Of time and longitude – clockwork and moonlight

Yes, I know – you’ve probably heard that the problem of finding longitude at sea was solved by a Yorkshireman called John Harrison in the 1750s.  Once he’d found a way of making a watch that would keep the time accurately on board a ship, there was no need for astronomy, right?

No! Wrong – in two ways.  First, even if you had a chronometer that kept Greenwich time with perfect accuracy, you couldn’t obtain your longitude without also knowing the exact local time.  It was the difference between local time and GMT that revealed your longitude (until the ‘new navigation’ was developed by the French in the 1870s).  And the only reliable way of finding the local time was by making astronomical observations – with a sextant.  Be patient – in due course I’ll do a post explaining how all this works…

The second problem is that no clockwork chronometer is ever completely accurate.  The most you can hope is that its inaccuracies follow a regular, predictable pattern – like gaining five seconds every 24 hours.  But clockwork chronometers were seldom, if ever, that reliable.  They often ran irregularly (especially if the temperature changed much) and not infrequently stopped for no obvious reason.  Since an error of less than a minute might result in a catastrophic navigational error, you badly needed some way of checking how fast or slow your chronometer was running.

Until the advent of the ‘atomic clock’ in the second half of the 20th century, time was actually defined astronomically.  So if you wanted to check (or ‘rate’) your chronometer you had to look to the sun, moon and stars.  In the eighteenth and nineteenth centuries the great observatories – at Paris and Greenwich, for example – devoted much of their energy to producing tables of the movements of the sun, moon and stars just so that mariners could find the time reliably at sea.

There were several different ways of achieving this goal, but the standard method involved measuring the angular distance between the moon and the sun – with a sextant.   This ‘lunar distance’ method was tricky to perform, but until accurate time-signals generated by the observatories were available at all major ports (based on signals sent by electric telegraph from the mid-19th century) every offshore navigator had to master it.  It wasn’t until the early 20th century that radio time signals were available even in mid-ocean, and only at that point did ‘lunars’ ceased to have any practical value.

So, even if you had a chronometer, you definitely still needed a sextant!  And that remained true until the 1940s when the pressures of war led to the development of the first reliable electronic navigation aids.

More of this later…

How navigation and mapmaking depend on each other


It’s hard to believe that until the second half of the 18th century there were no accurate charts of the world’s oceans.  Why not, you may wonder?  The answer is simple but perhaps not obvious.  Because it was impossible to navigate accurately out of sight of land.

To make a good map, you have to be able to determine exactly where you are – and where everything else is in relation to you.  On dry land this could be achieved as early as the 17th century by measuring an accurate baseline and then using the geometrical system known as triangulation to build on it.  Eventually a network of triangles gave the mapmaker a grid of fixed points extending across the landscape.  But this method didn’t work on the open sea.  In fact even important features like the Scilly Islands – which are actually visible from Land’s End – were charted in the wrong place until the end of the 18th century when the Ordnance Survey triangulation system finally embraced them!

Until the 1750s there was simply no reliable way of fixing a ship’s position when out of sight of land.  By measuring the height of the sun at midday it was relatively simple to find out how far north or south of the equator you were, but it was impossible to find out how far east or west you had travelled.  In other words, finding a ship’s latitude was possible, but finding its longitude with any precision was not.

Only when this problem was solved – in two very different ways, almost simultaneously, did it become possible to map the world’s oceans.  And both methods depended on the sextant!


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The early navigators

People have been using the sun, moon and stars to navigate by for a very, very long time.  And not just people but other animals too – like birds, butterflies, seals, and turtles – to name but a few.  This is a really exciting area of research and new discoveries keep piling in.  Maria Dacke and others have recently shown in an ingenious experiment that a dung beetle can roll its ball of poo back to its nest using the light of the Milky Way as a guide.  My guess is that our pre-human ancestors were pretty good at this kind of basic celestial navigation hundreds of thousands of years ago, though their purposes were probably a bit different!

What we do know is that the builders of Stonehenge and many other ancient monuments around the world are carefully aligned so as to highlight key celestial events like the summer and winter solstices – that mark the longest and shortest days of the year.  And the amazing Nebra Sky Disc – discovered in Germany in 1999 – shows that our Bronze Age ancestors understood the complicated relationship between the solar and lunar years.  It was made about 3,500 years ago. http://www.bibliotecapleyades.net/arqueologia/nebra_disk.htm

And the Greeks knew a thing or two.  The amazing Antikythera Mechanism – discovered by sponge divers about 100 years ago – dates from around the end of the 2nd century B.C.  It  is the most sophisticated mechanism from the ancient world.  Nothing as complex is known for the next thousand years. The Antikythera Mechanism operates as a complex mechanical “computer” which tracks the cycles of the Solar System:  http://www.antikythera-mechanism.gr/project/overview

We know very little about how the Greeks and Romans navigated at sea, though they travelled widely and not just in the Mediterranean.  It’s always assumed that they didn’t use any specialised equipment.  But the Antikythera mechanism makes you wonder…  If they knew that much about the behaviour of the heavens, would they not have put their expertise to good navigational use?

