Meridian
meridian
A meridian is an imaginary semicircle running from the North Pole to the South Pole on the Earth's surface, serving as a reference line for longitude and forming the foundation of the global timekeeping system.
Longitude
longitude
Overview
Longitude is the angular distance measured east or west from the Prime Meridian, which passes through Greenwich, England, at 0 degrees. It ranges from 0 to 180 degrees east and 0 to 180 degrees west. Together with latitude (the north-south coordinate), longitude uniquely identifies any point on Earth's surface.
Relationship to Time
Because Earth completes a 360-degree rotation every 24 hours, each 15 degrees of longitude corresponds to a one-hour time difference, and each single degree corresponds to four minutes. Japan, at 135 degrees east, is theoretically 135 / 15 = 9 hours ahead of the Prime Meridian (UTC+9). In practice, however, time zone boundaries follow national borders and administrative regions rather than meridians, so longitude alone cannot reliably determine a location's actual time zone.
History of Measurement
While latitude could be determined relatively easily from the altitude of Polaris, measuring longitude at sea remained the greatest challenge in navigation until the 18th century. Accurate longitude requires knowing the time at a reference point while at sea. In 1761, John Harrison completed his marine chronometer H4, enabling the first reliable determination of longitude during ocean voyages and dramatically improving the safety of maritime navigation.
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Related Terms
Time Zone
timezone
A region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
Standard Time
standard-time
Standard time is the officially adopted civil time of a region, defined as an offset from UTC and established by law or convention to serve as the basis for all social and commercial activity in that area.
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