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: Checking a sextant calibration.
    From: Jared Sherman
    Date: 2003 Oct 6, 23:26 -0400

    Paul,it has been pinted out to me that when I said "position" I confused all 
    the navigators on this list, who use that word in the jargon meaning of 
    "exactly where am I".
    As I am not a navigator I meant it only in the common sense and I should have 
    said "line of position" since one reading and reduction will only place you 
    along one line of position. (Which to me, is still "a position" in the larger 
    sense. If nothing else, I know I am on the rim a a circle drawn on Planet 
    Earth, and sometimes that is all the precision you will get.)
    By taking one sight and reducing it, you will get one "position" in that 
    general sense. By all means, complicate the exercise by taking multiple 
    sights or otherwise getting a real position. I did not refer to this and 
    should have made clear I was dismissing it, because it is unnecessary to the 
    Get one line of position, or position, as you prefer. Now recompute that 
    position by any means you please, using a bogus observed height (i.e. a 
    different sextant scale reading) that is off by 20".
    If you have simply reduced one sight, your new LOP will be off by small 
    distance. If you have used you sights or other means to fully calculate a 
    real position, you will have done a lot more work--but you will still find 
    the final result is going to be off by a small amount. In any case, that 
    amount will demonstrate the effect of a sextant which is in error by 20". In 
    the case of a single reduction, the line of position will move as the 
    distance along the intercept moves. How far does that distance along the 
    intercept move? About 1/10th of a mile per tenth of a minute (6 seconds). My 
    reduction program said 0.1 mile per 0.1 minute until I changed by .3 minute 
    and it changed by .4 miles, someone else can tell me why that is right or 
    wrong, probably program rounding.
    The point was just to say that one can take one reading--without even doing a 
    series for a proper position--and then diddle the numbers for that one 
    reading, to imitate a badly calibrated/manufactured sextant. If the 
    calibration card is missing and the sextant is "guessed" to be +-20" from the 
    manufacturing of the arm and scale, the fast single reduction then repeated 
    with a bogus number, will show that even if there is a +-20" error on the 
    scale, the final position (or position line) will be off by only a small 
    Which comes back to the simple question that is all I was addressing. If one 
    does not know the calibration error for the arm, and one does not know if the 
    sextant is built +-10" or 9" or even 30", one can still be confident that 
    errors of that magnitude are insignificant and readily ignored. Or, one can 
    consider them extremely significant, depending on one's own opinion.
    But with two simple calculations, one can see with one's own eyes exactly how 
    large/small a difference those seconds make, and draw one's own conclusions.
    That's all I was attempting to demonstrate, I apologize if I did not make that 
    clear enough and my illustrations were misleading.

    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