The long and troubled task
of Finding "The Longitude"

D Vautier
6/2019

Travelers who finally ventured by sea beyond the Mediterranean soon found out that the earth was really big and a ship could easily get lost.  By 1500 mariners had worked out a number of reasonable ways to determine latitude using the astrolabe and cross staff.  By measuring the height of the sun and applying a rough estimate of declination they could compute latitude. Mariners then had to used dead reckoning, the stars, the sun, coast lines, log-ships, wind speeds, and currents to help them find a rough longitude but it remained a continuing and troubling mystery for a long time.  Maps continued to contain areas of dragons and sea serpents.

Real worldwide exploration was said to have begun at the end of the 1400s and rapidly increased after that.  This sudden increase in ocean going discovery brought along with it all the accidents which were most likely to happen in uncharted waters and with such poor navigational aids and few accurate maps. It was especially bad because sailors had little idea of where the dangers were.  When they ran into shoals or rocks there was no way to chart them and to warn others. Things were even worse with the smaller caravels that were never designed for the rigors of open ocean travel and were mercilessly subjected to the harsh abuses of ocean storms, long periods at sea and calms. Full oceans were a lot bigger by far then the familiar old Mare Nostrum.

The first notable ocean traveler was Bartolomeu Dias who sailed around South Africa but did not know what he had done until on his way back he was able to recognize the Cape of Good Hope. Of course 4 years later Columbus easily followed the latitude and got to America.  It was said that he did bring an astrolabe and a cross staff.  A few years later in 1498 Vasco Da Gama took three ships and was able to get to India based on maps created by Dias.  He did it by good seamanship, luck, and following the coast. Also the only instruments he took were the astrolabe, the ship-log and the cross staff.

Astronomers gradually developed better ways to find latitude with higher quality cross staffs and back staffs, more accurate declination tables and eventually fine instruments such as octants and sextants, but for almost 400 years after Columbus, longitude remained an unsolvable mystery.  This period lasted from the beginning of full ocean exploration by Dias in 1488 until into the 1800s.  The almost mystical quest for “the longitude” became synonymous with griffins, unicorns, sea serpents and philosophers’ stones.  It was truly the stuff of legends and folklore, a superstition, a joke, a funny and humorous unattainable goal to all including astronomers and scientists alike.  The great Newton himself said it was unsolvable. 

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• this lithograph scene is from Hogarth. Notice insane man looking for the longitude (Cronin, 20).

But ships continued to be lost to uncharted waters and mysterious forces.  Sea maps continued to contain unknown areas marked simply as “here be dragons”.  Spain, England, Portugal, Holland and France had large navies and all desperately needing better sea maps.  In 1598 Philip III of Spain offered a reward of 100,000 crowns to anyone who could solve “the longitude” (Milner, 261). The States-General of Holland soon offered 10,000 florins for a solution.  These large rewards in cash brought interest from noted astronomers and scientists such as Huygens and Hooke but everybody soon lost interest as the task appeared unsolvable under their existing technology.  By 1700, more than a hundred years later, the problem was still nowhere closer to a solution.  Unworkable and crazy ideas were offered such as the chained-ship theory and the wounded dog theory.  In desperation the authors of the chained-ship theory published and printed articles and pamphlets thereby gaining so much attention and notoriety that the merchants of London demanded Parliament to do something about it.  So in 1714 the British government finally signed into law the Longitude Act which offered generous cash awards for basic research and development to help solve the longitude problem.  It is no understatement to suggest that the longitude Act of 1714 represented one of the most successful peacetime efforts on the part of any government anywhere to foster the development of basic research in astronomy, science and precision watch-making.  During the course of it’s existence the Board of Longitude distributed over £100,000 to many worthy projects.

Scientists laid the groundwork for the establishment of a prime meridian somewhere on earth that represented longitude 0.  Then they could measure everything east or west of that line in hours, minutes, and seconds using the sun or moon as it went overhead.  This was no easy task with a sun moving over the earth so fast.  At sea it was not quite as critical since the time could be off 30 seconds and land could still be visible but just to get 30 second accuracy was neigh impossible at that time.

So just where to set the prime meridian depended on which country was the strongest sea power or who had the best scientists or who could develop the best techniques.  The prime meridian was moved several times but eventually wound up at Greenwich, England even though France continued to claim Paris for many years.

Since the establishment of the Royal Observatory at Greenwich in 1675, the race was on between the astronomers and the watch-makers or “mechanics”.  The astronomers felt that the longitude could be solved by use of the moon and stars. The "mechanics" felt that an accurate sea clock could be made.

Lunar-Distance method

But astronomers were determined to solve the longitude problem by the measurement of lunar distances from key stars at different times of the month.  John Flamsteed was appointed the first Astronomer Royal at the Greenwich Observatory and his goal was to compile a catalog of stars and their locations in relation to the moon.  Flamsteed labored at this job for almost 50 years and never completed it.  In 1725 the lunar tables were finally published. They were not really complete until Tobias Meyer of Germany finished them (sort of) in 1755 (Cronin, 16).  To get longitude three simultaneous angles were required, one from moon to star, star to horizon, and moon to horizon.  Hence it required three skilled sailors and a clear night sky.  The lunar-distance method was proven to be fairly accurate to about ½ degree by Nevil Maskelyne who succeeded Flamsteed as England’s Royal Astronomer from 1765-1811.

There were many other calculations needed to get this procedure to work dealing with atmosphere, temperature.  The calculations could take up to four hours.  Any mistake along the way would render the reading incorrect.

The lunar-distance method was not easy.

Chronometer method

The chronometer finally began to become a reality when John Harrison came to London.  Under his leadership and inspiration a host of other clock-makers developed sea clocks that eventually proved inexpensive enough so that almost all English vessels were equipped with them. 

With a good chronometer finding longitude turned out to be incredibly simple.  The most critical part was determining within a fraction of a second when the sun passes over the local time meridian, that is when the sun is highest in the sky.  After all the sun is raging over the earth at almost a mile a second so each second represents about a mile.  Actually it’s 1.1 miles at the equator and less as you go north.  I suspect that there could have been two or three sextant shooters and possibly two or three sea clocks on board any one of the earlier seagoing ships.

Once the time of sun transit is recorded to the second, then the equation-of-time is added, the result being longitude.