Longitude Zero

On a recent flight from Frankfurt to Atlanta I whiled away the seemingly endless hours by reading The Da Vinci Code. I am reasonable tolerant of esoteric subjects, as long as they are thought provoking, and Dan Brown’s book certainly qualifies on that point. I was intrigued when in chapter 22 the author spins his yarn around what he calls The Rose Line, a polished strip of brass inlaid in the floor of a Paris church, and supposedly placed there to mark a prime meridian. “Long before the establishment of Greenwich as the prime meridian,” Dan Brown asserts, “the zero longitude of the entire world had passed directly through Paris, and through the Church of Saint-Sulpice. The brass marker in Saint-Sulpice was a memorial of the world’s first prime meridian…” – OK all you novelists, just get it down on paper and this surveyor will see what he can do with it –

Non plus ultra Hercules is supposed to have carved into the rocks at Gibraltar, but as a natural son of the all-powerful Zeus he should have known better. Greek seafarers soon discovered the Fortunate [Cape Verde] Islands and in his Geographia Ptolemy proposed to use them as a location for the prime meridian, the zero meridian from which all other meridians are reckoned, reasoning that here and not at the Pillars of Hercules was the end of the habitable world. Meridians were a brainchild of Eratosthenes (c.276-c.194 B.C.), who had calculated the size of the earth, but his meridians were arbitrary and not uniformly spaced. It took Hipparchus in the second century B.C. to develop a geographic coordinate system that required a starting point. 

Claudius Ptolemy (fl. about 150 A.D.) put mapmaking on a scientific basis. To him, the world was a sphere 18,000 miles in circumference that could be enclosed in a network of gridlines derived from a 360- degree circle and divided into degrees, minutes, and seconds. North was at the top, one of two points where all meridians met, and knowing its latitude and longitude would accurately place any point on that sphere. The prime meridian on the twenty-seven maps of his Atlas of the World passes through Alexandria in Egypt, and his atlas remained the best available information for well over a millennium. 

Few maps of the Middle Ages survive and fewer still show coordinates. To hide their lack of knowledge mapmakers filled the voids on their charts with fanciful creatures and intricate arabesques. Any real progress in geography had to wait for the voyages of the Portuguese and Spaniards towards the end of the fifteenth century. Therefore it was in Lisbon where mapmakers revolutionized their art (it was not yet a science) and began to show meridional lines. The prime meridian was usually drawn through Cape St. Vincent at the southwest extremity of Portugal. Later it moved to various islands off the northwest coast of Africa in order to place the zero line away from land and out into the Atlantic. The Canary Islands too became a favorite location; Mercator in 1538 used Tenerife on his celebrated map of the world, while in a 1634 edict France’s king Louis XIII put the zero-line through that group’s westernmost isle at Ferro (Hierro). On Ferro it remained on French maps until the close of the 1700s.

When seafarers were certain that the Cape Verde Islands were located a little further west than were the Canaries, some cartographers used Isla del Fuego (Foco) in that group. The Azores too was a commonly used starting point, especially the island of Pico. It had become customary to number the meridians eastward around the globe, so that the North American Continent usually fell between 230 and 330 degrees, but by the end of the 1700s the Spanish were numbering east and west from Cadiz. Alexander von Humboldt, a Francophile, after his celebrated voyage through Spain’s American colonies (from 1799 to 1804) published an atlas that numbered the meridians west from Paris, but not, as we shall see, from any church in that city.

With the rise of nationalism it became a matter of patriotism, mixed perhaps with some measure of practicality, to place prime meridians within the national territory of the mapmaker. Spain used Cadiz, Toledo, and nine other cities at one time or another. The Italians favored Rome, Pisa and Naples, the Russians St. Petersburg; Copenhagen, Oslo and many other cities have all been used as starting points. English navigator John Davis in 1594 used St. Michaels in the Azores because he thought there was no compass variation at that point.

Davis’ landsman John Seller was in 1676 the first to use the meridian of London, with no particular point in that city selected. Only a century later an Ordnance Survey under General William Roy specifically used the cross on the dome of Saint Paul’s cathedral as zero point. General Roy, a military engineer, had a penchant for surveying and mapmaking and often surveyed for no other reason than his own amusement, fixing the positions of prominent points in and around London to determine their relationship to the Royal Observatory in Greenwich. In 1784 he was put in charge of a survey to tie Greenwich to the observatory in Paris. 

The adoption of the meridian of Greenwich is in no small part the work of Nevil Maskelyne (1732-1811) astronomer royal, who in 1765 began publishing an annual Nautical Almanac. In his tabulations he figured all lunar-solar and lunar-stellar distances from the position of his Greenwich observatory. The British Admiralty soon used his tables to calculate their positions at sea, and by the early 1800s the longitude of Greenwich was used by navigators of nearly all nations, while on land it first appeared in 1794 on a Map of England and Wales. The adoption of the Greenwich meridian was by no means universal; as late as 1881 and on topographic surveys alone there were at least fourteen different prime meridians in use throughout the world, including our own meridian of Washington.

On September 28, 1850 an Act of Congress provided “That hereafter the meridian of the observatory at Washington shall be adopted and used as the American meridian for astronomic purposes and … Greenwich for nautical purposes.” The Washington meridian passes through the center of the dome of the old Naval Observatory on 24th Street and Constitution Avenue and is 77°03’02.3” west of Greenwich. During the 62 years of its existence meridional boundaries of eleven western states were affected by the Act, including New Mexico’s boundary with Arizona, run at 32° west of Washington. The Act was repealed August 22, 1912.

French astronomer and mathematician Pierre Simon Laplace (1749-1827) first proposed the adoption of a single prime meridian for all countries, an idea closely tied to the adoption of a common standard of time with interconnected time zones. The French were the first to put mapmaking on a truly scientific footing. On the day of the summer solstice in the year 1667 members of the Académie Royale began with observations for the location of a new observatory, the center of which was to become the official meridian of Paris. Yet it took three quarters of a century before it was first used on a map of France in 1744. The French tried unsuccessfully to sell it to the rest of Europe, and as late as the1890s the French delegation to the Sixth International Geographical Congress was still arguing for the adoption of the Paris meridian for world maps.

At the third International Geographical Congress held in 1881 in Venice a proposal was made to create a time system for the world in which minutes and seconds were uniform across the globe, time differing only by hours at 15 degree intervals, based on Greenwich as the starting point. No country was more interested in such a reform as the United States with its rapidly expanding railroad system. At the International Meridian Conference of 1884, held in Washington D.C. at the behest of President Chester A. Arthur, the adoption of the Greenwich Meridian as the prime meri
dian for the entire earth was approved by representatives of 22 nations. The French were miffed and abstained when the vote was taken.

A post script for all you GPS fans: The zero meridian used by satellite navigation systems (e.g.WGS84) is not fixed on the ground and at present is about 100 meters to the east of Greenwich. This arrangement allows for the ever-present continental drift.