Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Joshua Slocum, Victor Slocum, and lunars
    From: George Huxtable
    Date: 2009 Mar 3, 21:38 -0000

    Frank quoted from my mailing-
    
    "I know of only one successful small-boat lunar
    observation that a listmember has made in ocean conditions, compared with
    the many land observations that have been reported here. Neither I nor (as
    far as I know) Frank has ever even attempted it. Perhaps he will tell us
    about the sea-experience that he relies on, to allow us to decide whether to
    take such pronouncements seriously."
    
    That request, about the sea-experience, has not been answered.
    
    ================================
    
    I had written-
    
    "Indeed, it's so tricky to observe useful lunars from the unstable deck of a
    small craft, that it calls for sea conditions that are so infrequent as to
    make the method impractical, most of the time. "
    
    He, and I, were discussing Slocum's navigation, in Spray. That's the sort
    of small craft that we were considering. However, Frank seems
    to have other ideas, writing-
    
    "PRIMARY SOURCE EVIDENCE, GEORGE. Go to the logbooks. Go to the accounts
    written in the era. I've written about all of this on NavList before. For
    example, there's the logbook of the schooner "Weymouth" from 1823. There's
    also Crowninshield's yacht "Cleopatra's Barge" in 1817 which was visited by
    that famous lunarian expert Baron von Zach who was astounded to discover
    that the whole crew knew and worked lunars. Then there's the Baltimore
    clipper "Erin" in 1807 which was captured and navigated by Basil Hall. Or
    how about the "Hero" out of Stonington (right next door to Mystic) which was
    a little sloop just over forty feet length and forty tons which was famously
    captained by Nathaniel B. Palmer when he independently discovered Antarctica
    in 1820. It's not hard to find historical examples of small ocean-going
    vessels where lunars were used."
    
    Spray, at 32 ft length, displaced 15 tons. I understand  that Cleopatra's
    barge was something like 82 feet long, and 192 tons. Frank mentions a sloop
    of 40 ft and 40 tons, but doesn't disclose the size of the other vessels he
    refers to. Not very relevant, then, to the problems of taking lunars on a
    craft the size of Spray, and not providing the primary evidence that Frank
    claims. He will have to do better than that.
    
    ===========================
    Frank wrote-
    "It also explains why Victor mistakenly suggests that the circum-navigation 
    was accomplished because his father used lunars. That was a huge error."
    
    Big deal. I will quote, below, the relevant passage from "Capt. Joshua 
    Slocum", by Victor Slocum, 1950, and readers can make up their own minds 
    about the hugeness of the error.
    
    Here are his full words on the topic, from page 356 onward-
    
    "Dr David Gill, Royal Astronomer, invited the Captain next day to the famous
    Cape Observatory. An hour with Dr Gill was an hour among the stars.He showed
    the Captain the great astronomical clock of the observatory and he in turn 
    was shown the tin clock on the Spray. They went over the subject of standard 
    time at sea and how it was found from the deck of a little sloop at sea 
    without the aid of a clock of any kind.
    
     At the time my father went to sea, lunar observation was not an unusual
    method of obtaining Greenwich Time for longitude. It was very interesting as
    it gave the observer a complete independence of extraneous circumstances,
    like putting those cups on your ears and listening for electronic advice
    from the ether -all very well as long as it works. There is still, on
    cloudless days and nights, the moon going around like a hand on a clock to
    give you your Greenwich Time. And a sailor who cannot make use of it is not
    an altogether competent navigator, for independence is a rule of the sea.
    The clock up aloft was the moon, which together with the sun and several
    stars whose coordinates were tabulated, furnished the required data. Local
    time was computed by the usual means of getting the hour angle.
    
    Three observers were generally employed in a '1unar." The senior observer
    took the angular distance, say, between the moon and the sun. Observers No.
    1 and 2 would simultane�ously take the altitudes of the sun and the moon. To
    facilitate the lunar method, her true angular distance from anyone of the
    other available heavenly bodies was given in the Nautical Almanac for the
    beginning of every third hour, Greenwich Mean Time; the time answering to
    any intermediate angular distance being arrived at by inspection of a table
    of propor�tional parts. The difference between the computed times was the
    longitude required.
    
    The degree of accuracy to be found in this method depended entirely on the
    observer's skill in measuring the lunar distance with the sextant. If the
    observer was sharp enough to make a contact of the two bodies coming within
    hall a minute of arc of the truth, he could compute his longitude thereby
    within fifteen minutes of a degree. Additional accuracy could be had,
    however, by taking the mean of a series of angles, both east and west of the
    moon, being sure to always use the same sextant; in this way, errors caused
    by both mechanical and personal equation could be made to compensate each
    other. Results warranted the continuance of this laborious method until the
    present-day perfection of the chronometer had been reached, together with
    its invaluable adjunct, the daily Marconi time signal.
    
      The Captain's astronomical conversation with Dr Gill throws a light on the
    navigation of the Spray which has never been very well understood, owing
    perhaps to the Captain's purposeful vagueness on this point. Even
    professional navigators have taken his tin clock joke seriosly. My father
    meant that he employed the same methods in navigating the sloop as he had on
    all of his former vessels."
    
    ================================
    
    It's clear that here Victor, a master mariner himself, was giving his 
    readers a little lesson in how lunars worked. That wasn't necessarily 
    related to Slocum's voyage; for example, the reference to three observers 
    couldn't apply to a single-hander. And  the "present-day perfection" that he 
    describes, relates to Victor's own era, and that of his readers, and not 
    Slocum's, before the days of worldwide Marconi time signals.
    
    Remember, Slocum was competent in obtaining logitude from lunar distance, 
    and indeed did so, if only once on the circumnavigation (which Frank, to his 
    credit, pointed out, as he seldom ceases to remind us). Neither Victor, nor 
    Frank, nor any of us, were privy to that conversation with the astronomer, 
    but it's easy to imagine lunar distances being a matter of mutual interest, 
    if only a historical interest. Indeed, Victor might usefully have explained 
    to us the dead-reckoning on which Slocum mostly relied for his longitudes. 
    But he was reporting on a conversation in an observatory, where the details 
    of his towed log would not have occasioned much interest.
    
    An undue emphasis on lunars for longitude, then? Agreed. A "huge error"? 
    Come off it, Frank.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    --~--~---------~--~----~------------~-------~--~----~
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site