American Prime Meridians

Abstract

AITHITH the realization of the ancient Greeks more than twenty centuries ago that the earth had a spheroidal surface, the logical VY vposition a datum line from which north and south measurements of latitude might be made was clearly established. However, the nature of things provided no such obvious location a primary line to which east and west determinations of longitude might be referred. As a consequence, each map maker chose what seemed to him an appropriate point through which to draw his prime or reference meridian. Eratosthenes, perhaps the first scientific cartographer, based his zero line of longitude on Alexandria; Hipparchus, on the island of Rhodes; Ptolemy, on the westernmost known bit of land, the Canary Islands. During the sixteenth and early seventeenth centuries accumulating evidence indicated that at a number of points in the vicinity of the Azores, the Canaries, and the Cape Verde Islands the compass needle showed no variation from true north. Assuming these points of zero declination to be oriented along a true north-south line, Mercator and his contemporaries believed that the problem of locating a naturally established prime meridian had at last been resolved, and for many years at this period the prime meridian wandered about in search of the isogonic zero line.' When the basic fallacy of this search was finally understood, the cartographers of the various countries, prompted by considerations of both national pride and practical utility, reverted to the use of locally defined zero meridians. A large number of the more important initial meridians were drawn through leading national observatories. An admittedly incomplete list of those in use by i870 would include Greenwich, Paris, Ferro, Naples, Christi-