Cartier presented some very original watches at SIHH in January 2014, and the Rotonde de Cartier Day and Night represents an unusual treatment of the moon phase. Usually, moon phase watches have a circular disc with two moons on it, which rotates behind an opening in the dial with two semicircular shapes, so that in a 59-day cycle, first one moon waxes and wanes, then the other. Usually the time of the lunar month is approximated to 29.5 days, with a gear with 59 teeth that moves forward one tooth a day..
The Rotonde de Cartier Day and Night is different. Time is indicated not with hands on a 12-hour dial, but by a rotating central disc that can be seen only in the upper half of the dial. On one side of the disc, a depiction of the sun, with a ray-like pointer, marks the hours from 6 am to 6 pm, and on the other side, the moon is treated in the same way. So the watch also shows day/night as well as the time. The crescent-shaped extremity of the blued hand points to the lunar phases on the lower edge of the watch, and it is retrograde: when it reaches the end of the cycle, it flicks back to the beginning.
The case is in pink gold or palladium, and it is a fairly large watch at 43.5 mm. The detailing is superb, and includes a blue sapphire cabochon set in the crown that coordinates with the blued steel hand. The 9912 MC calibre movement has a power reserve of about 48 hours. Price about $37,000.