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250,000 stadia. Unfortunately, we do not
know the length of the stadium he used.
Hipparchus (190-120 B.C.) may be re-
garded as the founder of observational as-
tronomy. He measured the obliquity of the
ecliptic, and agreed with Eratosthenes. He
altered the length of the tropical year from
365 days, 6 hours to 365 days, 5 hours, 53
minutes–still four minutes too much. He
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