Weather Permitting - GPS Declassified: From Smart Bombs to Smartphones - page 31

ter was in sight. There are times in the year when Jupiter is on the opposite side
of the sun from the earth.
Christopher Huygens, a leading seventeenth-century astronomer and math-
ematician, devised the first pendulum clock in 1656. John Harrison, as memo-
rably recounted by Dava Sobel in Longitude: The True Story of a Lone Genius
Who Solved the Greatest Scientific Problem of His Time , built on the work of Huy-
gens and others in devising methods to offset the effects of heat and humidity
on clocks. Harrison built increasingly sophisticated clocks in his attempt to
win the £20,000 prize. His first clock, h1, was taken on a sea trial in 1736. It
performed well, but the Board of Longitude insisted that a transatlantic trial
was needed. His last clock, h4, a compact, ive-inch-diameter sea watch, was
taken by his son on such a trip in 1761.
The lunar method requires an accurate measurement of the moon's angle
in relation to specific stars. Lunar tables were completed by 1767, the same
decade in which Harrison's h4 received two official sea trials. Both methods
were used by navigators between 1770 and 1850. Royal Navy captain James
Cook took a replica of Harrison's h4 on his voyages of discovery and called it
“our faithful guide through all the vicissitudes of climates.”
The competition between the astronomical and the lunar approach ulti-
mately came down to the price of the marine chronometer, since finding lon-
gitude with a chronometer was a faster and easier process and could be done
every day provided you had clear weather. Frank Reed, who leads weekend
celestial navigation demonstrations at Mystic Seaport, in Connecticut, spoke
about lunars during the 2012 conference “After Longitude—Modern Naviga-
tion in Context” at the National Maritime Museum in Greenwich, England.
He commented that the best time for “shooting” lunars is when the moon is
half-full (first quarter or third quarter). It is done in daytime.
Inertial navigation, sometimes called dead reckoning, estimates position
based on speed and direction. Thus a mariner on the equator sailing due west
two hundred hours at five miles per hour ends up one thousand miles, or if-
teen degrees, farther west since the circumference of the earth is about twenty-
four thousand miles at the equator. During the first half of the nineteenth
century, inertial navigation estimates could be adjusted for currents to make
them accurate for about two weeks at a time. This worked well with lunar
updates (adjusting for slight variations in the sun's path) every other week.
Thus the longitude estimate from inertial navigation was updated by using the
lunar method.