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…

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