The Navigators toolbox-the chronometer

 

[1]

Background

Whilst latitude had been known and calculated for centuries the same cannot be said for measuring longitude, that essential east and west component of a ship’s position. The early navigators exploring and discovering new worlds relied on what became known as latitude or parallel sailing. Not knowing their longitude they followed a southerly course, often along a known coastline before sailing a constant latitude east or west to their destination. Christopher Columbus’s first voyage illustrates this method.

[2]On his first voyage (the blue line on the map), in 1492, he followed the known voyage to the Canary Islands where he carried out repairs before setting out on a westerly parallel course until he discovered an island in the Bahamas. For his return voyage he went northerly until he reached the latitude of Lisbon when he started his easterly latitude sailing. It is important to remember that navigation was relatively crude in this period and charts covered only a small part of the Atlantic Ocean so latitude sailing from a known departure point was a relatively low risk navigational strategy.

On later voyages, as his knowledge of the Atlantic grew and his charts improved, he took more direct routes as shown in his fourth voyage.

This type of navigational voyaging continued as finding longitude at sea proved an elusive target.

Some 200 years later the search for a method for determining longitude became acute.

Sir Cloudesley Shovell as Commander in Chief of an English naval fleet of 21 ships was returning from activities in the Mediterranean and experienced bad weather on the northward leg of the voyage through the Bay of Biscay without satisfactory plotting of his position. Relying on soundings he believed he was north and west of Ushant on the northern French coast and therefore turned to a NE’ly course to enter the English Channel.[3]

However he was much further north and west, and his course now led him to disaster on the outlying skerries of the Scilly islands off SW England.[4]

In the evening of 22^nd. October 1707 the fleet struck the outer rocks, and four ships were lost with an estimated loss of life of between 1500 and 2000 men including their Commander in Chief.

Much analysis of the logs of surviving ships placed the navigational errors on poor navigation with poor equipment. For instance only four of the surviving 112 compasses from nine ships were serviceable.[5]

This disaster perhaps accelerated the search for a method to calculate longitude as in 1714 a group of merchants and mariners petitioned Parliament to solve the longitude problem. This resulted in the Longitude Act of 1714 offering a prize of £20,000 for a method that could determine longitude to an accuracy of half a degree.[6]

However already in 1713 two mathematicians had applied for financial support from Parliament as they had found method to find longitude at sea.

A letter written in a newspaper by William Whiston and Humphrey Ditton stated:

“‘We are ready to disclose it to the world, if we may be assured that no other persons shall be allowed to deprive us of those rewards which the public shall think fit to bestow for such a discovery; but do not desire actually to receive any benefit of that nature till sir Isaac Newton himself, with such other proper persons as shall be chosen to assist him, have given their opinion in favour of this discovery.’” (“The original copy of the 1714 Longitude Act in the Parliamentary Archives”)[7]

So already before the Act came into force the scientific academic community was laying claim to the discovery. Note that the assistance of Isaac Newton to assist. The problem was that the method was completely impractical requiring ships at known locations firing rockets at fixed times. On 25^th. May of that year Parliament considered the application and by July the act became established, and a Board of Longitude created made up of scientists and astronomers who would oversee submissions for the prize. So the scene was set for astronomers to develop a method of calculating the moons motion to establish Greenwich Mean Time. The problem was that it was a complicated process thought beyond the capabilities of seafarers although Captain Cook did commend it on his first voyage albeit he had an astronomer on board at the time to carry out the calculations.[8]

The battle to win the prize

There were two challenges to be overcome, one technical and one political.

The technical issue

This relates to understanding the nature of longitude.[9] Circles of longitude are “great circles” as their diameter is the same as that of the earth and they all pass through the poles. They are often called meridians, and their angular distance apart determines their location, and they are measured east and west of the “prime meridian” which is set at 0 degrees and which today is Greenwich in the UK.

[10]

This meridian divides the earth into two hemispheres east and west.

As the earth rotates 360 degrees in 24 hours, so each 15 degrees of longitude passes in one hour.

So it seems relatively simple to measure longitude. The seaman determines his local time from observations of the sun and compare it with the time back home at the same moment he derived local time. The difference in time can then be calculated as a difference in longitude knowing the longitude of the home base. For example, if the time is 1300 local time when the suns meridian passage is measured it is 1200 GMT (Greenwich Mean Time) at Greenwich the ship is 1 hour ahead of Greenwich meaning 15 degrees east.

