It is remarkable that while the terrestrial measurement was thus grossly inaccurate, the observation of latitude as deduced from the gnomon at Alexandria was a very fair approximation to the truth: a fiftieth part of a great circle being equivalent to an arc of 7°12', thus exceeding by about 7' only the true interval between Alexandria and Syene, while falling short of that between Alexandria and the real Tropic by about 30' or half a degree.
It appears indeed almost certain that Eratosthenes himself was aware of the imperfection of his data, and regarded the result of his calculation only as an approximation to the truth. Hence, as mentioned above, he felt himself at liberty to add 2,000 stades to the 250,000 obtained by his process, in order to have a number that would be readily divisible into sixty parts, or into degrees of 360 to a great circle.
After all it must be admitted that the calculation of Eratosthenes, considering the disadvantages under which he labored, came surprisingly near the truth. His measurement of 250,000 stades (the immediate result of his calculation) would be equivalent to 25,000 geographical miles, while the actual circumference of the earth at the equator falls very little short of 25,000 English miles. The error in excess therefore amounted to less than one-seventh part of the whole.
Once the value of 252,000 stades was accepted, it was feasible also to work out the circumference of any parallel circle. Thus Eratosthenes calculated that the parallel at Rhodes, 36°N., was under 200,000 stades in circumference. To obtain the equivalent in stades of one degree of latitude he had only to divide by 360, i.e., 700 stades; to obtain the equivalent of one degree of longitude at Rhodes he could divide, say, 195,000 stades by 360, i.e., 541.67 stades. Thus was established the basis of a fairly accurate system of coordinates for any sectional mapping of the Mediterranean based upon the Rhodes parallel.
Having thus laid the foundation of what has been called in modern times "geodesy" - the determination of the figure and dimensions of the earth, considered in its entirety, as a part of the system of the universe, Eratosthenes next proceeded to consider that portion of it which was in his time geographically known, or supposed to be inhabited. And here it must be observed that the relation between the habitable world, which was alone regarded as coming within the scope of the geographer (properly so called), and the terrestrial globe itself, was, in the days of Eratosthenes, and even long afterwards, a very different one from that which we now conceive as subsisting between them. Ever since the discoveries of the great Portuguese and Spanish navigators in the 15th and 16th centuries opened out to us new continents, and extensions of those already known, far beyond anything that had previously been suspected or imagined, men have been accustomed to regard the "map of the world" as comprising the whole surface of the globe, and including both the eastern and western hemispheres, while towards the north and south it is capable of indefinite extension, till it should reach the poles, and is in fact continually receiving fresh accessions. With the Greek geographers on the contrary, from Eratosthenes to Strabo, the known or habitable world was conceived as a definite and limited portion of the earth's surface, situated wholly within the northern hemisphere, and comprised within about a third of the extent of that section. Towards the north and south a was conceived that the excessive cold in the one case, and the intolerable heat in the other, rendered those regions uninhabitable, and even inaccessible to man. That there might be inhabitants of the southern hemisphere beyond the torrid zone, or that unknown lands might exist within the boundless and trackless ocean that was supposed to extend around two-thirds of the globe from west to east, was admitted to be theoretically possible, but was treated as mere matter of idle speculation, much as we might at the present day regard the question of the inhabitants on Mars.
In his Geographica Eratosthenes discussed the best method of drawing a map of the inhabited area of the earth as known. The first task of the geographer therefore, according to the notions then prevailing, was to determine the limits and dimensions of the map of the world which was to form the subject of his special investigations. This question, which was taken up by Eratosthenes at the beginning of his second book, had already been considered by several previous writers, who had arrived at very different results. On one point indeed they were all agreed, that the length of the habitable world, from west to east, greatly exceeded as breadth, from north to south. Democritus, two centuries before Eratosthenes, had asserted that it was half as long again as a was broad, and this view was adopted by Dicæarchus, though recent discoveries had in his day materially extended the knowledge of its eastern portions. The astronomer Eudoxus on the other hand maintained that the length was double the breadth; Eratosthenes went a step farther and determined the length to be more than double the breadth, a statement which continued to be received by subsequent geographers for more than three centuries as an established fact. According to his calculation the length of the known world from the Atlantic to the Eastern Ocean amounted to 74,000 stades, while as breadth from the parallel of the Cinnamon Country [Ethiopia/Somaliland] to that of Thule [Iceland ?] did not exceed 38,000 stades.
Therefore, as with earlier map construction, the length of the oikumene greatly exceeds the width, though by what proportion depends on how much of the northern, eastern and southern extremities was regarded as inhabited. It is clear from Strabo that Eratosthenes used an orthogonal projection. Rather than a rectangle, he thought of the oikumene as tapering off at each end of its length, like a chlamys [short Greek mantle]. Moreover Strabo tells us that to the above total of 74,000 stades Eratosthenes, using another mathematical ploy, added 2,000 at each end, to prevent the width being more than half the length. On the parallel of Rhodes, this total of 78,000 stades corresponds to about 140° longitude, which is roughly the distance from Korea to the west coat of Spain.
