Weather Permitting - GPS Declassified: From Smart Bombs to Smartphones - page 28

fiftieth meridian. This method has limitations. North of the Arctic Circle, the
sun cannot be seen at times during the fall and winter.
Longitude is much more difficult to determine. The British Parliament in
1714 passed the Longitude Act, which authorized a series of rewards for the
person who perfected a method of determining it and established a commis-
sion, the Board of Longitude, to select the winners. The main competitors were
clocks built by John Harrison, a self-taught clockmaker, versus various astro-
nomical solutions.
Geometry has been used for millennia to measure distances. Eratosthenes,
a Greek mathematician and founder of the discipline of geography, first cal-
culated the circumference of the earth circa 240 bc. He knew that at noon on
the summer solstice in Swene (modern Aswan, Egypt), a city located near the
Tropic of Cancer, the sun was directly overhead, whereas in Alexandria the
sun appeared at an angle south of the zenith. That angle equaled one-iftieth
of a circle. Eratosthenes reasoned, assuming that Alexandria was due north
of Swene (it is actually slightly northwest), the earth's circumference is fifty
times the distance between the two cities.
Harrison's timepiece, which developed into the marine chronometer, tells
a mariner his longitude relative to the embarkation port based on its time com-
pared to local noon (the sun's highest point in the sky). If the mariner's clock
reads 1:00 p.m. at local noon, he is one-twenty-fourth of a day or fifteen degrees
(one-twenty-fourth of a 360-degree circle) west of the embarkation port.
Accurate maps and charts are important for showing hazards. This is impor-
tant even in the age of gps. The ferryboat Pride of Canterbury , which travels
between Dover, England, and Calais, France, hit the 1917 wreck of the ss Mah-
ratta on January 31, 2008. The submerged wreckage was shown on the elec-
tronic sea chart but not at the magnification the Pride of Canterbury was using
when the collision occurred. 6 Thus gps by itself is not enough. One must also
be aware of software idiosyncrasies.
Navigational challenges include finding a point along a coast or finding an
island. Rear Adm. Daniel Gallery (1901-77) commanded a task force hunting
German submarines in World War II. He seized the u505 , the first ship cap-
tured by the U.S. Navy since the War of 1812, which now resides in Chicago's
Museum of Science and Industry. In a novel written after the Six-Day War in
1967, he described current navigation techniques including Loran, a ground-
based system of radio beacons, and the first space-based navigation system,
Transit. However, his characters find Tel Aviv by sailing at a bearing where