Weather Permitting - GPS Declassified: From Smart Bombs to Smartphones - page 29

they can be confident that they will hit land north of the city. Then they could
sail south and be confident that they would reach it. 7 Amelia Earhart, in 1937,
failed to find Howland Island. Her navigator, Frank Noonan, wanted to fly to
a point between Howland Island and Baker Island, which would have greatly
increased their opportunity to check their position, but Earhart flew directly
toward Howland Island and disappeared.
One myth is that ancient mariners hugged the coast. Amir Aczel, author of
The Riddle of the Compass , points out that “the greatest danger a mariner faces
is that of running aground.” 8 Thus, it was critical to master navigation over
open waters. In addition, more direct routes are generally quicker. Some meth-
ods Aczel discusses are sounding lines for measuring the depth of the sea bot-
tom, knowledge of the shore profile, and knowledge of the winds, current, and
the habits of various animals. 9 Geographical knowledge was codified over time
with maps and sea charts. These gave sailors navigating across great distances
interim points to check their accuracy and provided them with information
about ports.
Navigational Techniques
Navigational techniques can be classified as celestial, mechanical, map related,
and geographical. Celestial devices, as mentioned previously, use the sun,
moon, planets, or stars. They are most commonly used to estimate latitude.
The sun and the stars can also be used to locate east and west directions. Since
astronomical objects that are not over the poles move from east to west, they
also reveal the direction.
Mechanical devices were developed to improve the accuracy of these mea-
surements. The astrolabe dates back to about 225 bc. Astrolabes were flat metal
discs, typically about six to eight inches in diameter, made of brass, with mark-
ings representing astronomical features on a moveable faceplate. By rotating
the faceplate and a watch-like hand to align with degree markings around the
outer ring, users could determine the time of day or night and estimate sun-
rise, sunset, and the positions of stars. 10 Over time, more accurate devices were
developed, such as the cross-staf, a simple T-shaped instrument with a move-
able crossbar, used to measure the angle of the sun or stars above the horizon,
and the octant, a triangular-shaped device with sighting mirrors at the apex
and a curved bottom representing an eighth of a circle, or forty-ive degrees,
for determining latitude. 11 Royal Navy captain John Campbell invented the
sextant in 1757. It improved upon the octant by increasing the curved scale to