Hipparchus - Geography

Geography

Hipparchus’ treatise “Against the Geography of Eratosthenes” in three books is not preserved. Most of our knowledge of it comes from Strabo. Hipparchus thoroughly and often unfairly criticized Eratosthenes mainly for internal contradictions and inaccuracy in determining positions of geographical localities. Hipparchus insists that a geographic map must be based only on astronomical measurements of latitudes and longitudes and triangulation for finding unknown distances. In geographic theory and methods Hipparchus introduced three main innovations. He was the first to use the grade grid, to determine geographic latitude from star observations, and not only from the sun’s altitude, a method known long before him, and to suggest that geographic longitude could be determined by means of simultaneous observations of lunar eclipses in distant places. In the practical part of his work, the so-called “table of climata”, Hipparchus listed latitudes for several tens of localities. In particular, he improved Eratosthenes’ values for the latitudes of Athens, Sicily, and southern extremity of India. In calculating latitudes of climata (latitudes correlated with the length of the longest solstitial day), Hipparchus used an unexpectedly accurate value for the obliquity of the ecliptic, 23°40′ (the actual value in the second half of the 2nd cent. B.C. was approximately 23°43′), whereas all other ancient authors knew only a roughly rounded value 24°, and even Ptolemy used a less accurate value, 23°51′. Hipparchus opposed the view generally accepted in the Hellenistic period that the Atlantic and Indian Oceans and the Caspian Sea are parts of a single ocean. At the same time he extends the limits of the oikoumene, i.e. the inhabited part of the land, up to the equator and the Arctic Circle. These Hipparchus’ ideas found their reflection in the Geography of Ptolemy. In essence, Ptolemy’s work is an extended attempt to realize Hipparchus’ vision of what geography ought to be.