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the equinoxes , which takes place at the
rate of 52”.1 every year, was necessary for
the progress of accurate astronomical ob-
servations. It is due to a steady revolution
of the earth’s pole round the pole of the
ecliptic once in 26,000 years in the opposite
direction to the planetary revolutions.
Hipparchus was also the inventor of trigonom-
etry, both plane and spherical. He explained
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