That seems simple but you need a clock on board that is accurate because 1 degree of longitude equals 4 minutes of time but in terms of distance, one degree at the equator is 60 nautical miles reducing to zero at the poles. Such accuracy in clocks was not possible in the early 1700’s.[11]

The accepted method of determining longitude in the late 17^th. and early 18^th. century was by the measurement of the angular distance between celestial objects, ideally the moon and the sun, called the lunar distance method.[12] As the moon travels across the background of stars and the sun it was possible to take the measured angular distance, look up its value in a published nautical almanac and find the GMT (Greenwich Mean Time) for that measurement. In the example, finding the lunar distance between a star and the moon will give a time. [13] Once the GMT or any other reference time and location is determined the local mean time (LMT) of the sun’s meridian passage can be found from the Nautical Almanac[14] and the time difference between GMT and LMT can be translated into longitude.

This is a simplified version what, is a long and complicated calculation but central to the method is the accurate measurement of time. In trials of the lunar method at sea in 1764 mariners stated it took around 4 hours to determine longitude.[15]

The political dimension

The competitive nature to find a method by the lunar distances to measure longitude was European wide amongst the astronomers of the day. France, had worked to refine tables and produce the Nautical Almanac to assist in the measurement of celestial objects as a basis to determine longitude.[16]

King Charles II promoted the UK’s bid for astronomical supremacy with the building of the Greenwich Observatory in 1676 and the appointment of an Astronomer Royal, John Flamsteed[17]. So the scene was set for scientists to research methods to determine longitude.

 Over the following 100 years subsequent Astronomer Royals sought to research and promote the Lunar Distance method and capture the prize for determining longitude at sea.

A Board of Longitude was responsible for determining whether any method submitted to them met any of the three criteria:

A £20,000 prize if the method had an accuracy of ½ degree of longitude

A £15,000 prize if the method had an accuracy of 2/3 degree of longitude

A £10,000 prize if the method had an accuracy of 1 degree of longitude[18]

Neville Maskelyne,[19] the fifth Astronomer Royal who was the most ardent advocate of the lunar distance method carried out numerous voyages to promote this method and claim the prize.

The challenger

John Harrison was a carpenter born in Yorkshire in 1694 with a fascination for clocks. At the age of 20 he built his first pendulum clock made entirely of wood.[20]

He continued to experiment with clock-making and around 1722 was commissioned to build a tower clock for Sir Charles Pelham. That clock still runs some 279 years after.[21]

[22]

With further experiments he refined his clocks so that by 1727 they had an error of one second in a month compared with contemporary watches of the time that had errors of one minute every day![23] If he could design a clock with such accuracy that would work in the harsh maritime environment, he could challenge the scientists and claim the Longitude prize.

This led him to enter the race to find longitude, a time-based calculation, in competition with the scientists approach with the lunar distance method.

Over the next 40 years he designed and built 4 clocks. The first attempt, H1, was a monster of a clock weighing 34 kilos and housed in a 1.2m casing.[24]

It went on trial at sea and performed well. The results were presented to the commission in 1737, and they were pleased[25]. Harrison, however, was critical of the watch and suggested he could improve on it. The commission agreed and gave him finance to continue.[26] For the next period of around 20 years Harrison produced further improved clocks H2 and H3, neither of which made it to the seaborne trials.

During this period two things emerged to challenge Harrison’s work. First, around 1730 John Hadley produced a device that could more accurately measure angular distance, the octant.[27] This would benefit both the measurement of lunar distance and the measurement of the altitude of the meridian passage of the sun.

Neville Maskelyne also continued to refine the lunar distance method of determining longitude.

[28]Harrison produced his fourth clock, H4, called a sea clock around 1755. What was remarkable about it was that it was only slightly larger than a pocket watch. So the scene was set for another dual with the scientists using the lunar distance method.

A final test of both methods was conducted in 1764 by voyages to Barbados to determine its longitude. Harrison was now in his 70’s and his son, William, took over the care of the sea clock on the voyage.

