As voyaging increased into the uncharted regions of the open ocean, mariners brought back their findings to be retained in written records and represented on maps. To create accurate maps cartographers needed more than recounted tales and estimated distances. It became apparent that a grid system dividing the earth into coordinates of latitude and longitude was necessary. Certain obvious reference points for latitude were used by mariners from the earliest of times. The equator and the north and south poles were constant, as was the swing of the sun from its most northerly declination of 23°28′ (Tropic of Cancer) at the summer solstice, through the equinox to its most southerly declination (Tropic of Capricorn) at the winter solstice. Dividing the globe with horizontal, circumferential lines–called parallels of latitude–was a logical construct. Hipparchus of Rhodes (c. 167-127 BC), one of the greatest of Greek astronomers, had marked off the earth’s surface at the equator into 360 parts–the “degrees” of modern geography. Ptolemy followed this plan, subdividing each of the degrees into partes minutae primae (“minutes” of arc) and partes minutae secundae (“seconds” of arc). He constructed circumferential, vertical lines equally spaced at the equator and passing through both poles. These are the meridians of longitude. Each 15° interval represents one hour of the twenty-four hours taken by one full rotation of the earth on its axis. Thus, a grid was constructed whereupon any point on the earth’s surface could be located.
In his "New General Chart of the Atlantic or Western Ocean and Adjacent Seas," de Fleurieu placed great emphasis on geographic accuracy as determined by astronomical observations for latitude and longitude. On this English edition of one of his charts, two prime meridians--the starting line of zero degrees longitude--are indicated by longitude scales at the top of the chart: one, located at the Greenwich Observatory in England; the other, positioned in Paris. A third scale at the bottom of the chart shows Time in hours and minutes, in effect another way of indicating longitude. Even latitude, with its fixed reference points of the equator and poles, is marked off in two different scales. Latitude on the left-hand margin of the chart is measured in degrees, while latitude on the right-hand margin is expressed in marine leagues. Many other features of the ocean--all of importance to mariner--are indicated: magnetic variation; ocean currents; soundings (or lack of); bottom character; fog-bank limit; tidal effects; and, landmarks as viewed from seaward.
Charles Pierre Claret de Fleurieu
A New General Chart of the Atlantic or Western Ocean and Adjacent Seas
Copper engraving, 49.1cm x 69.9cm
London: Sayer and Bennet, 1777
Even though the calculation of longitude was accomplished, meridians of longitude lacked any predetermined, natural starting point (prime meridian) for measurement, such as the equator provided for parallels of latitude. The earliest maps used Alexandria, that great seat of ancient learning, as the prime meridian. Other important geographic points such as Rhodes, Carthage, Strait of Gibralter (the Pillars of Hercules), and Rome were used. Ptolemy used Ferro, westernmost of the Canary Islands (Insulae Fortunatae, or Fortunate Islands, as they were called), for his prime meridian because it was the most westernmost land then known. Following Ptolemy, European geographers, such as Coronelli, also used Ferro. This prime meridian placed London at 18° East longitude.
Mare Del Nord
Copper engraving, hand-colored, 44.9cm x 59.9cm
By the eighteenth century, maps and charts used many different prime meridians, defined by prominent landmarks, capitals, or astronomical observatories. Thus, English charts placed the zero meridian at the Lizard (England's most southwesterly point) or London, designated as through St. Paul's Cathedral, or the Greenwich Observatory. Sometimes the principal observatory of a nation became the locus for the prime meridian. Here, on Bellin's chart of the east coast of North America, five different prime meridians of longitude are shown: Paris, London, the Lizard, Teneriffe and Ferro (Islands of the Canaries archipelago). The inexactitude of geographic positions and lack of an agreed upon prime, or zero, meridian continued to hamper navigators computing their course between places of known latitude and longitude. Obviously, a universally accepted starting point for the meridians of longitude was necessary to convert relative values into absolute values. Worldwide acceptance of Greenwich, England as the datum point (0°) for measurement of longitude did not occur until 1884.
Carte Reduite des Costes Orientales de l'Amerique Septentrionale
Copper engraving, 54.6cm x 88.6cm
Paris: Department de la Marine, 1757
The measurement of distances at sea differs from that on land. The statute, or English mile, was originally the Roman linear measurement of 1000 paces--about 4,854 feet. Today, in the United Kingdom, some parts of the Commonwealth, and the United States, the statute mile used for land measurement has been standardized at 5,280 feet. The nautical, or sea mile, is based on the circumference of the earth. Each degree of a 360 degree circle around the Earth contains 60 nautical miles. The degree is further sub-divided into 60 minutes of arc, therefore, one minute of arc of latitude equals one nautical mile. At 24,859 miles for the Earth's circumference, one nautical mile equals 6,076 feet. Distances at sea, however, were usually expressed in leagues rather than in nautical miles. Portuguese navigators estimated one degree of latitude at 17 1/2 leagues, making a single league equal to 3.43 nautical miles. This measurement remained in use throughout the fifteenth and sixteenth centuries. Initially, Spain used the same measurement, then changed to 16 2/3 leagues in a degree, making one league equal to 3.67 nautical miles. English mariners computed 20 leagues to the degree; this made one league equivalent to 3.0 nautical miles.
A Chart of the Coast of America
Copper engraving, hand-colored, 42.2cm x 54cm
Marking charts with limits of the sun's declination to determine latitude, and lines of latitude and longitude as a grid on which to locate position, were not the only guides navigators used in crossing the open seas. Yet another invisible line divided the ocean. Neither astronomical, nor mathematical in origin, this line was a political one, placed by Papal Bull (decree) and mutual consent of the two great Iberian powers--Spain and Portugal. As their mariners ventured westward into the uncharted Atlantic to seek riches and expand their sovereigns' empires, they encountered new lands, and sought validation of possession from the Vatican. Regarded by Christian nations as the ultimate global sovereign power, the Papacy of Rome had the right to divide the newly discovered world. Spain and Portugal justified their requests for ownership because "as crusaders, they were doing the Church's work and therefore entitled to some secular perquisites." Each island group discovered required new lines to be drawn across the Ocean Sea (as the Atlantic Ocean was then called) marking the territorial limits of Spain and Portugal. None of the divisions created by a series of papal bulls was totally acceptable; either they gave away too much land to one power, or took away dominion from a power whose ownership had already been established. Finally, for the sake of peace and accord, Spain and Portugal resolved their differences independent of the papacy. They reached a compromise in the 1494 Treaty of Tordesillas, wherein a line was drawn from the Arctic to the Antarctic Poles at a distance of 370 leagues west of the Cape Verde Islands. Each power agreed that all lands on the eastern side of the line belonged to the King of Portugal and his successors, while all other lands belonged to the King and Queen of Spain. Dividing the world between themselves prevented incursion into the southern oceans of the vessels of other nations. If England and France wished to reach the East Indies, they could do so only by finding a route north of Canada (the Northwest Passage) or over the top of Norway and Russia (Northeast Passage). At times, the line of Tordesillas, known as Meridianus particionis, was used as a prime meridian.
Antonio de Herrera y Tordesillas
Descripcion de las Yndias Ocidentalis
Copper engraving, 22.5cm x 31.8cm
From: Descripción de las Yndias Ocidentales (Spain, 1622)