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: Artificial horizon question
    From: George Huxtable
    Date: 2009 Apr 20, 23:19 +0100

    John wrote-
    "I will either get a piece of black glass (and devise a means of leveling it
    as I have read in past posteings) or at least try some sort of
    non-reflective treatment for the pan bottom."
    Beware. Whaver sort of base you fix it to must be smoothly and finely
    adjustable in tilt, and very rigidly mounted, right down to ground level.
    Sometimes, just bringing your feet, carrying your own weight, close to its
    base for reading can affect tilt. And you need a very light and sensitive
    spirit level, such that when you apply it to the glass plate, and when you
    remove it, there isn't any perceptible change in tilt.
    "I don't particularly suspect that I'm getting a double image from the oil
    pan, however.  When sighting the sun, I use a colored cover over the
    artificial horizon pan, and different colored filters between my eye and the
    sun, and I get two distinctly colored images, say green from the sky and
    yellow from the pan."
    I'm not sure I understand that picture. Are you certain that you are not
    getting reflections from the surface of the "colored cover"? What is the
    optical quality of that cover?
    "It is these that can't seem to be brought very near each other
    horizonatally.  That is, until for one luck second they slip right over one
    another, and if I'm quick with the micrometer I can get my shot beforfe they
    agaion wobble apart."
    I can't see what causes this instability with time, unless the liquid
    surface is being disturbed . Do you get that instability when looking at the
    two images of Sun or star with the index set to zero, in normal view without
    an artificial horizon? If you do, then you have a sextant with a loose and
    wobbly mirror.
    Or does it vary as you look into different parts of the artificial horizon
    assembly, as your eye position shifts about, because it's impossible to
    stand stock-still? In which case, any shift may be the result of reflections
    off different parts of the surface of your "colored cover", if that's not
    exactly plane. Normally, any draught-excluding cover has two angled
    surfaces, like a gardener's cloche. Is yours like that or is it a single
    horizontal sheet?
    The trouble with reflecting off water, compared with Mercury, is that only a
    small fraction of the light is reflected, so the wanted image is weak.
    Because of that, other reflected images, such as that off the cover, or off
    the base, can be equally intense.
    What's the magnitude of this effect? The Sun is about 30' diameter, how much
    are these unstable shifts, compared with that?
    "By the way, I had also meant to ask:  Should I ignore altitude correction
    when using an artificial horizon?  Clearly no correction for semi-diameter
    should be needed, because I am superimposing full discs of the sun and its
    reflection, center-over-center, rather than touching the upper or lower limb
    to the horizon.  But I guess I can't grasp the principle behind altitude
    correction sufficiently to decide whether it counts in this situation."
    The way you're doing the job, superimposing the two discs, there's no
    correction for semidiameter. However, it's more usual, and gives better
    answers, to place the two discs one above the other, so that the upper limb
    of one brushes the lower limb of the other If next, you reverse the order,
    and average, that corrects for semidiameter..
    There is no correction to make for dip. Correct for index error before
    halving the answer. Correct for refraction after halving.
    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

    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