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.
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