Calculating Longitude

To: Dava Sobel

 

Ms Sobel:

 

I recently re-read "Longitude", having first encountered it whilst reading O'Brian's Aubrey-Maturin novels. Not having much of a scientific bent, your book was - and is - a delight, and this is my third go-round.

 

I did have one question about the use of Chronometers to determine longitude: since the timepieces prior to Harrison's era were inconsistent to one degree or another, could not the desired accuracy required to calculate longitude have been attained by employing some number of timepieces whose inaccuracies had been documented beforehand? For example, if you had one timepiece known to run two minutes fast per day, and another that ran, say, 90 seconds slow, would they not each be reliably as fast or slow, and thus, reconcilable not only to each other, but to Greenwich time?

 

My Layman's sense would be that a minimum of three such chronometers reconciled on a regular basis would be sufficient to calculate longitude to a very fine degree. Further, that relatively cheap timepieces could have been employed, since their inaccuracy would in this instance be a virtue.

 

Is there anything to my theory?
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Dear Peter Smith,

Thank you for your nice note. I loved the Aubrey-Maturin novels, and am delighted to know a fellow fan of theirs is re-re-reading Longitude.

You raise an interesting question. In truth, there's nothing wrong with your idea in theory, except that it was not practiced, so far as I know. My sense is that no one had enough faith in the available timekeepers to bother checking their accuracy the way Harrison tested his clocks. Having several such timepieces aboard ship would have been both costly and of questionable value.

As you know, however, once Harrison's invention proved the virtue of the timekeeper method, ships carried several at a time. This is how the noon report came to say, "The chronometers have been wound and compared." Even the good ones differed from one another, but in manageable or predictable ways.

Again, thanks for sharing, and all best regards,
Dava Sobel
dava@davasobel.com