A real moon phase disc carries two moon faces on one wheel and advances one tooth per day. That means the moon you see drifts across the aperture a little each night, disappears for roughly a day and a half at new moon, and comes back on the other side. If you can find a video of the watch running, watch the cutout for a few seconds. A disc that snaps to a sun during daylight hours is a day/night indicator dressed up as a moon phase. Nothing wrong with that if the price reflects it, but it is not the complication you think you bought.
Movement and calendar layout
Most replicas here use an automatic base with a calendar module on top, which is how the original works too. That module drives three things at once: the day and month windows near 12, a date pointer hand, and the moon aperture. Check that the date hand points at the correct number rather than sitting between two, and that the day and month windows flip together at midnight instead of staggered by hours. Correctors on the case band should push in and spring back. If a corrector feels dead, the module is likely decorative.
- Day and month windows aligned at the same height, no tilt or clipping
- Date pointer with the right tip color reaching the outer track
- Moon aperture centered at 6 and cleanly cut, no ragged edge
- Crown screws down and the stem engages without grit
Dial, case and bezel
The dial is where replicas either look flat or look expensive. On the genuine watch the indices sit raised and the lume plots fill them edge to edge. On copies, look for printed lume that stops short of the frame and hands that feel thin at the base. The case should measure close to 43mm across with a sapphire or ceramic bezel insert that does not rattle when you tap it. Water resistance claims on any replica sit somewhere between optimistic and fiction, so treat it as splash resistant and keep it out of the pool.
What a fair price looks like
Three tiers show up in this model. Entry versions in the low hundreds have the fake day/night disc and a stamped case. Mid tier, usually a few hundred, gets a real moon wheel and a better dial but a rougher module. Top tier costs more and adds a cloned movement layout, sapphire crystal with proper coating, and a bracelet with solid end links. If a seller cannot tell you which tier the moon phase belongs to, that answers the question.
Inspection checklist before you pay
- Ask for a short clip of the moon disc advancing over several days, not a single photo.
- Confirm the day and month change together at midnight.
- Check date pointer alignment at 15, 25 and 31.
- Look at macro shots of the bezel insert and caseback engraving.
- Ask about the crystal, sapphire and anti-reflective coating or mineral.
- Confirm strap or bracelet options and lug width before shipping.
Buy the complication, not the badge. A clean 43mm case and a moon wheel that actually moves will beat a sharper logo every time you look down at your wrist.