North is usually located at the top of a map. But this is simply a convention, determined by a historical twist of fate.
Medieval European mapmakers generally placed east at the top, possibly to orient their maps toward the rising sun. Islamic maps of the Middle Ages, on the other hand, generally show south as up, placing the sacred city of Mecca proudly near the top of the world.
We prefer a northerly orientation, just as we prefer to read English from left to right—tfel ot thgir naht rehtar. But Arabic and Hebrew are read from right to left, Chinese is read down a column, and these directions work just as well. Maps with south, east or west at the top would do just fine, if we could get used to it.
The convention of making maps with north up top probably comes from the great Alexandrian geographer Ptolemy (c. 90–168 C.E.). When Ptolemy’s Hyphegesis Geographike (Guide to Geography) became widely known in the 15th century, mapmakers followed his lead by placing north at the top. Since the world Ptolemy knew was about twice as wide (east-west) as high (north-south), it was convenient to view it on a wide rectangular sheet. And the earliest maps based on Ptolemy’s records were drawn on parchment, which is easier to unroll sideways. Ptolemy also suggested that the north-south position of a place could be calculated by measuring the angle between the horizon and the North Star. Therefore, all maps using Ptolemy’s coordinates give special importance to north.
Unfortunately, Ptolemy’s second-century C.E. Geography was lost to European civilization for more than a millennium. Toward the end of the 14th century, a copy of the work in a Greek codex (a manuscript bound into a book with leaves, rather than rolled up like a scroll) was found in Constantinople and brought to Florence; a Latin translation was completed in 1406. The modern science of cartography began with this reintroduction of Ptolemy’s Geography to Western civilization.
Ptolemy knew that the sky held the key to accurate mapping. He was not the first to realize this; ancient travelers had long reached their destinations by making observations of the sun and stars. Nor was Ptolemy the first to suggest that the heavens be used to make world maps. More than 200 years before Ptolemy the Greek astronomer Hipparchus (c. 190–126 B.C.E.), who developed an accurate method for calculating the distance to the moon, insisted on using astronomy for mapping. Ptolemy’s contribution was to employ astronomic reckonings systematically to plot locations on a map using two coordinates—which he named latitude and longitude.
Like most learned Greeks of his day, Ptolemy was convinced that the earth was a sphere. Anyone living near a sea would have seen ships “heaving into sight.” As the ship approached, its high sails always came into view first. This is what one would see if the ship were coming up over the opposite side of a hill.
Aristotle (384–322 B.C.E.) found additional evidence for a spherical earth in the phenomenon of the lunar eclipse, when the moon disappears behind the shadow cast by the earth: “The sphericity of the earth is proved by the evidence of our senses, 007for otherwise lunar eclipses would not take such forms; for in eclipses the dividing line is always rounded. Consequently, if the eclipse is due to the interposition of the earth, the rounded line results from a spherical shape.”
Greek astronomers also knew that one star, the North Star, remained almost stationary in the night sky, whereas the other stars revolved around it in a counter-clockwise direction. This would be true if the North Star were on the axis of an immense astral sphere that rotated around a fixed earth, as Ptolemy thought (or if the North Star appears above the axis of a rotating earth, as we know today). The sphericity of the earth seemed confirmed by travelers’ reports that the North Star appears higher in the sky the further north one goes. Observations of the sun told the same story: As one moves north, summer days last longer and the noonday shadow cast by an upright object lengthens.
Following these clues, Greek astronomers developed a model of a spherical earth with a north-south axis, around which the heavenly sphere rotates. The sphere could be split in half at the equator to form two equal hemispheres, one northern and one southern. Ptolemy arbitrarily labeled the equator zero degrees of 008latitude. Since a circle has 360 degrees, the maximum degree of latitude is 90 (at the poles, one quarter of the way around the earth from the equator).
