The short answer

The Rotonde de Cartier Earth and Moon hides its best trick behind a pusher: press it and a moon disc travels across the dial, then slides back. That on demand moon display is the reason the watch exists, and it is also the part almost no replica reproduces. So buy a fake Rotonde Earth Moon for the dial, the case proportions, and the finish, not for the complication. Before you pay, ask for a short video with the pusher being pressed in real time. If the moon does not move, you are buying a printed aperture, and the watch should be priced and judged like a decorative homage.

What the original does, and why it matters here

The genuine watch is a large, dressy Cartier with a second time zone readout, a moon display that appears on demand rather than running on a constant cycle, and a movement built around a module that handles the sliding disc. Replica makers attack the outside first. A convincing case profile, a clean dial, and good printing are all achievable at low cost. A working on demand moon module is not. Understanding that split tells you where to spend your attention and where to accept a compromise.

What to look for

Case and dial

Look at the case flank in the photos, not the front. The Rotonde shape lives in the curve of the lugs and the thickness of the band between the crystal and the caseback. Ask for a straight side profile shot. Dial side, check the alignment of the printed minute track, the sharpness of the numerals, and whether the moon aperture sits centered in its window. Off center apertures and fuzzy printing are the two fastest tells on this model.

Movement and the moon display

Ask which base caliber the seller uses and how the moon function is driven. A date driven moon disc that advances in roughly 29.5 days is a real mechanism, though it is not what the original does. A disc that only moves when a pusher is pressed needs a module with a spring and a lever, which costs more and shows up in thicker cases. A moon that never moves is a painted plate. None of these are automatically bad, but the price you accept should match the mechanism you receive.

Pushers, crown, crystal, and strap

The pusher should have a crisp, short travel and return cleanly. Mushy pushers that sit flush with the case usually mean the function is cosmetic. The crown should screw down if water resistance is claimed. Crystal matters more than most buyers expect: sapphire with inner anti reflective coating keeps the dial legible at an angle, while mineral glass washes out under indoor light. Check the strap lug fit. Gaps at the lugs make an otherwise solid case look cheap.

Parameter bands worth checking

• Case diameter: the real watch is a large piece. Replicas appear from roughly 40mm to 47mm. If a listing claims the larger size but the wrist shots look compact, ask for a caliper photo.
• Thickness: expect roughly 12mm to 17mm. A moon module with a sliding disc adds height. An unusually thin case with a moon aperture is a warning sign.
• Lug width: 22mm to 24mm is workable and easy to re-strap. Odd widths limit your options.
• Movement: a decorated automatic from a known base is the realistic target. Hand wound versions exist but are less common in this shape.
• Crystal: sapphire is the expected standard at the upper tier. Mineral glass belongs on budget builds.
• Water resistance: treat any rating below a screw down crown as splash resistant only, whatever the listing says.
• Plating: for gold tone cases, thicker plating lasts longer. Thin plating wears through on lug edges and crown guards first.

Pitfalls

• Stock or borrowed photos. If the same images appear across several sellers, you are looking at catalog shots, not the watch you will receive.
• Pusher claims without video. A still photo cannot prove a sliding disc works.
• Dial layouts that add or move subdials. Compare against reference images of the genuine model before you commit.
• Misused Swiss labeling. Swiss Made is a protected term with a legal definition tied to where value is created, so a replica claiming it is describing nothing.
• Warranty language with no service path. A warranty is only useful if the seller names who does the work and where.
• Gem set bezels sold as precious stones. Assume treated or synthetic stones unless proven otherwise.

How to vet a seller in one pass

Ask for three things: a timestamped video of the watch running with the pusher pressed, a photo of the movement through the caseback, and the return window in writing. Sellers who answer all three with new material are worth dealing with. Sellers who reply with the same catalog images are not. Pay with a method that gives you a dispute path, and photograph the parcel as you open it.

FAQ

Will the moon display actually work?

On most replicas, no. A working on demand moon disc is rare. Some builds use a date driven disc that moves slowly across the month, which is a fair middle ground.

Is a smaller case acceptable?

Only if you accept it as a different watch. The silhouette changes noticeably below the genuine diameter, and the dial proportions follow.

Can I swim with it?

Do not. Even with a screw down crown, gaskets on replica cases are not tested to a standard. Keep it away from water and steam.

Should I buy the cheapest version?

No. On this model the cheapest tier usually combines mineral glass, a painted moon, and thin plating, which fails within a season.