Designing the World

The first cartographic method of Ptolemy (ca. 100-ca. 178) is a flat representation of the then known world mainly designed to preserve the measurement of the geographic coordinates of localities. In Ptolemy’s day, in the second century CE, the known world—called oikumene in Greek—extended 180° in longitude from the present-day Canary Islands to Indochina. In reality, however, the area occupies only 120° of the Earth’s circumference.

The latitudinal extension was about 80 degrees, from the legendary island of Thule, near the Arctic polar circle, to what was known as the anti-Meroë parallel, 16°25’ south of the Equator. The transformation of the terrestrial globe into a flat surface inevitably creates distortions. To keep distances between places unchanged, Ptolemy accordingly uses an intermediate geometric figure: a semi-cone that surrounds the Earth and touches it on the parallel passing through the island of Rhodes. He then transfers the latitudes measured on the meridian arc to the straight line generating the semi-cone. The length of the line from the top to the anti-Meroë parallel is 131° and a half.

For consistency with the geometric characteristics of the sphere, whose longest parallel lies on the equatorial circle, the boundary of the semi-cone below the Equator is bent inward, as if to form the trunk of an upside-down semi-cone. In this way, as in the sphere, the anti-Meroë parallel has the same radius as the symmetrical parallel passing through the city of Meroë. The transfer of geographic coordinates from the semi-cone thus allows Ptolemy to “peel” the Earth, as it were, turning the surface of the known world first into a cone, and then, simply by folding it out, into a flat shape.