In this system, which is remarkably like our own, it only takes a simple calculation to locate a place on the earth’s north-south axis: Measure the angle of the North Star above the horizon, make an adjustment, and you will know how many degrees you are above the equator. (The ancients also used other methods, such as the length of the day, to determine latitude.) Ptolemy was able to pinpoint the locations of about 400 places whose latitude was calculated by astronomic observation.
But it takes two coordinates to plot a place on a map. Cartographers need to know not only the north-south orientation (latitude) of a site but also its east-west orientation (longitude). Determining longitude proved extremely elusive—so much so that accurate calculations of longitude, especially at sea, were not made until the late 18th century.
The Greek geographers did develop a theory for determining longitude, again based on the sphericity of the earth. A complete revolution of the heavenly sphere around the earth—360 degrees—takes 24 hours; thus the sphere rotates 15 degrees every hour (360/24 = 15). High noon, in other words, moves 15 degrees westward every hour. If, for example, Ptolemy had known that at high noon in Alexandria it is 11:00 a.m. in Carthage, he could have plotted Carthage 15 degrees to the west.
The problem is that Ptolemy had no way of knowing, at a single instant, the times of day in other places. But he did suggest an ingenious solution to the problem: If an astronomical event—such as an eclipse—could be observed simultaneously in several places, local times (with respect to high noon) could be recorded and the time differences could be used to record longitude. Ptolemy stated that this had in fact been done during a 331 B.C.E. lunar eclipse, observed in Carthage, North Africa and Babylonia. But for some reason he did not make use of this information (some scholars doubt the observation was ever made). In the Geography, Ptolemy makes no other attempt to calculate longitude through astronomic observation. He was unable to put into practice this excellent method for determining east-west orientation. In his text, longitude is derived from estimates of distance taken from travel reports.
Ptolemy’s Geography taught Renaissance Europe how to use latitude and longitude to chart the locations of places. This method quickly became, and has remained, standard in scientific mapmaking. Today, navigators rarely look to the sun or North Star to find their way; but even the most up-to-date navigational technologies, such as the GPS guidance systems that rely on satellite information, continue to use latitude and longitude.
Despite all these far-sighted innovations, the great Alexandrian geographer had his limitations. He had no information about equatorial and southern Africa, so he represented this area simply as Terra Incognita. He did not recognize that India is a peninsula, and he grossly exaggerated the size of the island of Taprobane (Sri Lanka). His representation of the Indian Ocean as an inland sea is unfortunate, since it ruins an otherwise reasonable sketch of Indochina.
One of Ptolemy’s errors may even have induced Christopher Columbus to sail west. Ptolemy misjudged the circumference of the earth, assigning 50 miles to each degree; in this, he followed the North African geographer Posidonius (130–50 B.C.E.). Ptolemy should have used an earlier, remarkably accurate estimate by the Alexandrian mathematician Eratosthenes (270–196 B.C.E.) of about 70 miles to a degree, making the earth’s circumference 25,200 miles. (Why Ptolemy ignored Eratosthenes is one of the mysteries of ancient science.)
This error misled explorers who were guided by Ptolemy’s judgment—in particular, Columbus, who probably believed there were only 50 miles to a degree. Columbus also thought that by sailing west he would only have to go half way around the world, 180 degrees, to reach the Asian coast. But Ptolemy had made another error: He had miscalculated the location of China. In fact, Columbus would have had to sail westward 240 degrees to reach the spice lands of the East. So Columbus believed that his journey was shorter than it really would have been. Had he known the daunting truth, would he have set sail at all?
North is usually located at the top of a map. But this is simply a convention, determined by a historical twist of fate. Medieval European mapmakers generally placed east at the top, possibly to orient their maps toward the rising sun. Islamic maps of the Middle Ages, on the other hand, generally show south as up, placing the sacred city of Mecca proudly near the top of the world. We prefer a northerly orientation, just as we prefer to read English from left to right—tfel ot thgir naht rehtar. But Arabic and Hebrew are read from right to left, […]