Weather Permitting - GPS Declassified: From Smart Bombs to Smartphones - page 27

such as roads or cities. And repeated astronomical observations can be taken
from a stationary land position, whereas an unanchored ship is always mov-
ing. However, navigating across a featureless desert is much like sea travel.
Alexander the Great is believed to have visited the oasis of Siwa in Egypt. Dan-
ish scholar Torben B. Larsen gives a sense of how the ancients viewed the
risks: “The overland journey was, according to the historian Callisthenes, a
dangerous one. Alexander's party exhausted its water supply, but divine inter-
vention produced a sudden downpour. A sandstorm caused them to lose their
way, but divine intervention, Callisthenes says, sent two crows to lead them
safely to Siwa.” 4 Even today, mariners seeing nonmigratory birds know that
land is nearby.
Every point on the earth can be specified by latitude and longitude. Lati-
tudes are circles running east-west parallel to the equator, which is zero degrees.
The North Pole is ninety degrees north; the South Pole is ninety degrees south.
Longitudes run north-south, perpendicular to latitudes, but the starting point
is arbitrary since there are no unique positions such as the equator or the poles.
Given the British Empire's dominance in the nineteenth century, the British
made the arbitrary decision to place the prime (zero) meridian through Green-
wich in England. Using the previously defined navigational challenge as mov-
ing safely from point A to point B, the estimate of your current position on a
map has some uncertainty, and the destination, point B, also has been mea-
sured with some uncertainty. During the Apollo 8 mission to lunar orbit, nasa
found that the estimated position of the Command Module differed signifi-
cantly depending on which ground stations were used. Managers at Johnson
Space Center in Houston discovered errors in the coordinates of three remote
island tracking stations. “The anomalous measurements from Canary, Hawaii
and Guam were consistent with geodetic errors [in the positions of these sta-
tions] of up to 300 metres,” recalled Pat Norris, a former Apollo navigation
manager. 5 The distance that makes up a degree of latitude is constant whereas
the distance for a degree of longitude is greatest at the equator and is zero at
the poles.
North of the equator, latitude can be measured by the altitude of Polaris,
also called the North Star, adjusting for the fact that it is not precisely over the
pole. An alternative is measuring the sun's altitude at local noon, adjusting for
the season. The sun lies directly over the equator at local noon on the first day
of spring and the first day of fall. If the sun is forty degrees above the horizon
at local noon on those days, a ship's navigator can calculate that he is at the