Longitude is an imaginary line that runs from the North Pole to the South Pole and is used to describe the location of a place on Earth east or west of the zero or prime meridian. In 1884, the International Meridian Conference adopted the Greenwich Meridian as the universal prime meridian or zero point of longitude. Longitude is measured in degrees ranging from 0° at the Prime Meridian to +180° eastward and -180° westward. Each degree of longitude is further sub-divided into 60 minutes, each of which is further sub-divided into 60 seconds.

The prime meridian, and the opposite 180th meridian (at 180° longitude), separate the Eastern and Western Hemispheres. As you travel west from Greenwich England towards the United States, your longitude is designated in degrees (minutes and second) west longitude. New York City is located in the Western Hemisphere at approximately 74° West longitude, or simply 74° W. Once you reach 180° W the longitude will change from degrees west to degrees east and begin decreasing from 180° east until you once again reach Greenwich England at 0°. Conversely as you travel east from Greenwich towards Europe and Asia, your longitude is designated in degrees (minutes and second) east longitude. Moscow is located in the Eastern Hemisphere at approximately 37° East longitude, or simply 37° E. The Aleutian Islands chain in Alaska stretches from approximately 164° West longitude crossing 180° longitude; and extending to 172° East longitude at Attu Island. Click here to see a map of the Aleutian Islands.

Longitude can be calculated by knowing the time difference at a given location and Coordinated Universal Time (UTC). Since there are 24 hours in a day and and the Earth is divided in to 360 degrees of Longitude, the sun moves across the sky at a rate of 15 degrees Longitude per hour (360° / 24 hours = 15° / hour). So if the time zone a person is in is four hours ahead of UTC then that person is near 60° longitude (4 hours X 15° per hour = 60°). The word near was used because the point might not be at the center of the time zone; also the time zones are defined politically, so their centers and boundaries often do not lie on meridians at multiples of 15°. In order to perform this calculation, a person needs to have a chronometer (watch) set to UTC and needs to determine local time by solar observation or astronomical observation. The details are more complex than described here: see the "Wikipedia" articles on Universal Time and on the Equation of time for more details.

   Page Index
 ◊ Prime Meridian (0° longitude)
 ◊ International Date Line (180° longitude)
 ◊ Royal Observatory, Greenwich England
 ◊ Distance between lines of longitude
 ◊ John Harrison; developed the first successful chronometers
 ◊ Further Reading

For more information about longitude, visit the following sites:

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Prime Meridian (0° longitude)

The Prime Meridian is also known as the International Meridian or Greenwich Meridian line of longitude that, by international convention, runs through "the primary transit" instrument (main telescope) at the Royal Observatory, Greenwich England and is defined to be 0 degrees.

Heading south from the North Pole, the Prime Meridian passes through the following countries:

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For more information about the Prime Meridian, visit the following sites:

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International Date Line

The world is divided into 24 time zones, each time zone is 15° of longitude wide (360° divided by 24 hours – the approximate length of a day). A new day begins at International Date Line The International Date Line (IDL). The International Date Line is located at 180° line of longitude, exactly one-half way around the planet from Greenwich, England.

Each Time Zone is measured relative to Greenwich, England. The military, meteorologist and the aviation industry refer to Greenwich Mean Time as Zulu time. As you travel from west to east, and travel from one time zone to the next, the time you will set your watch forward by one hour as you enter each new time zone. Tine zones to the east of Greenwich England are denoted by a positive number. Example, if it is noon in Greenwich, it will be 1:00 pm in the time zone to the east of Greenwich, or GMT + 1. The first time zone to the east is said to be +1, the next is +2 until you reach the time zone just west of the International Date Line at +12. When you cross the International Date Line, you subtract one day and the time zone is now -12. The next time zone is -11. This pattern continues until we once again reach the prime meridian at time zone zero.

As an example, London England is in time zone zero while Los Angeles, California is in time zone -8. This means when it is 2 pm (or 14:00) in London, it is only 6:00 am in Los Angeles (14:00 - 8 = 6). If we traveled east from London to Wellington, New Zealand which is time zone +12, then it will be 2 am the next day in Wellington, New Zealand (do you really want to see the math? OK 14 + 12 = 26, since 26 is greater than 24, then the time is 26 - 24 = 2 on the next day).

The International Date Line makes detours around political boundaries because the date to the east of the line is one day earlier than that to the west of the line. The line deviates to pass around the far east of Russia and various island groups in the Pacific because no country wants to have its citizens functioning on two different dates. Time zones also often follow political boundaries for the same reason. Some countries have adopted non-standard time zones, usually 30 minutes offset. Two countries that have adopted this standard are Afghanistan and India. This allows all locations within the country to have the same local time.

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For more information about the International Date Line, visit the following sites:

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Royal Observatory, Greenwich England

The Royal Observatory located in Greenwich England is the oldest scientific institution in Britain, founded for navigational purposes in 1675 by Charles II. John Flamsteed, Astronomer Royal, was appointed to work at the Royal Observatory and to get the help of the heavens in finding longitude at sea by calculating "time" from the movement of the heavens. Flamsteed spent forty years plotting the stars in the sky.

The Royal Observatory main contributions have been in navigation, timekeeping, determination of star positions, and almanac publication. In 1767 it began publishing The Nautical Almanac, based on the time at the longitude of Greenwich; its popularity among navigators led in part to the Greenwich meridian's being made Earth's prime meridian and the starting point for international time zones in 1884.

For more information about the Royal Observatory, Greenwich England, visit the following sites:
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Distance between lines of longitude

The distance between each line of longitude changes as you move north or south. To visualize this think of the earth as a peeled orange with the lines between each wedge of the orange as the lines of longitude. At the equator (0° latitude), each degree of longitude is about 70 land miles apart (The circumference of the earth is 24,859.82 miles divided by 360 degrees equaling 69.06 miles per degree at the equator). At 34° north or south, each degree of longitude is 57.3436 miles. Close to the poles at 80° North or South, there are only 12 miles between each degree of longitude.

For more information about Calculating the distances between two places on earth, visit the following sites:
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John Harrison

On October 22nd, 1707 a British fleet led by Sir Cloudesley Shovell struck the rocks of Gilstone Ledges on the Scilly Isles, about 2000 men were lost in the accident. This tragedy led the British Parliament to authorize the British Longitude Act in 1714. This created the Longitude Prize in the amount of 20,000 pounds reward (£10,000 for any method capable of determining a ship´s longitude within one degree, £15,000, within 40 minutes, and £20,000 within one half a degree) for anyone who could devise a practical method of determining longitude at sea. The estblishment of the Longitude Prize led to a race between men of education (Astronomers) and men that labored with their hands (mechanic's/Clock makers) to invent an accurate method of determining the time at sea. Eventually John Harrison (1693-1776) designed and built the world's first successful chronometers.

Between 1730 and 1759, John Harrison designed 4 chronometers designated H1 to H4. It was Harrison´s H4 watch that proved to be three times better than required to win longitude prize.

For more information about John Harrison, visit the following sites:

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Further Reading:

  • Longitude : The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time by Dava Sobel.
  • Tools of Navigation by Rachel Dickinson.
  • The Discovery of Longitude by Jonathan Medwin.

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