Longitude

Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including: Ferro, Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, Philadelphia and Washington. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude.

History of the measurement of longitude

The search for a solution

The measurement of longitude is important to both cartography and navigation. Historically, the most important practical application of these was was to provide safe ocean navigation. Knowledge of both latitude and longitude was required. Whereas latitude was easy to determine by celestial navigation using the elevation of the pole star or of the sun at noon, for longitude early ocean navigators had to rely on dead reckoning. This was inaccurate on long voyages out of sight of land, and these voyages sometimes ended with shipwrecks.

Related Topics:
Cartography - Navigation - Latitude - Celestial navigation - Pole star - Dead reckoning

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The discovery of how to measure longitude accurately was among the important discoveries of the 1600s and 1700s. The first effective solution for mapmaking was achieved by Giovanni Domenico Cassini starting in 1681, using Galileo's method based on the satellites of Jupiter. For application without a professional astronomer at hand, and in particular measurement at sea, the problem was more difficult; see Dava Sobel's book: Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time for a good historical overview. This genius was John Harrison.

Related Topics:
1600s - 1700s - Giovanni Domenico Cassini - 1681 - Galileo's - Satellites - Jupiter - Dava Sobel - John Harrison

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The Longitude Act and Harrison's chronometer

The tragic wrecking of the British fleet led by Sir Cloudesley Shovell led to the British Longitude Act, which created the Longitude Prize for anyone who could devise a practical method of determining longitude at sea. This was eventually achieved by John Harrison with his chronometer; the timepiece in question was the one later known as H-4.

Related Topics:
Cloudesley Shovell - Longitude Prize - John Harrison - Chronometer

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Harrison's son led a voyage aboard a ship from Portsmouth, England to the Caribbean port city of Bridgetown, Barbados with the H-4 aboard. Harrison demonstrated a method of determining longitude by keeping the exact time of day for Britain, while using astronomical observations to find the exact local time on the ship as it sailed to the island of Barbados. In this way he able to determine the position of the ship relative to Barbados whose longitude was known. The calculation of the ship's position was only 10 miles in error when it arrived.

Related Topics:
Portsmouth - England - Caribbean - Bridgetown - Barbados - Britain

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Later developments

Exchanges of chronometers between observatories, to determine the precise differences in local time, used in conjunction with the observation of the transit of stars across the meridian became a standard way of determining longitude. Another method was the observation of occultations of stars at different observatories. From the mid 19th century, instead of exchanging chronometers, telegraph time signals were used; radio time signals followed in the early 20th century. Satellites were used for measurements from the 1970s and 1980s - see GPS.

Related Topics:
Transit - Meridian - Occultation - 19th century - Telegraph - Radio - 20th century - Satellite - 1970s - 1980s - GPS

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