Posted by: Rob Viens | May 7, 2012

FitzRoy’s Chronometers

May 7th was a quiet day for Darwin as he prepared to be left alone (more or less) in Rio while the Beagle went back to Bahia to recheck some of the survey data.  He writes:

“Went on board & spent the day there, in the evening brought with me a few things which I wanted before the departure of the Beagle.” (May 7)

Keep in mind that the primary mission of the Beagle was to survey the waters of southern South America and create nautical charts.  So when FitzRoy discovered a discrepancy in is data (as compared with earlier surveys) he felt that it was imperative that he recheck his measurements.  Especially since a mistake this early in the trip would mean that the rest of his measurements would be called into question.

You see FitzRoy was sort of a stickler for accuracy (to put it lightly).  To find that there may have been a mistake in his measurements must have driven him absolutely crazy.  So even though he did not have express orders to go back to Bahia, there was no question that it was the right thing to do. (FitzRoy was definitely an independent thinker, and throughout the voyage he took the initiative to do what he thought was right – even if it was not specifically in his orders. Sometimes he paid for these choices – literally by being stuck with the bill for some expensive purchases.  But that is a story for another day.)

At the beginning of the 18th century, accurately measuring latitude was relatively easy, but longitude was a different story.  In 1714, the British government issued the Longitude Act, offering a huge case prize for anyone who could solve the question of how to make an accurate measurement of longitude (they were sick of having ships get lost at sea). The problem was ultimately solved about 50 years later by John Harrison who spent the better part of his life trying to create a clock that kept very accurate time.  This chronometer was, in a sense, the most accurate pocket watch ever invented at the time. (The story is well told in Dava Sobel’s excellent book Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time.)

John Harrison and his fifth prototype (the H5) competed in 1772 (from Wikipedia Commons):

John HarrisonJohn Harrison's Chronometer H5

Why a timepiece?  Well, the idea behind the chronometer is that if you compare the relative time between two places on Earth you can also measure how far apart they are (in degrees of longitude).  Longitude is measured from the prime meridian, which runs through Greenwich, England.  So every ship by the 19th century carried a chronometer that was set at the beginning of the voyage at Greenwich Time.  As the ship sailed across the ocean it could compare the local time with its chronometer and use that difference to calculate its longitude.

In a crude sense the math goes like this – there are 360° in a sphere (so effectively 360° of longitude).  There are 24 hours in a day.  So if you sailed completely around the world your clock would be off by 24 hours.  Hence every hour difference is equal to about 15° (360°/24 hrs) of longitude.

Now lets say you sail across the ocean from England to Rio de Janeiro (like the Beagle did).  In Rio you could use the position of the sun to determine when it was exactly noon, and compare that with your chronometer set at Greenwich Time (which would read about 3 pm).  There are about 3 hours different between the two locations, so 15° x 3 hrs equals a difference of about 45° of longitude. If you measure to the exact minute or even second you can get an even more accurate answer (and if fact, the actual longitude of Rio is 43° 11′ 47″ west of the prime meridian). Whew – I didn’t even peek before making the calculation – it works 🙂

Longitude lines (from Wikipedia Commons):

longitude lines

My calculation gives a crude answer, but FitzRoy (and the Admiralty) wanted the most accurate and up-to-date measurements of longitude that were possible at the time– so the captain needed to make sure that he had an extremely accurate measurement of the difference in time.

So how did he assure that his chronometers were keeping accurate time – redundancy, of course.  FitzRoy brought 22 chronometers on the Beagle!  So if one started to slow down or speed up (by even several seconds), the agreement of the other 21 would allow the captain to notice and correct the mistake.  As long as he kept up with maintaining his chronometers, FitzRoy had one of the best ships for measuring longitude in the world. Keep in mind that in order to work as a survey tool, FitzRoy’s chronometers had to keep near perfect time for nearly 5 years – and at sea to boot!

One of the Beagle’s chronometers (in the British Museum via Wikipedia Commons):

Chronometer from the Beagle

When outfitting the Beagle, Francis Beaufort suggest that 18 chronometers would provide enough redundancy for accurate measurements, but FitzRoy did not want to mess around – what if a couple got broken.  So he upped the number to 22.  He knew the Lords of the Admiralty would not cover the cost of all of them but was surprised when they said that 5 would suffice and he should return the others.  Yes – bureaucrats at their best – outfit a multi-year voyage round the world, but skimp on the instruments that it needs to do the job.

Well, in the end, FitzRoy did what he thought was best and took them all, even though it meant that he paid for most of them out of his own pocket.  A good insight into the type of man Robert FitzRoy was. (RJV)

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  1. […] you recall, FitzRoy’s Chronometers where a key tool for doing accurate survey work along the coast (which is why he brought 22 of […]


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