A small, well-defined crater named Airy-0, near the planet’s equator, was designated as the starting point for the 360 degrees of Martian longitude.
The satellite Mariner 9 began photographing Mars on November 13, 1971, sending back thousands of detailed pictures of the planet’s surface on which to base a map.
Using the information captured by this mapping project, the scientists at the U.S. Geological Survey’s Center of Astrogeology at Flagstaff, Arizona, were able to refine their calculations so that the line for the prime meridian goes through the exact center of the crater, which is about three-tenths of a mile in diameter.
The zero-degree crater was named Airy-0 in honor of George Biddell Airy, a British astronomer who lived from 1801 to 1892. He became Astronomer Royal in 1835, and among his many accomplishments was the building of the Transit Circle telescope in the Greenwich Observatory’s Meridian Building in 1850.
The cross-hairs in the eyepiece of the telescope define zero degrees longitude for Earth.