Saturday, March 31, 2018

Chronometers, Almanacs, and Computers

A quick note: my name is Mike Romero, and I'm a Historic Interpreter at Colonial Williamsburg.  The postings I make on this site are my own personal opinions and research, and do not necessarily reflect the views of Colonial Williamsburg.  With that said, enjoy the read!

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An engraving depicting the destruction of Admiral Shovell's
flagship HMS Association off the Scilly Isles in 1707.
On October 22, 1707 (November 2 by the modern calendar), a British fleet under the command of Sir Cloudesley Shovell was sailing home, following several successful actions in the Mediterranean.  The return was plagued with constant squalls and poor visibility.  Shovell and his senior officers believed they were just entering the English Channel, in a position to safely weather Ushant...unfortunately, the fleet was nearly 60 nautical miles off course, and bearing straight for the Scilly Isles.  The error was not realized until the last moment, when four ships smashed into the rocks and sank, a fifth just barely steering clear.  Nearly 2000 men would perish in the disaster, owing primarily due to the imprecise navigational techniques of the time.  This incident was one of several factors leading to the passage of the Longitude Act of 1714 offering prizes up to £20,000 for a reliable and practical means for determining longitude at see to within half a degree.  Calculating latitude north and south of the equator was relatively easy by taking a noon sighting of the sun, but longitude was a much more difficult problem to solve.  (Latitude lines are perfectly parallel, while longitude lines grow steadily closer together from the equator until they converge at the poles.)  To judge the various proposals that came in and dispense prize money as appropriate, a Board of Longitude was established, the commissioners of which included professors of mathematics and astronomy along with the First Lord of the Admiralty and the Astronomer Royal.

John Harrison's H4 chronometer, known
more commonly as "The Watch," completed
in 1759.  Harrison would eventually be
awarded £10,000 for the Watch, a copy
of which would accompany Captain
James Cook on his second voyage of
exploration between 1772 and 1775,
One of the more promising proposals to come before the Board of Longitude were the chronometers designed and constructed by John Harrison.  As early as the 1530's, navigators thought of using timekeepers to compute longitude; simply compare the apparent time at the ship with the apparent time at a particular location (such as Greenwich or Paris) and there you have it...every hour of time difference between the two locations being equal to 15° of longitude. The difficulty was in constructing a clock that could run steadily and remain accurate over long periods at sea; imprecise construction, difficulties with friction and wear, changes in temperature, changes in motion, changes in dampness, and a variety of other factors could and did cause clocks to speed up and slow down and otherwise throw off one's reckoning of local apparent time and the corresponding longitude.  Harrison's chronometers, beginning with the unveiling of H1 in 1735 revolutionized this approach.  Each of his first three chronometers included some form of innovation that allowed clocks to run more accurately...components made from two different metals that expand and contract at different rates to counteract the effect of temperature change, components that worked together without friction and thus did not require lubricant that would eventually wear out, and numerous others.

The first three chronometers were relatively large, but H4 (the watch that would bring Harrison the most notoriety) was only 6.5 inches across at its widest and weighing only three pounds (compared with the two foot tall and 60 pound H3)...easily transported and stored at sea.  In 1761, H4 underwent a trial at sea to Jamaica under Harrison's son William, and only lost five seconds of time in a passage of 81 days...making it accurate to within 1.25 minutes of longitude, WELL within the half a degree required to win the £20,000 prize.  The Board of Longitude prevaricated for years, insisting that H4 undergo additional trials on land and that Harrison prove that H4's performance was not simply a fluke.  He was eventually paid a prize of £10,000 after turning over all four timekeepers, submitting H4 to a supervised dismantling and explanation of its workings, and committing to construct two additional watches on H4's principles to prove that his methods were indeed sound.  Sadly, Harrison was in his 70's and had grown steadily resentful of his treatment by the Board of Longitude.  He completed another watch known as H5 (which was tested personally by King George III himself), and was an awarded an additional £8750 by Parliament, but never won the full 'official' longitude prize before his death in 1776.  It would fall to future clock makers such as Larcum Kendall, John Arnold, Thomas Earnshaw, and Thomas Mudge to continue the development of marine chronometers through the end of the 1700's and into the 1800's.

A paragraph or two is by no means sufficient to fully illustrate the contributions John Harrison made to navigation; numerous articles, books, and even a television miniseries has been made on the subject.  (I have plans to discuss Harrison in more depth in future blog posts myself.)  In many instances, the antagonist in a Harrison story is Nevil Maskelyne, a reverend turned scientist turned Astronomer Royal.  While Harrison was busy refining his various timekeepers, Maskelyne became a proponent of the lunar distance method (measuring the distance between the moon and another celestial body, and then attempting to determine the apparent time at the observer's location and Greenwich to calculate longitude) while awaiting the transit of Venus across the Sun from the island of St. Helena in 1761. John Harrison and his son William would blame Maskelyne with increasing vehemence in the years to come for the Board of Longitude's reluctance to award the full prize, changing conditions for marine chronometers to win, and even seizing Harrison's first four chronometers as public opinion that only worsened when Maskelyne was appointed Astronomer Royal (and thereby one of the Commissioners of the Board of Longitude) in 1765.  Despite being vilified somewhat unfairly by Harrison supporters and historians, Maskelyne seems to have supported the chronometer method as well.  Shortly after becoming Astronomer Royal, Maskelyne writes to his brother in India, "The Board of Longitude are also desirous to encourage the making of watches after Mr. Harrison's method.  They have engaged a person to make one.  I have had the drawings engraved here under my eye & shall publish them in a short space of time."

