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Updated: June 21, 2025
Some have maintained that the obelisks which they erected served the purpose of gnomons, for determining the obliquity of the ecliptic, the altitude of the pole, and the length of the tropical year. It is thought that even the Pyramids, by the position of their sides toward the cardinal points, attest their acquaintance with a meridional line.
We have, however, distinct records of two such spheres used by the Chinese about 2500 B.C. Gnomons, or some kind of sundial, were used by the Egyptians and others; and many of the ancient nations measured the obliquity of the ecliptic by the shadows of a vertical column in summer and winter.
He found the distance between the tropics less than 53° 6', and greater than 52° 96', which gives a mean of 23° 51' for the obliquity of the ecliptic. 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.
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