Global Positioning System Reference
In-Depth Information
long time for the nations of the world to agree that the origin of longitude
(the
prime meridian
) should be located at the Greenwich Observatory in
London, England. Prior to that agreement, each country with maritime
aspirations placed the prime meridian within one of its own cities (e.g.,
Cadiz, Naples, Paris). Such is the pride of nations that this agreement,
ironically, took longer to hammer out than did the longitude problem
itself: it was only in 1884, at a conference in Washington, DC, that inter-
national consensus was reached.
The main contenders for the longitude prizes were thus methods for
accurately estimating the passage of time. The two successful candidates,
to which we soon turn, were the marine chronometer method and the
lunar distance method. In total, the British Board of Longitude awarded
over £100,000 on various schemes during its century and more of exis-
tence. Of this total, some £23,000 was given, with some reluctance and
considerable delay, to John Harrison for his marine chronometer; and
another £9,500 went to other contributors to this method, including the
watchmakers John Arnold, Thomas Mudge, and Thomas Earnshaw. The
board awarded £5,000 to lunar distance contributors and £615 to Jesse
Ramsden, whom we met earlier, for his dividing engine. Thus, the bulk of
the prize was awarded to proponents of the marine chronometer, and so it
seems as if this instrument provided the better solution. That would even-
tually prove to be the case, but in the late eighteenth and early nineteenth
centuries, the situation was not nearly as clear-cut as it is sometimes made
out to be today.
MEASURING LONGITUDE: EARLY CONTENDERS
In the seventeenth century two eminent astronomers had suggested solu-
tions for the problem of longitude. Edmund Halley had proposed a method
that considered the celestial background of the moon. He looked for stars
that passed close to the moon or that were occluded entirely by it. He made
observations in support of this method and tried it out at sea. It seems not
to have worked, as he dropped the idea and nobody else advocated it. This
method is similar in spirit to the lunar distance approach except that
Halley restricted attention to stars that were, in angular terms, very close
to the moon. Indeed, Halley became a supporter of the lunar distance
method, and some of his observations contributed to it. Another idea of his
involved the geomagnetic field. He hoped that magnetic deviation or varia-
tion (the di√erence between true north and magnetic north) might indi-
cate longitude. In principle this might work if the magnetic field lines were
