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    Re: Reducing back sights
    From: George Huxtable
    Date: 2004 Aug 12, 00:08 +0100

    Dave Weilacher wrote-
    >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?
    Response from George.
    Yes, that's fair enough. In the afternoon, measuring angles greater than
    90deg from the Eastern horizon, Dave is free (as he did) to use the true
    upper-limb of the Sun (which happens to look as if it's a lower-limb in his
    sextant-view, BUT IT ISN'T). After subtracting index error and dip, then
    subtracting from 180deg (not 90- Bill Noyce made an uncharacteristic slip
    there) his sextant will have given him the apparent altitude of the
    upper-limb above the unseen Western horizon. Now he will have to subtract
    the corresponding refraction and also the semidiameter, which is just the
    same combination as for any normal sextant correction of the Sun's upper
    Alternatively, after noon, Dave could just as well have observed, from the
    Eastern horizon, the true lower-limb of the Sun, which actually involves
    switching to the Sun's opposite edge at noon (if there was a visible
    sunspot to show it up). Now, this happens to look in the sextant view as if
    it's the upper-limb, BUT IT ISN'T. After subtracting index error and dip,
    and then subtracting the answer from 180, that will provide the apparent
    altitude of the Sun's lower limb, above the unseen Western horizon. Again,
    he will have to subtract the refraction corresponding to that apparent
    altitude, but this time ADD the Sun's semidiameter; which is again exactly
    the same combination of corrections as for a normal lower-limb Sun sight.
    So for these sextant backsights, the only thing that has to be done
    differently is to subtract the angle from 180deg, after correcting for
    index error and dip in the normal way. I think that the problem bugging
    Dave Weilacher is an unreal one. However, it's essential not to be confused
    by the appearance in the sextant view, and to make sure that an upper-limb
    observation really is treated as an upper limb observation, no matter what
    it may look like in the sextant.
    Another matter, which an observer will become aware of as soon as he tries
    to take such a backsight, is that as he rocks the sextant, he won't see the
    usual picture of the Sun dipping down to just kiss the horizon when he has
    it right. Instead, as he rocks the sextant, the Sun appears to ascend from
    below the horizon, and what he is aiming for is for the appropriate limb to
    just brush the horizon at its highest point, not its lowest.
    Bill Noyce and I seem to agree, but have we convinced Dave, I wonder?
    contact George Huxtable by email at george@huxtable.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.

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