On their return and examination of the results the Commission were convinced that Harrisons Sea clock, the Chronometer, was the winner as the clock only lost five seconds on an arduous 81 days at sea.[29]

Captain Cook had only praise for the watch calling it “our trusty friend, the watch”.[30] So from a demand by seafarers in 1714 for a reliable method of determining longitude to approval by one the foremost navigators of the time vindicated Harrisons struggle to demonstrate his prowess as a clock maker for maritime use.

So the birth of the chronometer was assured but it was expensive, and it took another 100 years before it was found regularly on merchant ships.

[31]Fast forward to the 1960’s when I was at sea and the chronometer was an essential instrument for determining the ship’s position. It was most often housed in gimbals in a glass case to the left of the chart table. In the ships I sailed on there was always two chronometers. It was wound regularly at the same time of day and the responsibility was that of the second officer whose main duty was as navigator. One of the important tasks was to check the daily error of the chronometer and this was done by listening to the BBC Greenwich Time signal or “pips” as they were called. Six electronically derived signals that precisely measured the time on each hour.[32]

Postscript

The measurement of time has always been crucial to fixing location and even today with GPS it requires an accurate satellite time signal to fix position. Threats to GPS position setting have been interrupted by GPS jamming where the satellite time signal is compromised. This is a serious breach to GPS position fixing and new methods are required to provide an accurate time signal. [33]One British research project will place a local time signal generator in your watch or mobile phones so removing the need for a satellite time signal. We must wait and see if GPS jamming can be avoided.

References

American Scientist. ‘The British Longitude Act Reconsidered’, 6 February 2017. https://www.americanscientist.org/article/the-british-longitude-act-reconsidered.

‘Greenwich Time Signal’. In Wikipedia, 8 January 2025. https://en.wikipedia.org/w/index.php?title=Greenwich_Time_Signal&oldid=1268182904.

‘H4 | Royal Museums Greenwich’. Accessed 4 March 2025. https://www.rmg.co.uk/collections/objects/rmgc-object-79142.

‘History of Royal Observatory Greenwich | Home of GMT’. Accessed 28 February 2025. https://www.rmg.co.uk/royal-observatory/history.

‘John Harrison’. In Wikipedia, 1 March 2025. https://en.wikipedia.org/w/index.php?title=John_Harrison&oldid=1278206504.

‘Kronometer’. Accessed 24 January 2025. https://digitaltmuseum.no/011022541437/kronometer.

‘Letter Claiming Discovery’. Accessed 24 January 2025. http://www.royalobservatorygreenwich.org/articles.php?article=1312.

‘Longitude Found - the Story of Harrison’s Clocks | Royal Museums Greenwich’. Accessed 24 January 2025. https://www.rmg.co.uk/stories/topics/harrisons-clocks-longitude-problem.

Longitude Prize. ‘The History’. Accessed 24 January 2025. https://amr.longitudeprize.org/the-history/.

‘Lunar Distance (Navigation)’. In Wikipedia, 28 January 2025. https://en.wikipedia.org/w/index.php?title=Lunar_distance_(navigation)&oldid=1272314357.

‘MV Akdeniz - Photo Page 2’. Accessed 4 March 2025. http://ssmaritime.com/Akdeniz-5.htm.

‘Nautical Almanac’. In Wikipedia, 25 February 2025. https://en.wikipedia.org/w/index.php?title=Nautical_almanac&oldid=1277512307.

‘Nevil Maskelyne’. In Wikipedia, 15 December 2024. https://en.wikipedia.org/w/index.php?title=Nevil_Maskelyne&oldid=1263169488.

‘Octant (Instrument)’. In Wikipedia, 10 January 2025. https://en.wikipedia.org/w/index.php?title=Octant_(instrument)&oldid=1268610567.

‘Plane GPS Systems Are under Sustained Attack - Is the Solution a New Atomic Clock?’, 3 March 2025. https://www.bbc.com/news/articles/cq6yg204pvmo.

‘Royal_Observatory.005_-_Greenwich_(London).Jpg (4000×3000)’. Accessed 3 March 2025. https://upload.wikimedia.org/wikipedia/commons/5/55/Royal_Observatory.005_-_Greenwich_%28London%29.jpg.

‘Scilly Naval Disaster of 1707’. In Wikipedia, 2 December 2024. https://en.wikipedia.org/w/index.php?title=Scilly_naval_disaster_of_1707&oldid=1260750007.

Sobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. First. Bloomsbury USA, 2007.

‘The Quest for Longitude and the Rise of Greenwich - a Brief History’. Accessed 28 February 2025. https://www.royalobservatorygreenwich.org/articles.php?article=1290.

‘Voyages of Christopher Columbus’. In Wikipedia, 17 January 2025. https://en.wikipedia.org/w/index.php?title=Voyages_of_Christopher_Columbus&oldid=1269955043.

‘What Is the Prime Meridian, and Why Is It in Greenwich?’ Accessed 28 February 2025. https://www.rmg.co.uk/stories/topics/what-prime-meridian-why-it-greenwich.

 

 

 

 

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[1] ‘Kronometer’.

[2] ‘Voyages of Christopher Columbus’.

[3] ‘Scilly Naval Disaster of 1707’.

[4] ‘Scilly Naval Disaster of 1707’.

[5] ‘Scilly Naval Disaster of 1707’.

[6] ‘The History’.

[7] ‘Letter Claiming Discovery’.

[8] ‘The British Longitude Act Reconsidered’.

[9] ‘The Quest for Longitude and the Rise of Greenwich - a Brief History’.

[10] ‘What Is the Prime Meridian, and Why Is It in Greenwich?’

[11] Sobel, Longitude, 5.

[12] ‘Lunar Distance (Navigation)’.

[13] ‘Lunar Distance (Navigation)’.

[14] ‘Nautical Almanac’.

[15] Sobel, Longitude, 131.

[16] Sobel, page 7. (page 7)

[17] ‘History of Royal Observatory Greenwich | Home of GMT’.

[18] Sobel, Longitude, 53.

[19] ‘Nevil Maskelyne’.

[20] ‘John Harrison’.

[21] Sobel, Longitude, 68.

[22] ‘Royal_Observatory.005_-_Greenwich_(London).Jpg (4000×3000)’.

[23] Sobel, Longitude, 72.

[24] Sobel, 78.

[25] ‘Longitude Found - the Story of Harrison’s Clocks | Royal Museums Greenwich’.

[26] Sobel, Longitude, 83.

[27] ‘Octant (Instrument)’.

[28] ‘H4 | Royal Museums Greenwich’.

[29] Sobel, Longitude, 120.

[30] Sobel, 160.

[31] ‘MV Akdeniz - Photo Page 2’.

[32] ‘Greenwich Time Signal’.

[33] ‘Plane GPS Systems Are under Sustained Attack - Is the Solution a New Atomic Clock?’

The Navigators toolbox- the sextant

 

2-The sextant

The Navigators tool chest Is full of objects and competence that enable him to determine where the ship is on the maritime globe, whether it is within the sight of land or in the middle of one of the world's large oceans.

Not only that, he or she could interpret their visible maritime world whether it be sky, sea or weather to help them make a forecast and adjust their position, all without external support.

The sextant was one of the most important tools the navigator had to enable him to fix the ships position anywhere on the ocean.

They're also precious because it was a personal instrument that every budding junior deck officer needed to invest in. In the 1950s or 1960s you either bought a German Plath model or the Kelvin Hughes from the UK.

[1]

The principle of the sextant is relatively simple. It is used to measure the altitude of celestial objects like stars, planets, and the sun and moon. Altitude being the angular elevation of the object above the visible horizon. No visible objects or no visible horizon meant that there was  no use for the sextant!

The word sextant is derived from the Latin “sextus” meaning one sixth[2].  So a sextant can measure an arc of one sixth of a circle, namely 60 degrees.  Because it has double reflecting mirrors it can measue up to 120 degrees.

[3]

[4]The instrument comprises a movable arm over a arc inscribed with angular degrees. The top of the arm has a fixed mirror attached. On the body of the instrument is a second vertically split mirror, the left side of which is clear glass. A telescope is attached to view the contents of the mirror. Each mirror has a set a filters to reduce the sun's glare. Setting the radial arm to zero degrees, you point the instrument at the celestial object, say the sun, and then by releasing

the radial arm you slowly bring the reflected sun down towards the visible horizon in the clear glass of the fixed mirror. Once you can just place the lower edge of the sun on the horizon you have measured its altitude.

