Answer

A fake astrotourbillon is a rotating cage or arm that does not hold the escapement. The balance wheel and the pallet fork sit on the main plate and stay in one place. The arm spins for looks. On a real astrotourbillon the carriage carries the balance, the pallet fork and the escape wheel, and the whole assembly orbits the dial center once per minute.

What Cartier built in 2010

Cartier showed the Astrotourbillon in 2010 in the Calibre de Cartier case. Caliber 9451 MC places the escapement at the end of a long arm. The arm turns around the dial axis at one revolution per minute. The balance travels with the arm. The seconds indication comes from that rotation. No part of the dial moves at 60 seconds apart from the carriage.

What the copy does

Most replica astrotourbillons use an automatic base such as a 2824 or a 7750 clone. A worker adds a disc, a bridge or a bent arm above the dial. A wheel under the dial drives the arm from the seconds pinion or from a small extra gear. The balance stays at 6 or 9 o'clock. It ticks at 28,800 vph while the arm turns at 1 rpm. Two motions, two energy paths, one decorative part.

Cheaper versions skip the gear train. A quartz motor turns the arm. The balance is printed on the plate or fixed in a recess.

Checks you can run

  1. Film the dial for 70 seconds. Follow the balance wheel. If it holds one position, the arm is decoration.
  2. Look at the arm in a macro shot. A working carriage holds a balance cock, a pallet bridge and a stud holder. A fake arm holds screws, jewels and a printed logo.
  3. Count the jewels on the visible bridge. Decorative jewels sit in blind holes with no pinion under them.
  4. Check the seconds hand. Cartier routes seconds through the carriage. Many fakes keep a subdial at 6 and spin the arm from a separate wheel.
  5. Ask the seller to stop the arm with a finger. On a real astrotourbillon the watch stops. On a decorative arm the balance keeps running.

Market notes

Replica listings use the terms astrotourbillon and flying tourbillon for the same watch. A true carriage with the balance inside costs more, so it appears on higher price tiers. Photos hide the difference because a still frame cannot show which part moves. Ask for video, not images. If the seller sends only stills, treat the claim as unproven.