American Prime Meridians: The Early Years

In 1850, the United States Congress capped a decades-long process by officially setting a prime meridian for the new nation that ran through Washington, D.C. The law was limited in its application, excluding maritime uses. It was also limited in its duration, being effectively replaced in a little over thirty years by the international convention that established Greenwich, England, as a common prime meridian. Nevertheless, the project reflected enthusiasm about and efforts toward an American prime meridian, which had ebbed and flowed for a century. Much of the push had come from outside of the federal government, with an American prime meridian appearing on maps and in atlases and being discussed in geography textbooks and scientific societies. Congress had taken up the issue on numerous occasions, and presidents Thomas Jefferson and James Monroe had actively supported the project early in the nineteenth century. Not until the construction of the Naval Observatory in Washington in the 1840s, though, had it been possible to bring these efforts to fruition.

Unlike the equator for latitude, there is no obvious logic for choosing a prime meridian among the infinitude of longitude lines. All lines of longitude connect the two poles, meet the equator at a right angle, and are exactly the same length. Over the centuries, different mapmakers, astronomers, governments, and cultures have placed their prime, or first, meridian at more than a dozen different longitudes. At the beginning of the eighteenth century, most western European maps used Ferro, in the Canary Islands, for their prime meridian.

Over the course of the century, English maps often added a second set of meridian markings based on a prime meridian in London or Greenwich; some even replaced Ferro entirely. Increasingly, French maps set their prime meridian at Paris, while Spanish maps employed various different sites in that country for their prime meridian.

Within a few years of independence, Americans, in and out of government, began calling for their own prime meridian, centered on their national capital. Having one’s own prime meridian seemed, to some Americans, as much a matter of national sovereignty as issuing one’s own currency or maintaining one’s own army and navy. In 1784, just a year after the peace, geographer Jedidiah Morse included maps in his textbook with a prime meridian at Philadelphia. A more advanced geography book by Morse, five years later, made clear the nationalist implications of his choice: “The meridian of Philadelphia is the first for Americans; that of London for the English; and that of Paris for the French.”¹ In the early 1790s, the first published images of Pierre Charles L’Enfant’s plan for Washington, D.C., placed the Capitol building at 0.0 degrees longitude. By 1796, when the new national capital hardly existed in a material sense, one could already find a map of the nation’s mail routes that used Washington for the prime meridian.

What is striking, however, is that two of the most important American-made maps of the colonial era also included a zero meridian in North America. Dating from a period before any idea of American independence or any sense of American nationalism, they can hardly have reflected the same concerns that drove the push for an American prime meridian that began in the 1780s and culminated in 1850. Both Joshua Fry and Peter Jefferson’s Map of the Most Inhabited Part of Virginia (1753) and Lewis Evans’s General Map of the Middle British Colonies (1755) included Philadelphia as a prime meridian.

Evans, a Pennsylvanian, also marked the longitude west from London at the bottom of his map. But the actual lines of longitude on the map rely upon the Philadelphia meridian. The original edition of the Fry/Jefferson map did not indicate the longitude from London or Greenwich at all; only in the third state in 1755 was a London meridian added. If it was not a matter of incipient nationalism, then what explains these mapmakers’ use of a North American prime meridian? Each map or mapmaker provides clues for answering this question.

The Fry/Jefferson map, like the Evans map, included a second set of meridian markings even in its original state. Across the bottom, it noted the longitude west from a zero at the point where Currituck Inlet opened into the Atlantic Ocean—the eastern edge of the border between Virginia and North Carolina. That portion of the map must have been based upon the boundary survey that commissioners from the two colonies had undertaken in 1728, which was famously chronicled by Virginia’s William Byrd. But there is no evidence in Byrd’s two accounts of the survey or in the surveyors’ field journal of any effort to establish a zero at the inlet relative to some other meridian or to take even occasional longitude readings during the running of the boundary. When Fry and Jefferson were hired to extend the boundary line to the west in 1749, they almost certainly had a copy of the field notes from 1728 and of whatever map may have been produced from them. While there do not seem to be field notes of their 1749 survey, even without them we might guess that the more accurate of the two sets of longitude marks on the Fry/Jefferson map is the one at the bottom, with its zero at Currituck Inlet. It is certainly more accurate than the London markings, which were “deducted from that of New York at 74° 04’,” thus requiring a two-step process of converting from Philadelphia (or maybe Currituck Inlet) to New York and then New York to London.²

If the Fry/Jefferson map suggests the utility of a local prime meridian, Lewis Evans makes it abundantly clear. At the same time that he published his map, Evans also published what is often called his Analysis of it. Evans grounded his choice of Philadelphia for his prime meridian on the fact that it was “a fine City, situate[d] near the Center of the British Dominions on this Continent, and … far excel[ing other cities] in the Progress of Letters, mechanic Arts, and the public Spirit of its Inhabitants.” Beyond merely explaining his choice of Philadelphia over other North American cities, however, Evans also justified his decision to use an American prime meridian at all. “A Meridian here,” he argued, seemed “the more necessary, that we may determine the Difference of the Longitude of Places by Mensuration[—]a Method far excelling the best astronomical Observations.” “Always reckoning from remote Meridians,” in contrast, seemed likely to lead mapmakers “into several Errors.”³

Evans’s word “Mensuration,” meaning measuring, points to a crucial reason for having an American prime meridian even on a colonial-era map. As long as they progressed east or west along a line of latitude, surveyors could calculate how many degrees (or minutes) of longitude they had covered simply by knowing how many miles they had gone. This process would have allowed the commissioners who established the North Carolina/Virginia boundary, or Fry and Jefferson working with their notes, to fix longitudes from Currituck Inlet without any astronomical observations. Once the commission determined that the boundary followed 36° 31’ of latitude, they could have calculated that each degree of longitude was about 48 miles and each minute of longitude was about 4,224 feet. In effect, Currituck Inlet became the “prime meridian” for their purposes, even if it would never be zero for anyone else. Even without traveling due east or west, moreover, surveyors could keep pretty close track of changing longitude from a known zero by plotting a series of right-angle triangles along their line using different lines of latitude.

Recognizing the value of a local “prime meridian” for accurate mapmaking allows us to make sense of the use of Philadelphia on both the Fry/Jefferson and Evans maps, and of Currituck Inlet on the former. The nationalist sentiments that became so prominent in the push for an American prime meridian after the Revolution do not need to be projected onto the colonial era. This understand raises the question, though, how much did later mapmakers’ use of an American prime meridian arise from the same concern for accuracy?

Banner image credit: detail from Plan of the city of Washington in the territory of Columbia…

For Further Reading

Martin Brückner, The Geographic Revolution in Early America: Maps, Literacy, and National Identity (Chapel Hill: University of North Carolina Press, 2006).

Joseph Hyde Pratt, ”American Prime Meridians,” Geographical Review 32 (April 1942): 233-44.

John Rennie Short, Representing the Republic: Mapping the United States, 1600-1900 (London: Reaktion Books, 2001).

Charles W. J. Withers, Zero Degrees: Geographies of the Prime Meridian (Cambridge, Mass.: Harvard University Press, 2017).

About the Author

James E. Lewis Jr. is a professor of History at Kalamazoo College. He has written extensively on the politics, diplomacy, and political culture of the early American republic. His most recent book is The Burr Conspiracy: Making Sense of an Early American Crisis (Princeton, N.J.: Princeton University Press, 2017), which was a finalist for the George Washington Prize. On October 8th, 2024, Lewis gave a talk at the George Washington Presidential Library about his research on the American Prime Meridian, which can be viewed here.