Of course the sun rises and sets, but there is one altitude that is important to the navigator. That is when it has its maximum altitude, some time in the middle of the day. This is called the meridian passage because at that precise moment its bearing is south and the sun has reached its maximum altitude for that day. This altitude can help the navigator calculate latitude.

 

 

 

 

 

 

 

A simple diagram shows the process:

·         Measure the sun’s zenith altitude

·         The zenith distance is then 90 degrees minus the sun’s altitude

We then need to take account of the fact that the sun is inclined from the plane of its orbit around the sun by something called its declination and it varies with the time. We know this because  the sun is highest in our northern summer and lowest in winter.

[5]So we need the declination for exactly the time we took the altitude reading and this comes from another item in the navigators toolbox, the Nautical almanac where we can find the declination we want.

For northern latitudes, in the summer period the declination will be northerly so a calculation is: With an altitude of 47 degrees, the zenith distance is 90 degrees minus sun altitude = 43 degrees.

From the almanac the declination is 10 degrees north so latitude is 43 + 10 = 53 degrees north.

I should explain that this is a simplified example and for other latitudes and declination the calculation will vary. Also, there are a number of corrections to be made to the sextant and the observed sun’s altitude that the navigator must make.

 

References

‘Brown’s Nautical Almanac - Google Search’. Accessed 21 January 2025. https://www.google.com/search?sca_esv=68f5f289e15608b2&sxsrf=ADLYWIKGrRTYhttgFFByyiliSP6-e4e2DQ:1737457710086&q=brown%27s+nautical+almanac&udm=2&fbs=AEQNm0Aa4sjWe7Rqy32pFwRj0UkWd8nbOJfsBGGB5IQQO6L3J_TJ4YMS4eRay1mUcjRHkZxQmI-azE4-kvfPRbUUVQX-VdukAcnN3WyeChzMt5JDRBMtwKF_X1ZFdhHy6uNqBY-4IVOFfEGiyJu7GCSojJTrLQS8W8-G9DpcjCnldC9L40SzLN0&sa=X&ved=2ahUKEwikv_ys1oaLAxXnQlUIHaikMTUQtKgLegQIFRAB&biw=1136&bih=480&dpr=1.1#vhid=PK2xbiJKjwohgM&vssid=mosaic.

‘Definition of Sextant - Google Search’. Accessed 20 January 2025. https://www.google.com/search?q=definition+of+sextant&oq=definition+of+sextant&gs_lcrp=EgZjaHJvbWUyDggAEEUYORhGGPkBGIAEMggIARAAGBYYHjIICAIQABgWGB4yCAgDEAAYFhgeMggIBBAAGBYYHjIICAUQABgWGB4yDQgGEAAYhgMYgAQYigUyCggHEAAYogQYiQUyCggIEAAYgAQYogTSAQkxMDgxOGowajeoAgCwAgA&sourceid=chrome&ie=UTF-8.

Malm, Bengt. Bengt Demo. of Sextant. n.d. Photo.

‘NOVA Online | Shackleton’s Voyage of Endurance | How a Sextant Works | PBS’. Accessed 14 January 2025. https://www.pbs.org/wgbh/nova/shackleton/navigate/escapeworks.html.

‘Sekstant’. Accessed 20 January 2025. https://digitaltmuseum.no/011024215631/sekstant.

 

Acknowledgements

·         Bengt Malm, Ancient Mariner and volunteer colleague Norwegian Maritime Museum, Oslo

·         Camilla Nordeng, conservator Norwegin Maritime Museum, Oslo

 

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[1] ‘Sekstant’.

[2] ‘Definition of Sextant - Google Search’.

[3] Malm, Bengt Demo. of Sextant.

[4] ‘NOVA Online | Shackleton’s Voyage of Endurance | How a Sextant Works | PBS’.

[5] ‘Brown’s Nautical Almanac - Google Search’.

British Seamen’s Discharge Book

 Three generations of seafaring

A seamen’s discharge book is a record of the employment of a seafarer. All seafarers need one and it is issued by the maritime authority of the country the seafarer is a citizen of.[1]

I am lucky to have the discharge books of my grandfather, father and myself over a period of 80 years. Collectively they tell a story of ships, employment and voyages from the 1890’s to the 1970’s. This article will describe the changes in discharge book entries over three generations.

The hard bound cover of the book has changed little over this period.

