The Cassini Planisphere of 1696

ALEXANDER O. VIETOR

IN ANY study of tlie liistory of cartography, it soon becomes
obvious that to understand the subject fully, it is vital to
have a knowledge of sorts concerning the history of dis¬
covery and exploration and, almost more important than either
of these, a knowledge of the history of earth measurements. For
it is to the size of the earth and its shape that all scientific map
making is linked.

This rather self-evident observation was completely under¬
stood by the ancients who struggled with their limited technology
to try to form from terrestrial and celestial data an accurate pic¬
ture of the globe they inhabited. The closest to a modern calcula¬
tion of the earth's size was achieved by the Greek philosopher
Eratosthenes, who came within fourteen percent of the correct
circumference of the earth by measurements made in tlie Nile
Valley and the relative angles of the sun's shadow at widely sep¬
arated points along «'liat was roughly the arc of a meridian.

Knowledge of the size of the earth was likewise bound up with
the voyages of exploration in the 15th century and earlier. It was
in part due to the rejection by Columbus of Eratosthenes's figures
for those of Poseidonius, which were made some one hundred
years later and ■\vhicli postulated a globe roughly one-quarter too
small, that the discoverer tried to reach the Indies by sailing west.
In this, Columbus was only following a belief that was also held
by Claudius Ptolemy, the Alexandrine geographer of the second
century A.D., whose word became law to the philosophers of the
1400's.

From the beginnings of ocean navigation until the development
of the chronometer in the i8th century, the great and burning
question for navigators when making a passage was the distance