Prime Meridian

First meridian; the starting point of longitude; the meridian from which longitude is reckoned east and west as high as 180 deg.; the meridian established by a country from which to reckon longitude in laying out their charts. The French use the meridian of Paris as 0 deg.; the English that of Greenwich; the Russians that of St. Petersburg; the Spaniards that of San Fernando; the Americans that of Washington, etc. The meridian passing through the national observatory of a country is selected as the prime or first meridian of that country. It is to be explained that a chronometer must be regulated to the meridian of the country whose chart is being used for navigating the ship. A ship having a chronometer set to Greenwich time must use English charts; a chronometer set to Paris time must have French charts, etc.