MARGINALIA
Finding Out the Longitude
J. Donald Fernie
The Colledge will the whole world measure,
Which
most impossible conclude,
And Navigators make a
pleasure
By finding out the longitude.
Every [sailor]
shall then with ease
Sayle any ships to th' Antipodes.
—Anonymous, circa 1661
Modern astronomers, like most research scientists today, are
resigned to spending a significant fraction of their working hours
writing grant applications and pleading with government agencies for
the funding of ever-more-expensive research facilities. Could it
have ever been the other way around? Was there ever a time when
governments of their own accord established major observatories and
then set about staffing them with top-notch scientists wherever
these might be found? There was! It was the 17th century.
Needless to say, the governments were not acting out of any sudden
urge to understand the universe. The driving power was utterly
pragmatic—someone somehow had to find out how a ship at sea
could determine its longitude. As things stood by the mid-1600s,
navigators could readily find their latitude—that is, how far
north or south they were—but were never sure how far east or
west they might be. The result was that all too often they sailed
their ship in one direction when they should have gone in quite
another. The resulting shipwrecks led to the loss of thousands of
lives and cargoes worth fortunes.
Even as late as 1707 there occurred the famous episode of Admiral
Sir Cloudesley Shovel (I love that name), whose fleet ran through
totally overcast skies and violently stormy weather while returning
to England from Gibraltar. After 12 days of such weather, no one was
certain where they were. The fleet's navigators conferred and
concluded they were well west of the French island of
Ouessant—'Ushant' in British terminology—which lies off
Brittany and marks the southern entrance to the English Channel. It
was said that an ordinary seaman on the Admiral's flagship publicly
disagreed with this conclusion and was promptly hanged from the
yardarm for his insubordination. The fleet then turned eastward,
hoping, presumably, to sight the island and so enter the Channel.
Instead, during the night they ran headlong into the Scilly Isles
just off the southwest tip of England. Four ships and two thousand
men were lost, including the Admiral. Virtually every writer on the
subject of longitude cites this story as an example of how urgent
the need for a way of determining longitude had become. Ironically,
however, if you examine a map, you find that to have run into the
Scilly Isles this way, they must indeed have been well west of
Ouessant in terms of longitude. The error, though, was one of
latitude! They were actually about 1.5 degrees north of
where they thought they were. In any case, ships at sea sailing for
12 completely overcast days would have had no way of knowing either
their latitude or longitude with any certainty, right up to the
mid-20th century, when electronic aids like Loran became available
in some parts of the world.
This crisis in sea travel became ever more urgent as world
exploration and trading developed, so that a Royal Observatory in
Paris in 1667 and another in Greenwich in 1675 were established,
largely in the hope that they would lead to an astronomical solution
to the longitude problem. (There was considerable contrast in the
way the two observatories came about. The French lavished funds on
theirs, but the royal warrant authorizing the building of the
Greenwich observatory announced that "the paying of such
materials and workmen as shall be used and employed therein, [shall
come] out of such monies as shall come to your hands [from the sale
of] old and decayed [gun-]powder. " Also, the choice of
Greenwich as a site was at least partly influenced by the fact that
an old, disused building there would provide many of the necessary
building materials, and some wood, iron and lead would come from a
gatehouse demolished in the Tower of London. Moreover, the
astronomer appointed to the new observatory was poorly paid and had
to provide all necessary instruments, including telescopes, himself.)
Why was longitude determination such a difficult problem compared
with finding latitude? A very basic answer is that latitude is
measured north or south and so is independent of the earth's
east-west rotation, whereas longitude's determination by celestial
means is affected by that rotation. Latitude can be found, in
principle, from angular measures alone—say the angle of the
midday sun above the horizon—but longitude requires knowledge
of time. Thus if a mariner had a clock keeping Greenwich time and
found that it read 2 p.m. when the sun was at its maximum angle
above the horizon—the local noon—he would know that his
longitude was two hours west of Greenwich. The whole problem lay in
finding a clock that would keep time with sufficient accuracy over
the long voyages of the 17th and 18th centuries. The best
timekeepers of the age were pendulum clocks, but these were useless
on the heaving deck of a small ship, whereas spring-wound clocks
were relatively crude and hopelessly inaccurate for voyages over
many weeks or months.
There had been hopes that the variation of the compass—the
angle between the directions of magnetic north and true
north—might do the trick, since it was known to vary with
position on the earth. Could longitude be calibrated in terms of the
variation and perhaps latitude? By 1701 Edmond Halley had dashed
these hopes, showing that in the western North Atlantic, for
instance, the isogonic lines (of constant variation) run almost
east-west and so are independent of longitude.
Thus the turn to astronomy. Were there natural phenomena in the sky
whose happenings might be predicted with precision well ahead of
time, so that the observation of those happenings could then provide
the Greenwich time for correcting an onboard clock? Indeed there
were! Lunar and solar eclipses, for instance. And Galileo had for
years touted the idea of using the bright satellites of Jupiter,
which, as they revolve about the planet, undergo a variety of
eclipses, transits and occultations as seen from the earth. There
was also the moon itself; in the course of its monthly orbit about
the earth, it is seen moving against a background of stars, and if
its changing position in this pattern is predictable, then
observations of its position will tell the time. This scheme became
known as the method of lunar distances, referring to the angle
between the moon and various bright stars as a function of time.
But there were problems with all these schemes. Lunar and solar
eclipses were too rare to be of general use to a mariner, although
they were the principal means of establishing the longitudes of
remote ports and inhabited places generally, especially if the
results from a number of eclipses over the years were averaged. The
use of Jupiter's satellites, although fine in principle, had so many
practical difficulties that the method never was acceptable to
mariners. To mention just two problems: It was next to impossible to
hold a telescope steady enough on the heaving deck of a small ship
to time the eclipses accurately. Also, Jupiter is in the daylight
sky for months at a time, during which periods the method is simply
inapplicable. As for the moon moving against a background pattern of
stars, well, although it too is in the daylight sky for part of each
month, its angular distance from the sun could then be used. But the
moon's apparent motion is so complicated (largely because it is so
near the earth that higher-order dynamical effects become
discernible), that for many years predictions of its position versus
time were not accurate enough. There was the further drawback that
because of the moon's proximity to the earth, its position among the
stars when observed even at the same instant from different
localities is different because of a parallax effect. Thus the
calculations involved in finding one's longitude this way were
laborious and liable to error in the hands of the average mariner.
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