Longitude

Geographic longitude specifies the east-west position of a vessel on the Earth's surface. It is measured in degrees (°), arcminutes ('), and arcseconds (''), or decimal minutes, ranging from 0° at the Prime Meridian in Greenwich to 180° East (E) or West (W). Unlike latitude lines, meridians of longitude converge at the poles. Therefore, while one minute of latitude consistently equals one nautical mile (1852 meters), the physical distance of one degree of longitude varies from 60 nautical miles at the equator to zero at the geographic poles, calculated as 60 nautical miles multiplied by the cosine of the latitude.

Longitude is directly linked to time: because the Earth rotates 360° in 24 hours, 15° of longitude corresponds to a one-hour shift in local solar time (1° per 4 minutes). Determining longitude at sea was historically a major navigation challenge until John Harrison developed the marine chronometer, a breakthrough chronicled in Dava Sobel’s historical account *Longitude*.

In modern sailing, understanding how GPS works and the role of electronic navigation explains how receivers instantly calculate longitude alongside latitude. Mariners must distinguish geographic longitude from celestial or ecliptic longitude used in astronomy, as well as the book title. Reliable dead reckoning requires combining modern chartplotter coordinates with a traditional compass on a sailboat to cross-check headings and positioning.