Longitude

Longitude is the angular distance east or west of a reference meridian, measured in degrees around Earth. The internationally adopted reference meridian passes through Greenwich, England.

Latitude: Latitude is the angular distance north or south of the Equator, measured in degrees. Together, latitude and longitude specify a position on Earth's surface.
Prime meridian: A prime meridian is the meridian designated as zero degrees longitude. Longitude is measured east or west from this zero line.
Ptolemy: Ptolemy was a Greek-Egyptian astronomer and geographer whose second-century works shaped European geography. His maps and coordinates helped establish longitude as a geographic measurement.
Dead reckoning: Dead reckoning estimates a vessel's or vehicle's position by advancing a known position using direction, speed, and elapsed time. It can estimate longitude between observations, but errors accumulate without fixes.
Local meridian: A local meridian is the north–south great circle passing through a particular location and Earth's poles. It is not a universal zero line; longitude requires a shared reference meridian.
Meridian: A meridian is a line of longitude running from the North Pole to the South Pole. Longitude measures position relative to a chosen meridian.
Greenwich meridian: The Greenwich meridian is the internationally adopted zero-degree meridian, defined by the IERS Reference Meridian. It is the reference from which modern geographic longitude is reckoned.
Portuguese maritime exploration: Portuguese maritime exploration was the fifteenth- and sixteenth-century expansion of voyages along Africa and across the Atlantic. Ocean navigation made reliable east–west position increasingly urgent.
Marine chronometer: A marine chronometer is a precise portable clock designed to keep reference time aboard a ship. Comparing its time with local solar time gives a ship's longitude.
International Date Line: The International Date Line is an irregularly drawn boundary where the calendar date changes by one day. It lies near 180 degrees longitude, but political borders make its path differ from a meridian.

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