On the other side of the world Polynesian navigators were already starting to venture out across the Pacific at least two thousand years ago.  In their outrigger canoes they could make accurate landfalls on tiny islands after travelling as much as two thousand miles across the ocean.  They didn’t use any instruments or even charts.  They relied only on their senses – and a lengthy apprenticeship that started when they were children.  When western sailors like Louis de Bougainville and James Cook first encountered these prodigies they were astonished and baffled.  Luckily we now know a lot about how these Polynesians navigated because researchers started asking the right questions before their skills died out.  At the heart of their technique was an encyclopaedic knowledge of the stars.  They knew exactly where on the horizon each bright star rose and set and could steer a steady course by reference to them.  They were also adept at estimating how fast they were going and could ‘read’ the changing patterns of waves and swells.  These remarkable skills are still being practised: http://hokulea.org

Experienced navigators everywhere used to pay very close attention to the world around them.  The colour of the water and its depth, the presence of birds and their behaviour, clouds on the horizon – these and many other signs could give early warning of the presence of land, often long before it could be seen.


How GPS makes fools of us all…

One of the things that bothers me about our ever-increasing reliance on technology is the way it distances us from the natural world.  Yes, yes – it makes life easier in all sorts of ways and it would be crazy to turn our backs on it completely, but all that convenience does come at a cost.

This is especially true of GPS.  Have you ever had had the slightly weird experience of allowing the in-car ‘satnav’ to lead you astray on a journey to a place you know perfectly well how to find?  I certainly have.  It’s as if we’re so awed by its usual accuracy and reliability that we daren’t challenge its authority.  GPS can be disabling – reducing us to a state of child-like dependence and making us behave like complete idiots.  In fact it’s worse than that – it can actually weaken our ability to navigate naturally.

Inuit peoples in Northern Canada now worry that young people who have become reliant on GPS can’t safely find their way around in the Arctic wilderness and easily get lost – which is no joke up there.  Their elders, by contrast, just using natural cues like the sun and stars, the wind, the shapes of snow drifts,and the behaviour of the wildlife, can find their way in almost any weather.  Apparently they never get lost even if they sometimes have sit tight for a while when there’s a complete ‘white out’!

Over the last 20 years GPS has come to dominate marine navigation almost completely.  Integrated electronic navigation systems based on GPS are now installed even on small boats.  Basically they offer a ‘moving map’ display on which the vessel is represented by a little boat-shaped icon.  Linked to the GPS – and maybe also to radar and other instruments – these ‘Electronic Chart Display and Information Systems’ (ECDIS) have taken all the hard work out of navigation.  With a click of a mouse you can find out how far you have to go to reach your destination, what the best course is, how long it’ll take to get there, how much current is against you and much else besides.  It’s absolutely brilliant but it’s totally infantilising!  You are turned into a mere consumer of navigational information with no hand at all in the business of generating it.  A glance at the display gives you your precise latitude and longitude – down to the nearrest few meters – and  you don’t even have to raise your head to look at the world outside.

Maybe that’s fair enough if you’re a professional seaman who just needs to get safely from A to B by the fastest route, but there’s something a bit sad about recreational sailors relying on GPS and ECDIS – as they increasingly do.  After all, what’s the point of going sailing, if not to be experience the wonders of the natural world?   The sea is the last great wilderness and there’s nothing more rewarding than to find your way across it with a sextant in your hand.  To fix your position in mid-ocean by taking sights of unimaginably distant stars is a truly sublime experience.  As the first single-handed round-the-world yachtsman, Joshua Slocum, said after finding the Marquesa Islands just where he expected:

To cross the Pacific Ocean … brings you for many days close to nature, and you realize the vastness of the sea. Slowly but surely the mark of my little ship’s course on the track-chart reached out on the ocean and across it, while at her utmost speed she marked with her keel still slowly the sea that carried her. On the forty-third day from land,… the sky being beautifully clear and the moon being ‘in distance’ with the sun, I threw up my sextant for sights. I found from the result of three observations, … that her longitude by observation agreed within five miles of that by dead-reckoning.

To be continued…

The coming of GPS


When Colin and I set sail from Halifax, Nova Scotia, in late July 1973 there was only one safe way to navigate a small boat across the ocean – you had to use a sextant to measure the heights of the sun and stars above the horizon.  You also needed a chronometer to determine the exact time of each sextant sight.  With these two bits of information you could plot your position anywhere on the surface of the earth – and if you were skilful and the conditions were favourable the fix might be accurate to within a mile or two.  This was exactly the way Captain Cook navigated back in the 1770s.

By coincidence it was also in 1973 that the US Air Force began developing the satellite navigation system we now know as GPS.  It wasn’t the first such system but it represented a major step forward in terms of accuracy and sophistication.  And it was a really amazing technical achievement.   At its heart is a network of satellites in precisely determined orbits each carrying its own atomic clock.  GPS receivers pick up the faint signals from these satellites and use them to fix their positions in three dimensions.