As approximations to sizes and shapes of parts of the world, Eratosthenes first divided the inhabited world by a line stretching from the Pillars of Hercules [Straits of Gibraltar] to the Taurus Mountains and beyond, then subdivided each of these two sections into a number of irregular shapes, or sphragides, which literally meant 'an official seal' and later was extended to represent a plot of land numbered by a government surveyor, then by extrapolation to a numbered area on a map. India he suggested drawing as a rhomboid, Ariana [the eastern part of the Persian Empire] as approximating a parallelogram. We do not know the total number of sphragides and have shapes recorded only for some. Taprobana Island, a misplaced Ceylon/Sri Lanka, and the short-cutting of Africa and India in the south were the result of the misconception that the equatorial waters were too hot to be navigated.
Eratosthenes undoubtedly conceived, in accordance with the prevalent belief in his day, that the Ocean was found immediately to the east of India, and that the Ganges flowed directly into it. Just to the north of the Ganges the great mountain chain of Imaus, which he regarded as the continuation of the Indian Caucasus and the Taurus, descended (according to his ideas) to the shores of the Eastern Ocean; and he appears to have given the name of Tarnarus to the headland which formed the termination of this great range. From that point he supposed the coast to trend away towards the north-west, so as to surround the great unknown tracts of Scythia on the north, but sending in a deep inlet to the south which formed the Caspian Sea.
Of the northern shores of Asia or Europe he had really no more knowledge than Herodotus, but, unlike that historian, he assumed the fact that both continents were bounded by the Ocean on the north; a fact which is undoubtedly true, but in a sense so widely different from that supposed by Eratosthenes that a can hardly be held as justifying his theory. In fact the conclusion of Eratosthenes was mainly based upon the erroneous belief that the Caspian communicated with the Ocean to the north in the same manner that the Persian Gulf did to the south; a view which was adopted by all geographers for a period of three centuries, on the authority of Patrocles.
It was doubtless from the same authority that Eratosthenes derived his statement as to the dimensions of the Caspian Sea, as well as that concerning the outflow into it of the rivers Oxus and Iaxartes, which he asserts "... was well known to the Greeks". Yet the erroneous idea of its communication with the Ocean to the north sufficiently shows how questionable the information possessed by the Greeks really was.
His ideas of the geographical position and configuration of India were also in great measure erroneous. As mentioned above, he conceived it to be of a rhomboidal form, which may be regarded as a rough approximation to the truth, and he even knew that the two sides which enclosed the southern extremity were longer than the other two. But as he supposed the range of Imaus that bounded the country to the north to have its direction from west to east, while the Indus flowed from north to south, he was obliged to shift around the position of his rhomb, so as to bring the other two sides approximately parallel to the two thus assumed. Hence he conceived the projecting angle of India to have a direction towards the south-east instead of the south, and even supposed it to advance farther towards the east than the mouth of the Ganges. He appears in fact to have obtained, probably from the information collected by Patrocles, a correct general idea of the great projection of India in a southerly direction towards Cape Comorin, but was unable to reconcile this with his previously conceived notions as to its western and northern boundaries, and was thus constrained altogether to distort its position in order to make it agree with what he regarded as established conclusions. It was doubtless from the same source that he had learned the name of the Coniaci, as the people inhabiting this southernmost point of India; a name which hence forward became generally received, with slight modifications, by ancient geographers.
Physical geography, in the modern sense of the term, was still quite in its infancy in the days of Eratosthenes, and it cannot be said that he did much to impart to it a scientific character. In treating the mountain chains of Asia as one continuous range, to which he applied the name of Taurus, he may be regarded as having made a first attempt, however crude, at that systematic description of mountain ranges to which we now give the name of orography. He also arrived at a sound conclusion concerning the causes of the inundation of the Nile, a subject that must naturally have engaged the attention of a geographer resident in Egypt. On the other hand he stated a strange hypothesis, that the surplus waters of the Euphrates were carried by subterranean channels to Coele, Syria, and thence again underground so as to feed the streams which broke out near Rhinocorura and Mount Casius.
Eratosthenes also adopted, and apparently developed at considerable length, an idea first suggested by the physical philosopher Strato, that the Mediterranean and the Euxine [Black] Seas had originally no outlet, and stood in consequence at a much higher level, but that they had burst the barriers that confined them, and thus given rise to the Straits of the Bosphorus, the Hellespont and that of the columns. In proof of this theory he alleged the presence of marine shells far inland in Libya, especially near the temple of Jupiter Ammon, and on the road leading to it, as well as the deposits and springs of salt that were also found in the Libyan deserts.
This map of the known world was a very striking achievement and may be considered to be the first really scientific Greek map. Although the dimensions are not known exactly, as it was presented to the Egyptian court a may be assumed to have been fairly large. It must have been drawn as closely as possible to scale, and its influence on subsequent Greek and Roman cartography was tremendous. Indeed, with Ptolemy's inaccurate alterations to the overall dimensions of the world and the oikumene, it can be said to have affected world maps right down to the Age of Discovery.
LOCATION: (this map only exists as reconstruction)
Bagrow, L., History of Cartography, p. 33.
*Bricker, C., Landmarks in Mapmaking, p. 13.
*Brown, L.A.,The Story of Maps, p. 51.
*Bunbury, E., History of Ancient Geography, pp. 618-660.
*Dilke, O.A.W., Greek and Roman Maps, pp. 33.
Harley, J.B., The History of Cartography, Volume One, pp. 154-57,162, 381.
Raisz, E., General Cartography, p. 9.
Thrower, N.J.W., Maps and Man, p. 17.