Page 59 - BBC History The Story of Science & Technology - 2017 UK

A reconstruction of a
‘world’ map by fourth-century BC
geographer Dicaearchus, pupil of
Aristotle and predecessor of Ptolemy

drawing of the entire known part of the
world”. He then divided the globe’s circum-
ference into 360° (based on the Babylonian
sexagesimal system), with the known world
stretching from west to east through an arc of
177°, from the Canary Islands to Cattigara in
modern-day Vietnam. The known world’s
breadth was estimated at just over half its
length, from Thule (Iceland), 63° north of the
equator, to the region of ‘Agisymba’ (mod-
ern-day Chad), 16° south of the equator, a
latitudinal range of just over 79°.
Yet expanding the world in this way made
it difficult to project the globe onto
a flat surface. Ptolemy knew that no map
projection could ever represent the globe
without distortions, so he used Euclidean
geometry to offer two different methods of
making a world map. On the first cone-like
projection the meridians were drawn as
straight lines converging at an imaginary
point beyond the north pole, with the
parallels shown as curved arcs of different
lengths, centred on the same point. Ptolemy
explained that anyone could draw such a
map by using a swinging ruler and referring
to his tables of latitude and longitude in the
later books of the Geography.
He conceded that this projection had
the world map (see top right) of Aristotle’s circle. This led him to conclude that the its drawbacks: on a globe, parallel lines
pupil Dicaearchus of Messina, who worked globe had a circumference of 250,000 stades diminish south of the equator, but on
between c326 and 296 BC, shows how the (37,000–46,000km). Considering the Earth’s Ptolemy’s projection they actually increase
Greeks began to understand the size and circumference at the equator is 40,075km, in length. His compromise was to propose
shape of a world centred on Rhodes. his calculations were extraordinarily accurate. meridians forming acute angles at the
Ptolemy was also able to draw on some equator. This was fine for the Greeks, who
remarkably accurate calculations of the size From Vietnam to the Canaries regarded the habitable world as ending
of the Earth, including those of Eratosthenes. When Ptolemy came to write his Geography, somewhere in the Sahara, but it would prove
Using a sundial, Eratosthenes measured the he synthesised this mass of Greek learning a problem for 15th-century pilots when they
angle cast at midday on the summer solstice and drew a geometrical net of latitude and tried to sail down the African coast.
at both Aswan and Alexandria, which he longitude over the world, preferring the He therefore offered a second projection
believed were on the same meridian, 5,000 consistency of mathematics over the that was “similarly proportioned” to the
stades apart (a Greek stadion was between unreliable gossip of travellers’ tales (what the globe by drawing curved parallels and
148 and 185 metres). He calculated the angle Greeks called akoe, or ‘hearsay’). He began by meridians. The trigonometry was more
between the two places as one-fiftieth of a defining geography as “an imitation through complex, and Ptolemy confessed that it was

The Story of Science & Technology 59