Surveying - The Science of Navigation - page 62

lems without calculation, in the centuries before the telescope was in-
vented and before planetary motion was properly understood, by present-
ing a view of the relative positions of the sun, the moon, the planets, and
the stars at di√erent times. That is, the astrolabe showed what the sky
looked like at di√erent times and places. Astrolabes were usually made of
brass and were accurate only for a particular latitude. They were used to
tell the time of day, to determine when sunrise or sunset would occur, or to
point the faithful toward the direction of Mecca. 4
Astrolabes were astronomical instruments, not really navigational or
surveying tools, and I mention them here only because of their antiquity
and because they gave rise to the mariner's astrolabe during the period
leading up to the European Age of Exploration, to which we now turn.
Sunny Latitudes
Many of the navigational instruments that were derived during the first
200 years of European maritime exploration of the world 5 were designed
to estimate latitude from the height of the midday sun. We will examine
these instruments in this section. Mapmakers as well as navigators used
them, and they led to the development of more sophisticated instruments
employed in later times for land surveying as well as maritime navigation.
CROSS-STAFF
Also known as a Jacob's sta√ , the simplest form of the cross-sta√—which I
describe here—was used by mariners to estimate the height of the sun,
from which they could determine latitude. Land surveyors made use of a
more sophisticated version.
The cross-sta√ was first described in the fourteenth century, though the
instrument may be much older. It consisted of two straight rods set perpen-
dicular to each other. The larger main sta√ supported the smaller transom ,
which could slide along it, as suggested in figure 3.7. The angle subtended
by the ends of the transom at one end of the main sta√, where the mariner
placed his eye, was marked on the main sta√. A mariner who wanted to
estimate the height of the sun would align the lower end of the transom
with the horizon, as shown, and the upper end with the sun.
A simple instrument, the cross-sta√ was accurate to within a degree or
4. The astrolabe is discussed by Morrison (2007).
5. Roughly the fifteenth and sixteenth centuries and known as either the Age of Explora-
tion or the Age of Discovery.