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: Making an artificial horizon, and leveling thereof
    From: Gary LaPook
    Date: 2011 Jan 25, 17:24 -0800
    How is Alan determining the index error?

    gl

    --- On Tue, 1/25/11, George Huxtable <george{at}hux.me.uk> wrote:

    From: George Huxtable <george{at}hux.me.uk>
    Subject: [NavList] Re: Making an artificial horizon, and leveling thereof
    To: NavList@fer3.com
    Date: Tuesday, January 25, 2011, 2:04 PM

    on Jan 23, Alan provided some observations as follows-

    ==============

    Observer's position, GPS coordinates from which I've dropped tenths of a
    minute was 40 D 34 M North x 80 D 02 M West. No watch error. Index error's
    as noted. Lower limb shot in all cases. No extremes of either temperature
    or pressure so far as I remember.

    Date             WT           Hs           Index Error
    19 Aug 2010     16-52-39     72D 42.6M    2.5 M On
    19 Aug 2010     11-12-22     99D 18,4M    2.5 M On
    9 Aug 2010      09-51-35     74D 58.8M    1.2 M Off
    9 Aug 2010      16-16-02     90D 41.2M    1.0 M On
    26 July 2010    15-02-17     120D 46.0M   0.6 M On
    26 July 2010    18-16-50     50D 30.0M    0.6 M Off
    28 July 2010    17-02-15     78D 14.2M    1.0 M Off
    28 July 2010    09-51-00     78D 15.8M    0.4 M On
    17 Aug 2010     10-23-42     83D 58.4M    2.6 M On
    17 Aug 2010     16-01-6      92D 00.9M    1.3 M On

    Re sights on 26 July, looks like the time interval might be to close.

    ====================

    He ended- "Let me know what, if anything jumps out at you."

    Before looking at these observations in any detail, one aspect jumps out at
    me straight away; the variability of his index error, between 2.6' on the
    arc, and 1.2' off it, including on the same day (August 9th) errors of 1.0'
    on, and 1.2' off. These seem awful. I don't know what sort of sextant Alan
    is using, but they are MUCH worse that I get on my cheapo plastic Ebbco.
    However, I don't own a modern micrometer sextant, though I have used
    several from other people, so I have no real first-hand experience of what
    to expect in long-term stability.

    Is Alan a compulsive "sextant-worrier", perhaps? Does he twiddle at the
    mirror-adjustment screws, for each set of observations? That's bad
    practice, but it might explain this apparent instability. Otherwise, it's
    worth making sure there's no looseness in the positioning of the sextant
    mirrors. Press on the corners of each mirror, lightly, with something soft
    and plastic, and see if there's any detectable movement of the reflected
    image. If the adjusting screws have been slacked back too far, that can
    leave a wobbly mirror. If it always springs back to the same position, then
    it's as it should be.

    George

    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.




       
    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