Mars' Prime Meridian, such as: Longitude "0" (2 - Original NASA/MGS/MSSS CTX b/w Frame)

Beer and Mädler used a small circular feature, which they designated "a" as a reference point to determine the rotation period of Mars. The Italian astronomer G. V. Schiaparelli, in his 1877 map of Mars, used this feature as the 0 point of Longitude. It was subsequently named Sinus Meridiani ("Middle Bay") by Camille Flammarion.
When Mariner 9 mapped the Planet at about 1 Km (0,62 mile) resolution in 1972, an extensive control net of locations was computed by Merton Davies of the RAND Corporation. Davies designated a 0,5-Km-wide crater, subsequently named Airy-0 (within the large crater Airy in Sinus Meridiani) as the Longitude 0 point.
Airy-0 was imaged once by Mariner 9 and once by the Viking 1 orbiter in 1978, and these 2 images were the basis of the Martian Longitude System for the rest of the 20th Century. The MGS has attempted to take a picture of Airy-0 on every close overflight since the beginning of the mapping of Mars.