It was subsequently named Sinus Meridiani ("Middle Bay") by Camille Flammarion. Schiaparelli, in his 1877 map of Mars, used this feature as the zero point of longitude. They used a small circular feature, which they designated "a," as a reference point to determine the rotation period of the planet. This "International Reference Meridian" is now about 100 meters east of the Airy Transit at Greenwich.) For Mars, the prime meridian was first defined by the German astronomers W. (While visual observations with transits were the basis of navigation until the space age, it is interesting to note that the current definition of the prime meridian is in reference to orbiting satellites and Very Long Baseline Interferometry (VLBI) measurements of distant radio sources such as quasars. The transit circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. The prime meridian was defined by international agreement in 1884 as the position of the large "transit circle," a telescope in the Observatory's Meridian Building. Locations on Earth are measured in degrees east or west from this position. ![]() On Earth, the longitude of the Royal Observatory in Greenwich, England is defined as the "prime meridian," or the zero point of longitude. The Martian Prime Meridian - Longitude "Zero"
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