longitude

The prime meridian, defined as zero degrees longitude, passes through the Royal Observatory in Greenwich, London.本初子午线被定义为经度零度，穿过伦敦格林威治的皇家天文台。

Accurate measurement of longitude at sea was a persistent navigational challenge until the invention of the marine chronometer.在海上准确测量经度一直是导航方面的一大持久挑战，直到船用天文钟被发明出来。