The Observation Of Eratosthenes Respecting The Obliquity Of The Ecliptic
(Though Undoubtedly Not So Immediately Or Essentially Connected With Our
Subject As His Observation Of The Circumference Of The Earth) Is Too
Important To Be Passed Over Entirely Without Notice.
He found the distance
between the tropics less than 53 deg.
6', and greater than 52 deg. 96', which gives
a mean of 23 deg. 51' for the obliquity of the ecliptic. The observations of
Hipparchus (who flourished at Alexandria about 140 years before Christ, and
whom we shall have occasion to mention more particularly afterwards)
coincided with those of Eratosthenes. Plutarch, however, who died A.D. 119,
informs us, that, in his time, the gnomons at Syene were no longer
shadowless on the day of the summer solstice. As the interval between
Eratosthenes and Plutarch was only about 512 years, Bishop Morsley has very
naturally expressed his doubts of the accuracy of Plutarch's assertion. He
says, that the change in the obliquity of the ecliptic in this interval was
only 2' 36". "A gnomon, therefore, at Syene, of the length of twelve
inches, if it cast no shadow on the day of the solstice in the time of
Eratosthenes, should have cast a shadow in the time of Plutarch of the
length only of 9/1000th, or not quite 1/100th part of an inch. The shadow
of a perpendicular column of the height of 100 feet would have been 9/10ths
of an inch." As, however, the ancients do not appear to have constructed
gnomons of such a size, and as gnomons of inferior size would have given a
shadow scarcely perceptible, it is probable that Plutarch is mistaken in
his assertion; or, at any rate, that the very small variation which did
take place between his time and that of Eratosthenes (if it were observed
at all) was ascertained by means of the well itself, which would point it
out much more distinctly and accurately than any gnomon the ancients can be
supposed to have used.
We are also indebted to Eratosthenes for the first regular parallel of
latitude, and also for tracing a meridian. His parallel of latitude began
at the Straits of Gibraltar, and passed eastward through Rhodes to the
mountains of India; the intermediate places being carefully set down. His
meridian line passed through Rhodes and Alexandria, as far as Syene and
Meroe. Meroe, on this account, became an object of the greatest interest
and importance to all the succeeding ancient geographers and astronomers,
and they have taken the utmost labour and care to ascertain its latitude
accurately. Strabo informs us, that Eratosthenes constructed a map of the
world; but he does not give such particulars as will enable us to trace the
extent of his geographical knowledge. At the extremity of the world to the
east, bounded by the ocean, Thina was placed in the map of Eratosthenes, in
the parallel of Rhodes; a parallel which passes through the empire of
China, within the Great Wall. Eratosthenes, according to Strabo, (to whom
we are indebted for nearly all we know respecting this philosopher,)
asserts that Thina had been, previously to the construction of his map,
incorrectly placed in the more ancient maps. His information respecting
Meroe or Abyssinia, is most probably derived from Dalion, Aristocreon, and
Bion, who had been sent by Ptolemy Philadelphus and his successors into
that country, or from Timosthenes, who sailed down the coast of Africa as
low as Cerne. His information on the subject of India (which, however, as
far as regards oriental commerce, is very confused) must have been derived
from the Macedonians. There is little doubt that the library of Alexandria
afforded him access to all the knowledge which then existed respecting the
various countries of the globe; but the turn of his mind led him rather to
astronomical than geographical studies; or rather, perhaps, he directed his
labours and his talents to the discovery of the figure and circumference of
the earth, thinking, that till this was effected, the delineation of the
habitable world, and the relative position of different countries, must be
very inaccurate as well as incomplete. This opinion regarding Eratosthenes,
that he was more of a geometrician than a geographer, seems to be confirmed
by the testimony of Marcian of Heraclia, who informs us, that Eratosthenes
took the whole work of Timosthenes, preface and all, as it stood, and in
the very same words. If this account be accurate, it is probable that
Eratosthenes' knowledge of Thina, and his being able to correct the
erroneous position of this country in more ancient maps, was derived from
Timosthenes, who had commanded the fleet of Ptolemy Philadelphus on the
Indian Ocean.
If we reflect on the rude and imperfect state of science at this period,
the paucity and inadequacy of the instruments by means of which it might be
improved, and the superstitions and prejudices which opposed the removal of
error or the establishment of truth, we shall not be disposed to question
the justice of the panegyric pronounced by Pliny on Eratosthenes. This
author, after detailing all that was then known on the subject of the
circumference of the earth, and on the distances which had been ascertained
by actual admeasurement, or approximated by analogy or probable conjecture,
between the most remarkable places on its surface, adds, that Eratosthenes,
whose acuteness and application had advanced him far in every branch of
knowledge, but who had outstripped all his predecessors or contemporaries
in that particular branch which was connected with the admeasurement of the
earth, had fixed its circumference at 250,000 stadia; a bold and almost
presumptuous enterprize, (_improbum ausum_,) but which had been conducted
with so much judgment, and on such sound principles, that it commanded and
deserved our credit. Hipparchus, who was distinguished for his correctness
and diligence in every part of geometrical and astronomical science, and
who had specially exerted those qualities in his endeavours to correct the
errors of Eratosthenes, had been able to add only the comparatively small
extent of 25,000 stadia to the computation of Eratosthenes.
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