Global Positioning System Reference
In-Depth Information
FINDING LONGITUDE BY THE MARINE CHRONOMETER METHOD
The simplest method of measuring time is (no surprise) with a timepiece.
Reliable pendulum clocks had been developed in the late 1600s, and these
were accurate enough for the purposes of longitude estimation. But as was
the case with the Jovian moon method, this approach was not practical on a
ship. Pendulum clocks required a steady, unmoving base so that the pen-
dulum oscillations were uniform. On a pitching and rolling ship—a sailing
ship that would heel over at a considerable angle in crosswinds—pendu-
lum clock accuracy went overboard. The great Dutch scientist Christiaan
Huygens, working in France, tried for many years to get a pendulum clock
to keep accurate time at sea, but never succeeded.
The first timepiece that retained its accuracy at sea required a di√er-
ent operating principle—a spring instead of a freely oscillating pendu-
lum. This marine chronometer was the work of John Harrison, an English
watchmaker of humble origins but lofty aspirations and stellar capabilities.
Harrison completed a total of five handmade chronometers in his attempt
to win the Longitude Prize; these timepieces are now conventionally la-
beled H1 to H5. The first three, which displayed increasingly sophisticated
horological developments, all proved insu≈ciently accurate to merit sub-
mission to the Board of Longitude. H1 had been tested on a sea voyage from
England to Lisbon and worked better than any earlier sea clock, but it did
not satisfy the perfectionist Harrison. He obtained money from the board
to develop H2 and H3, but these designs also did not work well enough.
Harrison had begun work on H1 in 1730 and submitted H4 for considera-
tion in 1761. More than three decades of intense e√ort produced a very
small timepiece that was extremely accurate. In sea trials during a voyage
from England to Jamaica, H4 was found to be only 5.1 seconds slow after
more than nine weeks at sea.
16
The error in estimating longitude, based on
this time error, was about one and a quarter minutes of arc (corresponding
to a distance of about 2 km), which was well within the board's require-
ments for the maximum prize.
This prize was slow in coming; some members of the board favored the
rival lunar distance method,
17
and some may have had their own eyes on
16. On the return journey, which was not part of the trial, the weather was extremely
bad, and yet the watch still kept time su≈ciently accurately to fall within the most stringent
requirement for winning the Longitude Prize.
17. Nevil Maskelyne, the astronomer royal (chief astronomer of England), sat on the
Board of Longitude and dragged his feet over awarding the prize to Harrison. Maskelyne
was a very capable astronomer; among his achievements was an experiment to measure the
