Slide #112
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)
REFERENCES:
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.
*illustrated