The colours and wording may have changed on the covers but from the black edition of my grandfather through the red one of my father to my blue one, they all serve the same purpose, to catalogue the voyages of the holder and provide a character reference for each voyage.

You might be wondering why there are 4 books for three mariners. The twist and turns of our time at sea through two world wars will reveal many interesting events that will explain this apparent anomaly.

Let’s look inside my father’s discharge book. The first page holds personal details that identify the seafarer and their next of kin.

Much like a passport it provides evidence of identity. However, there is an interesting handwritten comment in the upper right corner.

Renewal, original lost when vessel sunk by enemy action!

Therefore, there are no earlier discharge book records of my father. More on this later.

Photographs were included later. My personal record page shows a gawky 16-year-old schoolboy before his first trip to sea.

  

The first British discharge books were issued around 1900[2] and were identified by a number until around 1920 when a decision was made to renumber then with the prefix R.

My grandfathers discharge book number is 163200 with a first entry of February 1901. My father had 2 books numbered R123850 with a first entry 9.7.39 and the second book also numbered R123850. The reason for two books is that one was lost on the SS Stonegate that was sunk in WW2. See the story here. My discharge book issued in 1957 is R677594.

However, my grandfather, FGW Randall, was indentured to a Whitby shipowner Richard Smailes for 4 years starting in 1898 at the age of 15 before discharge books were issued in the UK.

Subsequently his record of ship employment until early 1900’s was on individual paper certificates. Here is his first one. One interesting item is that normally you start a voyage in port and the fact is verified by the local marine office. Here it states his engagement was “At Sea” and there is no official stamp. It was unusual for someone to join a ship away from a port where there was a marine office. It can mean literally he joined at sea from another ship or boat or at a port without a marine office.

Entries in the discharge book are made by the Captain and the British Maritime authority at the port of engagement or discharge. There can be multiple entries on each page.

In the early discharge books a voyage entry took two pages, one for voyage details and another for the captain to report on the seaman. In my grandfather’s discharge book seen above you can see details of the ship, the date and port of engagement, the rank of the seaman, the port and date of discharge, a brief statement on the voyage and the signature of the master.

The second page was divided in two, each side relating to a voyage on the previous page and reporting on the seaman during the voyage. There are two aspects of character, “ability” (assume that relates to job specification for the rank of the seaman) and “for general conduct” probably relating to his conduct in a team/social environment.

There are two possible entries “VG” or “DR”. Very good or Decline to Report. The latter can have consequences for future employment.

Note that it is the master who makes this report, and it is endorsed by the marine authority at the discharge port.

30 years later my fathers discharge book was a little different.
Now both the record of a voyage and the character report on the same double page layout. Note the entry in the first row relating to the SS Stonegate. He was discharged at sea and under the column for the voyage description it states, “Ship sunk”. This relates to the article I referred to earlier. Note also the captain making the report is my grandfather!! They served on the same ship.

There is little change over the next 20 years until I went to sea as an apprentice officer.

The only element is that the masters never recorded the ships I was on during my indentured 4-year period. Hence, I have no record of the ships I served on, but I do remember I sailed to India and back a number of time whilst sometimes calling at the southern states of America after Port Said before returning to Liverpool.

 You may have noticed that the first image had four discharge book covers. Well, my father had a new discharge book after a long layoff from the sea. Now he was a Captain, the person who makes the reports in a seamen’s discharge book so why did he need a discharge book. The rules are clear everyone needs one so he was issued with one that only related to voyage descriptions, presumably because a Captain should report on his own character!

 

So the discharge book is an important certificate for its owner and a valuable source of information for those interested in writing about the voyages of seafarers. More on this later.

References

GOV.UK. ‘Get a Seaman’s Discharge Book or British Seaman’s Card’. Accessed 19 November 2024. https://www.gov.uk/get-seamans-discharge-book-or-british-seamans-card.

Ships Nostalgia. ‘Discharge Book Numbers’, 18 November 2005.

 

https://www.shipsnostalgia.com/threads/discharge-book-numbers.288819/.

 

Acknowledgement

I need to recognise the detailed search work of my brother Peter E. Douglas, the family archivist

 

 

 

 

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[1] ‘Get a Seaman’s Discharge Book or British Seaman’s Card’.

[2] ‘Discharge Book Numbers’.