It is close to Walthamstow Central and the Prime Meridian. It is defined as the mean solar time at Mars's prime meridian. It was chosen for adoption as the international Prime Meridian in 1884. But there was still a hope that a "natural" basis for a prime meridian existed. The Prime Meridian passes through Bromley. It is north of Greenwich on the Prime Meridian. Longitude is measured east and west from the Prime Meridian. Another significant crater is Afon, which marks Ida's prime meridian. The observatory is noted as the home of the prime meridian and Greenwich mean time. The longitude of physical features on Venus are expressed relative to its prime meridian. At the South Pole, grid north conventionally points northwards along the Prime Meridian. Many years later around the 19th century there was still concerns of the prime meridian. Most medieval Muslim geographers continued to use al-Khwarizmi's prime meridian. In 1851 Airy established a new Prime Meridian at Greenwich. This old astronomical prime meridian has been replaced by a more precise prime meridian.