Longitude is a geographical coordinate representing the east-west position of a vessel, measured in degrees, minutes, and seconds (0° to 180°) east or west of the Prime Meridian (Greenwich, England). These vertical lines, called meridians, converge at the poles and are crucial for calculating time differences, as 15° of longitude equals one hour.

Key Aspects of Longitude in Sailing:
Measurement: Longitude is the angular distance from the Greenwich Meridian (0°). It is often expressed in degrees and decima minutes.

Distance Calculation: Unlike latitude, the distance between degrees of longitude varies. At the Equator, 1° equals 60 nautical miles, but this distance decreases to zero at the poles.

Time and Navigation: Historically, calculating longitude was a major challenge requiring accurate timekeeping, solved by the marine chronometer to compare local time with Greenwich Mean Time (GMT).

Positioning: On a chart, longitude lines run top-to-bottom, used in conjunction with latitude (horizontal lines) for finding positions.