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
    Name or NavList Code:
    Email:
       
    Reply
    Re: Real accuracy of the method of lunar distances
    From: Fred Hebard
    Date: 2004 Jan 12, 16:06 -0500

    Frank,
    
    You are correct in pointing out that Chauvenet did not say that "at
    least one of the objects would need to be a star or planet."  That was
    my extrapolation.
    
    Yes, he was talking about eliminating errors.  He also explicitly
    mentions averaging separately leading and tailing (east and west) sun
    lunars.  Certainly this would not be useful at sea as they would occur
    at separate times of the month.  He may have been alluding to Bolte's
    and Behrens' data sets, or similar sets of data.
    
    I believe Chauvenet mentions star-star distance measurements in Volume
    II in the section on sextants, and derives the recipe for calculating
    them, including refraction.  I mentioned star-star distance
    measurements to John Luykx of Navtrak Nauticals (he passed away last
    year, was a Pres of the navigation foundation, American) as a method of
    determining eccentricity and other "uncorrectable" sextant errors.
    John said he thought it might take upwards of 500 observations or sets
    of observations to assemble enough data.  It would be tough.  I think
    one of those fancy 20x scopes would be useful here, to superimpose the
    stars accurately.  But holding them in the field of view would be
    physically demanding.  A motor-driven equatorial mount for the sextant
    would be very helpful!
    
    It would be interesting to see how the fiction that star-star distances
    do not need to be corrected for refraction worked its way into even
    Bowditch, I believe, as well as other modern sextant texts.  The old
    guys knew better.  Perhaps the modern guys are just thinking of using
    it for rough checks of sextant accuracy.
    
    Fred
    ------------------------------------------------------------------------
    Frederick V. Hebard, PhD                      Email: mailto:Fred@acf.org
    Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
    American Chestnut Foundation                  Phone: (276) 944-4631
    14005 Glenbrook Ave.                          Fax: (276) 944-0934
    Meadowview, VA 24361
    
    On Jan 12, 2004, at 3:32 PM, Frank Reed wrote:
    
    > Fred, you earlier wrote:
    > "One clear point that came out of Bolte's paper from the 1870s
    > referenced by Jan Kalivoda was that there's little difference in
    > precision between lunars taken with stars and lunars taken with the
    > sun. Additionally, taking two lunars, one in either direction from the
    > moon (and equally spaced), is more accurate than taking one. Then at
    > least one of the objects would need to be a star or planet. I vaguely
    > recall Chauvenet also making this second point. "
    >
    > In this message, if I understood you correctly, you were saying that
    > sights on both sides of the Moon would yield better results because
    > "at least one of the objects would need to be a star or planet." (this
    > was at the point when you were contesting my statement that Moon-Sun
    > lunars are easier to shoot accurately than Moon-star lunars). You
    > agree that that's not what Chauvenet was getting at, right? He was
    > talking about eliminating errors from inadequately determined index
    > correction and, if you're lucky enough to find objects the same
    > distance on either side of the Sun, eccentricity error. It's
    > interesting that Chauvenet comments that errors of eccentricity are
    > "not readily determined". Apparently, star-to-star sights did not
    > occur to him.
    >
    > Frank E. Reed
    > [X] Mystic, Connecticut
    > [ ] Chicago, Illinois
    >
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Get a NavList ID Code

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

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

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

    Email Settings

    NavList ID 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