The authors discuss the so-called ‘zenith star method’, first mentioned in Ptolemy’s Geography (ca. AD 150), from an astronomical and historical perspective. They reach the conclusion that the exact representation in some texts, i.e. that the distance between the two points of culmination is 1°, does not in fact concern a pair of stars culminating at the zenith but only one star which is measured at an angle of 1Æ from the zenith. This peculiar condition points to a historical measurement carried out by an unknown Greek astronomer: it makes use of the fact that the bright star Pollux (β Geminorum) culminated at Alexandria with an angle distance of 1° from the zenith or (which is equivalent) culminated at the zenith over a place 1° south of Alexandria (ca. 110 km). Although a scholium to Ptolemy’s Geography claims this, the unknown author of the experiment is in all probability not Hipparchus of Nicaea.
Hipparchus. Ptolemy. Circumference of the earth. Zenith star method.
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