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    Re: Resetting your clock at a known location
    From: Henry Halboth
    Date: 2015 Oct 21, 13:46 -0400

    It was fairly common practice to caculate chronometer error quite accurately when in a known position by use of the time sight. Thhis was often done when passing a well charted headland or even in port by use of an artificial horizon. By conparing the longitude determined by chronometer with that based on a bearing and distance from the headland the chronometer error could be determined with considerable accuracy.


    On Wed, Oct 21, 2015 at 9:57 AM, Bruce J. Pennino <NoReply_Pennino@fer3.com> wrote:
    Since we are “talking” historically, I have a question regarding resetting your clock when at a known location (or with a excellent fix , say from stars). This is discussed by Letcher & Karl. 
    Background: In August, a couple of hours before sunset when the sun was about due west, I pretended I did not know my time. I recorded an ordinary set of sights....really no special pains taken.  Knowing my location, by initially guessing a time ( I knew it was a couple of hours before sunset) I quickly calculated my actual time. I recollect it was in error by a minute or less. If I did it again the next day, I suspect I could eventually get to 15 or 20 seconds error. Did mariners in the 1800s do this? So much easier than lunars and it can be done with any celestial body. This is nothing but a bit more than a “time sight” with some tabular interpolation.  Anyone done this? How accurate was your time when at a  known position? Karl’s method of interpolation is straight forward.
    Just curious.


    From: Frank Reed
    Sent: Tuesday, October 20, 2015 9:17 PM
    Subject: [NavList] Re: Maury and Spooner on longitude at sunset

    In his post, Rommel wrote that he gave his friend "the link for Maury and sunset latitude" (presumably he meant longitude??). Does anyone know that "link" is? Did M.F. Maury at some point publish details of a method for using a sunset observation to get longitude? I don't recall seeing it in his 'Treatise on Navigation' published in 1845, but it's a common enough proposal.

    A longitude by time sight calculation (or more generally a line of position) from an observation of the setting sun is certainly nothing novel or extraordinarily clever. In any navigation class that I have taught, at least one in ten students of navigation independently suggest the idea. You can use the altitude of the Sun at sunset, namely 0° 00.0', either upper or lower limb, and work it as an ordinary sight. The advantage, of course, is that it requires no sextant. But when is one likely to have all the tools of the trade for celestial navigation yet, by chance, no sextant? At best it fills a minor role. On a more general level, the trouble with such sights is that refraction right at the horizon is sufficiently variable that a sight like this can't be depended on for anything better than an "emergency" line of position. It can easily be in error by ten miles from variable refraction alone.

    As for this claim that someone named Spooner thought of it first, well, sure, he may well have thought of it as some point. Many people have thought of this. It's a minor discovery, at best. The present source for the claim is a quaint book of collected Nantucket anecdotes published in the 1880s. Did Spooner publish his claimed discovery in any navigation resource? Apparently not. He merely boasted about it decades later. While it's certainly possible that he invented the idea on his own at some point, there's no evidence that I can see that he did anything with it, that he developed it into a proper technique, that he tested it out empirically over any length of time, or that he taught others to use this method. Therefore we're looking at a trivial and rather silly footnote on a minor topic in celestial navigation. Far from being some "wronged genius" in navigation, I see nothing here but an elderly mariner trying to convince his Nantucket neighbors that, once upon a time, he was really quite clever. Yes, indeed, as Bill suggested, "There once was a man from Nantucket, whose boast was so long he could..." Well, you know the rest.

    By the way, there were many experts in nautical astronomy (the name used for celestial navigation in the 19th century) with Nantucket roots. They have not been hidden from history. One of the most important textbooks on the subject in this period was authored by J.H.C. Coffin (he was born in Maine but his family is an old Nantucket lineage). I've got two copies of the book on my shelves, and it's listed among the many historical works in the index that I assembled here. Speaking of Annapolis, as we have been recently, Coffin's textbook was the principal textbook in celestial navigation used at the US Naval Academy in the late 19th and early 20th centuries. Coffin was William Chauvenet's successor there. It's nearly impossible to reconcile the prominence of J.H.C. Coffin in navigation education with the proposed tale of a Nantucketer wronged by Matthew Fontaine Maury and ignored by the "establishment". If Spooner's tale had any merit at all, he would have had no trouble getting a hearing. As I say above, it sounds like little more than an old goat boasting to his neighbors... 130 years ago!

    Frank Reed
    Conanicut Island USA

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