Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: Are higher altitude sights inherently less accurate?
    From: Alexandre Eremenko
    Date: 2012 May 2, 23:29 -0400

    Dear John Karl,
    I have one remark.
    What you are saying is apparently applicable to
    observations with natural horizon, while we were discussing artificial
    one. This also applies to the table you attached.
    Actually I was wrong when I mentioned "tilt" in my explanation
    about why the errors increase with the angle when observing with
    artificial horizon.
    With artificial horizon there is no tilt and there is no "swinging the
    arc". You just touch two sun images. If you are able to do this at all,
    the angle that you measure is automatically in a vertical plane.
    Because the ray from the sun and the reflected ray are always in the
    same vertical plane.
    However I know from experience that measuring a large altitude with
    artificial horizon is indeed tricky. Why this is so, I am not sure.
    Taking a Lunar distance at 130 degrees is also hard. It is hard
    even to keep the two objects in the field of view when the angle is too
    large, because the ray hits the index glass under very small angle.
    On Wed, 2 May 2012, John Karl wrote:
    > For once I disagree with Geoffrey.  He says, "The error how vertical your 
    sextant is becomes rapidly more important [at higher altitudes]. And in 
    consequence, errors in high altitude sights will increase."
    > The opposite is true.  The error with sextant "tilt" is max at 45d altitude 
    and decreases to zero at 90d altitude, for any given tilt angle, as the 
    attached table shows.
    > It is true that it's uncomfortable, even weird, swinging around in azimuth 
    as you rock the sextant looking for vertical.  Here's were the traditional 
    half-silvered horizon mirror can help.  Just keep the vertical division of 
    the mirror perpendicular to the horizon (as you pivot around awkwardly on one 
    foot) and your altitude error should be within reason, just as the attached 
    table shows.
    > Happy highs (altitudes),
    > JK
    > ----------------------------------------------------------------
    > NavList message boards and member settings: www.fer3.com/NavList
    > Members may optionally receive posts by email.
    > To cancel email delivery, send a message to NoMail[at]fer3.com
    > ----------------------------------------------------------------
    > Attached File: http://fer3.com/arc/img/119354.sextant-tilt-table.pdf
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=119354

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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