The short answer
When people say "fake earth and moon," they almost always mean the disc sitting behind a moonphase or day/night aperture. On a genuine watch that disc is a piece of metal, stamped or etched into relief, then painted in several passes and set deep behind the dial. On a replica, the same part is usually a thin foil or plastic wafer with a flat image of a moon, sometimes with a small Earth next to it, pressed on top. The giveaway is depth. Real artwork has it, printed artwork does not.
How the real thing is built
A proper moon disc runs roughly 0.2 to 0.3 mm thick, usually brass, with the craters formed by stamping before any paint goes on. Omega's Speedmaster Moonphase goes further and reproduces an actual photograph of the Moon, licensed from NASA, so you see maria, crater shadows, and a soft terminator line rather than a cartoon circle. Other makers, IWC among them, put two moons in the aperture so the display looks balanced at every stage of the cycle.
That is the reference point. Once you have seen a real disc under a loupe, replicas become easy to read.
What gives a replica away
- Flatness. No relief, no shadow inside the aperture. It reads like a sticker because it is one.
- Aperture edges. Dried adhesive, a burr, or a white ring where the opening was punched through the dial.
- The Earth. A saturated blue circle with green blobs. Real depictions show cloud cover, a soft coastline, and continents that are not the same size on every sample.
- Sky field. Genuine discs use a graduated deep blue or black. Cheap ones use flat black paper that goes grey under light.
- Centering. Moon off-center in the aperture, or the disc visibly wobbling as it turns.
- Cut. An oval opening instead of a clean arc, with paint bleeding onto the surrounding dial.
Does it actually move?
Most moonphase gearing runs a 29.5-day cycle, close to the true 29.53-day synodic month but not identical, which is why high-end perpetual calendars use a 135-tooth wheel to stay accurate for decades. A quick test I use: set the moon to full, note the date, then check again in about seven days. A working display should have advanced roughly a quarter of the way around. If nothing shifted, the disc is decorative and the pusher is doing nothing at all.
The caseback tells you more than the dial
Moon watches carry a lot of engraving on the back, and this is where replicas fall apart. Look for shallow laser etching instead of deep stamping, lettering that is too wide or too thin, and a medallion that sits proud of the surface. Genuine casebacks are machined first and finished after, so the edges stay crisp.
Before you buy, check these
- Ask for a macro photo of the aperture, not a full dial shot.
- Request the case thickness. Moonphase builds run 11 to 13 mm, and a thin case usually means a flat printed disc.
- Confirm the disc material in writing. Metal, or walk away.
- Test the pusher cycle yourself if you can hold the watch.
- Compare the moon face to a NASA lunar photo. If the craters do not match any real region, it is invented artwork.
None of this is about brand names. It is about whether the part in front of you was made by someone who cared how the Moon looks.