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landmarks on sea maps enabled navigators
to calculate location and direction with
increasing accuracy.
THE SIZE OF THE EARTH
Contrary to the fable that geographers of
Columbus’s time thought the Earth was flat,
the spherical shape of the globe was a gener-
ally accepted fact in 1492, the same year the
first known globe of the Earth was made by
German cartographer Martin Behaim (signifi-
cantly, Behaim’s globe reflected the miscon-
ceptions of his time and did not include the
Americas). Most of the Earth—roughly 70 per-
cent—is covered with water. In medieval
times, however, European scholars and map-
makers thought the reverse was true. This
assumption was based upon an obscure bibli-
cal passage that declared that only one-sev-
enth of the Earth was covered with seas. The
passage appears in one of a group of chapters
sometimes called the apochrypha, whose dis-
puted origins have caused them to be omitted
from various editions of the Bible over the
centuries. The passage was legitimate to
Columbus, who focused on the description of
God’s creation of the Earth:(^18) B Discovery of the Americas, 1492–1800
Navigating at Sea in 1492 =
Celestial navigation—using the stars, including the Sun, to determine lati-
tude or distance north of the equator—was possible in 1492 with the help of
handheld instruments such as the astrolabe and the marine quadrant. By
sighting the constant position of the North, or Pole, Star with an astrolabe,
navigators were able to roughly estimate their distance north of the equator
by determining the altitude of the star over the northern horizon. Similarly,
the weighted cord hanging from the top of a quadrant would line up along
varying degree marks on the instrument’s curved bottom edge when it was
sighted on the Pole Star. Ship movement made both the quadrant and the
astrolabe somewhat inaccurate. Furthermore, the Pole Star was not visible
from or below the equator. Building on the observations of early Arabian and
Jewish geographers, Portuguese astronomers of the 1480s had also compiled
complicated tables for determining latitude by using the position of the Sun
as it moved north and south of the equator with the seasons. The altitude of
the Sun at noon would be measured with a quadrant, then compared to tables
listing the position or “declination” of the Sun north or south of the equator
according to the date.
Although Columbus had an astrolabe and a quadrant, he depended prima-
rily on a system called “dead reckoning.” (It is believed that deadwas based
on the word deduced,meaning “traced from the beginning.”) Without a fixed
star on the east or west horizon to play the constant role the Pole Star repre-
sented in determining latitude, measuring longitude remained difficult until
the 1700s. In 1492 navigators using dead reckoning roughly determined how
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