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.
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