This map of the Atlantic Ocean, dated 1755 and typical of maps of the period, shows two scales of longitude. Above the scale running across the top of the map are numbers reading longitude from 0° at Greenwich, London. Below the scale the numbers read from 0° longitude at Paris. In addition, the vertical line passing through the Canary Islands at about 18° East of Greenwich reflects even earlier use of the Islands as a meridian based their being the most westerly known land.
MAP COURTESY THE NEW YORK PUBLIC LIBRARY
s hown o n m a ps. Ferro ( Hi erro), westernmost of the Canary Islands, or the Cape Ve rde Islands soon became the meridian starting point. Not all prime demarcations ori ginated at territories. Some nav igators, Chri stopher Columbus among them, tried to determine longitude by observing the change in magnetic variation of the compass. He noted that in the Azores there was zero magnetic variation. West of this, the variation became more westerly, while east of it, the variation became easterly in increasing amounts. Mapmakers were quick to use this information in depicting a new prime meridi an (initium longitudinis). Up until this time, cartographers, under the influence of that great authority Ptolemy, used the Canary Islands for the prime meridi an because they represented the boundary of the known world. But now they had a new reason fo r placement of a prime meridian , one based on nature itse lf, usin g the magnetic needle , where it points true north , as their g uide. It w as n ' t until the publi ca ti o n of Ge llibrand ' s Discourse Mathematical/
on the variation of the Ma gneticall Needle in 1635, that the imprac ti ca lity of thi s so lution was full y app rec iated. There were also prime meridians created purely by political ac tions. In the Treaty ofTordesillas in 1494, Spain and Portuga l officially divided the non christian world between them. This meridian , known as Meridianu s particionis, was taken as the prime meridian , and lay 270 leagues west of Cape Verde (rough Iy 20° west of Faial in the Azores). This line was to have important consequences 20
late r in prompting Eng land and Holland to search for a ro ute to the Orient by a northeast or northwest passage. In the 16th and 17th centuries, rewards were offered both by the government and private donorsofFrance, Holland, Venice and Spain for a solution to the problem of determining longitude. In 1598, Philip III of Spain offered the reward of 6,000 ducats, plus a life pension of 2,000 ducats. Despite this financia l stimulus, there were no forthcoming solutions. At the beginning of the 18th century, a bill was put before the British Parliament offering "a pub I ick reward for such person or persons as shall discover the longitude at sea." Known as the Act 12 Queen Anne, it became law on 8 July, 171 4. Under the act, rewards were to be: £10,000 if the method were acc urate to within I 0 , (60 nautical miles); £15 ,000 if acc urate to within 2/3° (40 nautical miles); £20,000 if acc urate to within 1/2° (30 nautical miles) The importance of this problem and the substanti al amount of prize money involved is reflected in the numerous proposals received-in all , fully 54 volumes of manuscri pt papers, now preserved at the Royal Greenwich Observatory. The problem of determining longitude was finally resolved by the perfection of the marine chronometer by John Harrison in 1761 , which provided a simple and practical method for establishing Greenwich Mean Time (GMT) at sea, beating out the other contender for the longitude prize, the lunar distance method. But the solution had little influence on the placement of the prime meridian . It would be another
hundred years before a standard prime meridian would be universally accepted. At the very beginning of the 17th century the most commonly used prime meridians were: A. Canary Islands-based on their historic precedence of being the most westerly land known . This reference placed London at 18°00 ' East longitude. B. Canary Islands -but spec ifi ca lly though Pice de Teide of Isla de Tenerife; for this lofty, volcanic mountain peak, rising 12, 180 feet above the sea, would be visi ble a long way off. London, 19°30' East longitude. C. Azores-particularly through the island of San Miguel. Gerardus Mercator favored thi s for his maps since he thought the true and magnetical meridian (the meridian with no variation) agreed in one place only, at the Azores. London, 24°40' East longitude. D. West Africa (westernmost portion) selected as a meridian when it became obvious that the reasons for selecting A. and C. were at fault. London, I 0° IO ' East longitude. E. Azores-using Ilha da Pico. At 7 ,6 13 feet, it is the highest mountain in the archipelago. This was preferred by the Dutch mapmaker, Jodicus Hondius. F. Cape Verde Islands-primarily used by the Portuguese. By the 18th centu ry, every nation publishing maps and charts used as the prime merid ian its most prominent landmark. Thus, English charts placed the zero meridian at either The Lizard or London (defi ned as through St. Paul' s Cathedral). As an example of how confusing this had all become, the French, in their sea-atlases, showed up to six different longitude scales on each chart. In addition, they showed different scales of distance-Engli sh leagues, French leagues, nautical miles, etc. Legend on the chart states that " this multiplicity of scales is to enable passing ships to communicate their supposed longitude the more readily." By the end of the 19th century, the principal observatory of each nation became the locus for the prime meridian . At this time, we find the following prime SEA HISTORY 66, SUMMER 1993
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