Eratosthenes' greatest published texts were his Geographica, which "offered a rational and critical argument for the layout and content of a world map" (Talbert, 115) and Measurement of the Earth, which - as the title suggests - outlined Eratosthenes' model for determining our planet's circumference. Unfortunately, both texts no longer exist but we are aware of them through the later writings of Strabo and Hipparchus.
Geographers in the ancient world used a tool - called a dioptra - to measure distances using the stars, "just as was done later with the great medieval instrument that was its direct descendant, the astrolabe" (Talbert, 141). Europe, Africa and Asia were the only continental landmasses known to the Greeks during antiquity. In their maps, Europe and Africa represented the western hemisphere and Asia was in the eastern hemisphere, with the eastern Mediterranean located at the centre of the map. Though he only published a textual 'image' of his worldview, the map below symbolises what the Earth looked like to Eratosthenes and many of his contemporaries in the latter decades of the ancient era:
[Eratosthenes's world map, similar to the one seen on p. 103 of Akerman, Ancient Perspectives; image source: http://www.henry-davis.com/MAPS/Ancientimages/112A.JPEG via Google Images]
"Building on Dicaearchus's diaphragma and rejecting a division of landmasses only by bodies
of water, he [Eratosthenes] split the oikoumene into two equal halves, with a parallel from the
Pillars of Hercules to the easternmost limit of the Taurus Mountains; hence, Dicaearchus's
symmetrical axis was reinforced (Strabo 2.1.1). Eratosthenes then subdivided his northern
and southern halves into "seals," of sphragides, irregular quadrilateral shapes resembling
document seals. Thus, India was rhomboidal, bounded by oceans on two sides, by
the Taurus Mountains to the north and the Indus River to the west; Ariana was a parallelogram
delimited by the Caspian Sea, the capes of Carmania (in southern Iran), and the Persian Gulf.
Eratosthenes divided his northwest region, Europe, on the basis of three promontories projecting
into the Mediterranean: the Peloponnese, Italy, and the Ligurian "promontory" of Corsica
and Sardinia. Even though his excessive generalizations were later subjected to harsh
criticism by Hipparchus and Strabo, his sphragides still represent a concerted effort
to compartmentalize, categorize, and impose order on the oikoumene" (Talbert, 102-104).
Eratosthenes was the director of the library at the Museum of Alexandria. It was during this time that he conducted his research for Measurement of the Earth, which would prove to be Eratosthenes' greatest achievement. Though the measurements and calculations are far too mind-boggling for me to comprehend, his methodology was quite straightforward. To simplify, Eratosthenes was aware of a well in Syene (now Aswan, Egypt) which the sun would completely illuminate the bottom of (no shadow was cast) at noon during the summer solstice, when the sun was at its zenith. Determining that Syene was on the same meridian as his home in Alexandria, he observed that the sun cast a slight shadow in Alexandria at the same time on the same day. With this observation, and a calculation of the distance between Syene and Alexandria, he could hypothesise that the Earth was indeed spherical, and determine its circumference. Though his calculations were slightly inaccurate, they were nonetheless remarkably close to the measurements we now know today (Talbert, 100-102; "Size of the Earth: Eratosthenes, 200 BC": http://www.earth.northwestern.edu/public/seth/107/Time/erathos.htm; Chodos, "This Month in Physics History: June, ca/ 240 B.C. Eratosthenes Measure the Earth," http://www.aps.org/publications/apsnews/200606/history.cfm).
["Size of the Earth (Eratosthenes 200 BC)," image http://www.earth.northwestern.edu/public/seth/107/Time/Image1.gif;
Eratosthenes' legacy in the field of geography and cartography lived on well into the Roman period, and indeed, lives on even today
(Chodos, http://www.aps.org/publications/apsnews/200606/history.cfm).
Sources:
"Size of the Earth (Eratosthenes 200 BC)": http://www.earth.northwestern.edu/public/seth/107/Time/erathos.htm.
Alan Chodos, ed., "This Month in Physics History: June, ca. 240 B.C. Eratosthenes Measures the Earth," American Physical Society 15:6 (June 2006): http://www.aps.org/publications/apsnews/200606/history.cfm.
Richard J.A. Talbert, ed., Ancient Perspectives: Maps and Their Place in Mesopotamia, Egypt, Greece and Rome. Chicago: The University of Chicago Press, 2012.