The biggest issue with Harrison's chronometers was their practicality, and whether an instrument as precise as H4 could be duplicated and made available to the general public.  Certainly, the basic multiplication needed to determine longitude from such a device was easy enough, but early marine chronometers were prohibitively expensive.  Larcum Kendall, the person referred to in Maskelyne's letter, is able to create several chronometers using Harrison's principles, but later says, "I am of the opinion that it would be many years (if ever) before a watch of the same kind with that of Mr. Harrison's could be afforded for £200."  For a bit of perspective, the most junior Post Captain in the Royal Navy could expect a daily wage of six would take him 667 days to accumulate enough money to purchase such a chronometer.  Kendall did attempt several less expensive variants, but these came with a corresponding drop in quality and a loss of accuracy.  Years later in the mid-1780's, at the height of competition between the chronometers of Arnold an Earnshaw, marine chronometers still cost between £65 and £80, whereas a mahogany sextant and the latest Nautical Almanac could be had for less than £6 in 1775...and the sun, moon, and stars were available free of charge.

Computation of a lunar distance observation for longitude, likely made by an
East Indiaman, 4 October 1772.
Speaking of the Nautical Almanac (properly known as the Nautical Almanac and Astronomical Ephemeris), this was the brainchild of Nevil Maskelyne and first published in 1766, shortly after he took office as the Astronomer Royal.  The Nautical Almanac contained twelve pages of calculations for each month, including data on the Sun, Moon, the most prominent stars, the movements of the Jovian satellites, eclipses and occultations, etc.  The point Maskelyne's almanac was to cut down on the time required to compute latitude and longitude at sea by calculating the positions and movements of celestial bodies in advance, with special emphasis on those required for the lunar distance method.  Paired with the Tables Requisite to be Used with the Astronomical and Nautical Ephemeris (another publication spearheaded by Maskelyne), the time required to "do your lunars" was reduced from nearly four hours to a much more practical thirty minutes.  Following its initial release in 1766, a new edition of the Nautical Almanac was to be published for every year, and these were eventually published as far as ten years in advance to be of more utility on long-term voyages such as the three year exploratory missions of Captain James Cook.  Initially, two computers were hired to calculate the tables for the first six months of a given year, two others to calculate the second six months, and a fifth person to act as a comparer and check all the tables for errors.  Pay for computers started at £70 per almanac in 1766, though this was increased to £75 the following year.

One of the more unexpected contributors to the Nautical Almanac was a woman named Mary Edwards.  She was the wife of a clergyman/mathematician/instrument maker who used computing work as early as 1773 to supplement his family's income.  John Edwards came to an unfortunate end in 1784, inhaling a lungful of arsenic fumes while experimenting with mirrors for a new telescope.  Desperate to provide for herself and her children, Mary wrote directly to the Astronomer Royal asking to continue computing for the Nautical Almanac.  There are strong indications that Mary was doing most of the computing for her husband prior to his death, and the Board of Longitude begins paying her openly starting in 1784.  Over the next few years, Mary's reputation for reliability and accuracy would grow; where other computers would take several months to finish eight weeks worth of tables, Mary tended to complete the work in less than half the time.  By the early 1790's, the Nautical Almanac was being published ten years in advance, so the Board of Longitude decided to briefly halt the computing work.  Mary appealed to the Board of Longitude for lost income, which was endorsed by Nevil Maskelyne itself, and was successful.  By 1809, Mary had been promoted to comparer, checking the work of the other computers; this came with an increase in pay from £225 per almanac to £250 (the dramatic increase in wages from the 1767 edition a direct result of the protracted wars with France).  She would serve in this capacity until Maskelyne's death in 1811, when she would have to appeal (again successfully) to the Board of Longitude and new Astronomer Royal for continued work.  Mary Edwards died herself in 1815, but not before passing her mathematical proficiency to her daughter Eliza, who continued as a computer until work on the Nautical Almanac was centralized as a form of civil service in 1832 and thus closed to women.

In the end, no one method ever "officially" won the Longitude Prize established by Queen Anne.  Many say that John Harrison deserves the honor, despite the prohibitive price of early marine chronometers.  Chronometers came into more prominence closer to the turn of the 18th and 19th centuries, though many mariners used them in tandem with the lunar distance method to be even more certain of their position at sea.  Well into the 20th century, until the advent of GPS, mariners could accurately plot their way across the oceans with a trusty chronometer, sextant, and almanac.  Notwithstanding "the fascinating modern age we live in," these navigational skills of the late 1700's still hold up today.

1.  His Majesty in Council.  Regulations and Instructions Relating to His Majesty's Service at Sea, The Eleventh Edition. (1772.)
2.  Howse, Derek.  Nevil Maskelyne: The Seaman's Astronomer.  (Cambridge University Press, 1989.)
3.  Pain, Stephanie.  "Lady of Longitude."  New Scientist 13 March 2004: Web.
4.  Sobel, Dava.  Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time.  (The Penguin Group, 1995.)