The Circumference Of The Earth
Was Conjectured, Or Ascertained, By Aristotle, Cleomedes, Posidonius, And
Ptolemy Respectively, To Be 400, 300, 240, And 180 Thousand Stadia.
It is
immediately apparent that these various measures have some relation to each
other, and probably express the same extent; measured in different stadia;
and this probability is greatly increased by comparing the real distances
of several places with the ancient itinerary distances.
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.
Enter page number
PreviousNext
Page 75 of 524
Words from 38865 to 39366
of 273188
