To measure distance, you need a starting point. For latitude, the starting point is naturally the Equator. However, for longitude, there is no natural starting line.
: All meridians meet exactly at the North and South Poles. meridian coordinates
To measure distance, scientists needed a baseline, just as the equator serves as the baseline for latitude. In 1884, the International Meridian Conference in Washington, D.C., established the Royal Observatory in Greenwich, London, as the site of the Prime Meridian. To measure distance, you need a starting point