Of course Colin and I knew nothing of these developments.  I suppose we may have been dimly aware that the US military were already using some kind of satellite navigation – as indeed they were – but the Cold War was still very much in progress and such technology was highly sensitive.  That was to change when the Iron Curtain was lifted in 1989 and the Soviet Union fell apart.  GPS was already publicly available in the 1990s and in 2000 the last controls were lifted so that the public had access to the best positional information.  Anyone who could buy a GPS receiver could now fix his or her position to within a few meters. It was quite incredible until it became so routine and familiar.

Since 2000, as the price (and size) of GPS receivers has dropped, this astonishing navigational technology has swept the board.  It’s everywhere: in our mobile phones, our cars, in planes and of course in boats.  We are even being told that we can put GPS transponders in our valuables or on our children so that we can keep track of where they are!  GPS has made life much easier and, in many ways, much safer.  But it has its shortcomings.  First of all, it’s very vulnerable to interference and can easily be jammed.  Secondly, it’s quite easy to ‘spoof’ the GPS signals and persuade a receiver that it’s somewhere where it’s not.  There are other risks too – charged particles entering the atmosphere from outer space can potentially fry the circuitry in the GPS satellites.   For all these reasons – and others – robust backup systems are now being developed.  One possibility is to revive and improve LORAN – a radio-based navigation system developed in World War II.

Of course the most robust alternative to GPS is the good old sextant.  It doesn’t even need electricity!  But to navigate reliably with a sextant requires practice – and lots of it.  It’s a real skill, and a deeply rewarding one.  And the saddest thing about the dominance of GPS is that so few people are now acquiring it.

More soon…

Sextant – A Voyage Guided by the Stars and the Men who Mapped the World’s Oceans

Sextant banner

Sextant is a book about the history of celestial navigation – fixing your position by the sun, moon and stars – and the crucial role this amazing instrument played in the exploration of the world. As well as telling the story of the author’s own voyage across the Atlantic in a small yacht back in 1973, it includes exciting accounts of some of the greatest voyages of discovery – led by the likes of Cook, Bougainville, La Pérouse, Vancouver, Flinders and FitzRoy.  The astonishing small boat journeys of William (‘Bounty’) Bligh, Joshua Slocum, Ernest Shackleton and Frank Worsley – all of which depended on the sextant – also feature prominently.

Sextant is an elegy to a wonderful technology that has now all but vanished.  Celestial navigation has now been supplanted by GPS – without anyone noticing, and without a tear being shed.

So my aims in writing the book are to remind people just how much we owe to the sextant and to the courageous people who used it to chart the world’s oceans, and to give some sense of how thrilling and rewarding it is to fix your position on the open ocean by reference to the heavens above – rather than just by pushing a button.

Long live the sextant!

Sextant is to be published in the UK by William Collins and in the US by William Morrow in the spring of 2014.

How it all started…

Lieutenant Colin McMullen RN in 1932Sextant has had a long gestation – it goes all the way back to the summer of 1973…

I was then a student and a family friend asked me to help him sail his 35-foot yacht back across the Atlantic from America.  How could I resist an offer like that?

The boat in question was old-fashioned, even then.  She was built throughout of wood – even her mast was wooden – and most of her fittings were made of bronze rather than stainless steel.  Her rigging was made of galvanised steel and she had a long, deep keel – great for stability but not for speed.  She was called Saecwen, sea-queen in Anglo-Saxon, appropriately enough, as she was a Saxon class boat, built by Priors to a design by Alan Buchanan in 1961.  She may not have been fast, and she was certainly leaky, but she was tough and seaworthy. And she had lovely lines: long, low and slim.

The ‘skipper’ was Colin McMullen – a retired Captain of the Royal Navy.  He’d been Gunnery Officer aboard HMS Prince of Wales when she put a crucial shell into the awesome Bismarck up in the Denmark Strait in 1941.  One of Bismarck’s fuel tanks was holed and she had to cut short her first and last combat mission.  Colin saw HMS Hood blow up a few minutes later, when straddled by one of Bismarck‘s deadly accurate broadsides.  All but a few of her crew of 1400 died.  Not many months later Colin was one of the last to leave Prince of Wales when she in her turn was sunk by Japanese bombers off the coast of Malaya.  He coolly swam off the bridge just as she turned turtle and went down.  Luckily there were no sharks around and he was safely rescued.   Later in the war Colin commanded destroyers escorting the Arctic convoys and he was also involved in the Dieppe Raid.  It was not his fault that the raid was a disaster.  He won two Distinguished Service Crosses. Here he is as a dashing young Lieutenant in full dress uniform back in 1932 

Colin was also an immensely experienced cruising yachtsman.  He’d been messing about in boats all his life, starting in Waterville, County Kerry, during the First World War, and by the time I got to know him well he was Commodore of the Royal Cruising Club – an organisation that acts as a magnet to small-boat-sailors (often eccentric) who enjoy exploring out of the way places, from the Arctic to the Antarctic – and everywhere in between.   I’d been longing to learn how to navigate using the sun and stars, and Colin was just the man to teach me.

To be continued…