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    Re: Reducing back sights
    From: Dave Weilacher
    Date: 2004 Aug 11, 12:36 -0700

    Hi George;
    I picture it this way.
    Instead of an octant, or a sextant, I'm out there with a halftent (I'm pretty 
    sure this would be called a puptent).
    Sun comes up in east I have east horizon.  Every hour I take a lower limb 
    sight using the east horizon only.
    LAN comes and goes.
    I am still bringing the same limb of the sun down to the same horizon even in 
    the afternoon.  With my puptent, I can do this all the way to sundown.
    Problem is that in the afternoon, the limb that I am bring down is the upper 
    limb, refraction wise, but appears as lower limb against east horizon through 
    the eyepiece of my puptent.
    Our marine almanac combines semidiameter and refraction together as one 
    figure.  Thats what makes our lower and upper limb corrections different.
    Do I have this pictured to your satisfaction?
    Dave W
    -----Original Message-----
    From: George Huxtable 
    Sent: Aug 11, 2004 12:17 PM
    Subject: Re: Reducing back sights
    Bill Noyce wrote-
    >George Huxtable asks:
    >> A further question now arises: what does Jim Thompson
    >> wish to use an octant backsight FOR?
    >I had a different reading of the question.  I assumed Jim was
    >using a sextant to measure the altitude of a body high in the
    >sky, but without a suitable horizon under it.  He's measuring
    >the altitude from the opposite horizon instead.  In that case
    >he has the procedure exactly correct.
    >David Weilacher raises a concern about differing refraction
    >between UL and LL corrections.  I think Jim's procedure works
    >correctly, as long as he remembers that what looks like a LL
    >measurement in the backsight is really measuring the (supplement
    >of the) altitude of the body's LL, and vice versa.  The key is
    >to perform all the corrections for the body itself *after*
    >subtracting the reading from 180.  Jim is correct that index
    >error and dip corrections should be applied *before* subtracting
    >from 180.  After correcting for dip and index error, and
    >subtracting the result from 180, the result is just what you
    >would have gotten from a normal forward sight after the same
    Response from George:
    Thanks, Bill, for putting me right. Clearly, I've been too involved in the
    historical stuff recently! When the expression "back sight" came up, I
    immediately thought of octants and their modifications. But it seems that
    Jim is preparing to indulge, with a sextant, in what I described as option
    4-. "At sea, he wishes to measure the altitude of a body above a horizon
    which is obscured by nearby land or by mist, though the horizon in the
    opposite direction is clear."
    This works with a sextant, as long as the altitude of the body is greater
    that 60deg or so. And indeed, there's no problem with a sextant in checking
    index error, as there would be with an octant in back-observation mode, so
    my comments about index error were irrelevant.
    So I concur with Jim, and with Bill. Correct for index error and dip in the
    normal way, then subtract from 180. This will give the elevation of the
    apparent limb above the true (but invisible) horizon beneath it, in the
    range 0 to 90 degrees. From now on, everything else is as normal. Allow for
    refraction in the normal way, by subtracting an amount corresponding with
    the measured altitude. Add parallax in the normal way, by adding it. Allow
    for semidiameter in the normal way, by adding (for lower limb) or
    subtracting (if upper). What you need to be VERY careful to note is
    whether, in the view in the sextant with the body behind your head, you
    measured the upper-limb or the lower-limb touching the horizon, because in
    that view, for back observations, the lower-limb will appear above the
    upper-limb! (I THINK that last sentence is correct, but somebody please
    confirm or deny it, because I haven't altogether convinced myself...)
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Dave Weilacher
    .US Coast Guard licensed captain
    .    #889968
    .ASA instructor evaluator and celestial
    .    navigation instructor #990800
    .IBM AS400 RPG contract programmer

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