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Alan explained clearly his procedure in obtaining index error, which it
seems to me can not be faulted (except in using a too-close foofline, as
we've discussed).

He also describes some similar inconsitencies in such measurements using a
true horizon, which, if that horizon line is sharp, should not happen, with
a good sextant.

It seems that the most plausible explanation may be the one that Alan
himself proffered, when he wrote-

"One is "old eyes" they being 78 years of age, and not as sharp a once they
were." As one who is approaching a similar age, and with more than my fair
share of eye-defects, I know just what he means. It seems that he and I
just have to resign ourselves to accepting a lack of acuity, which prevents
us from achieving again the precision that we once enjoyed.

And if observations of the index-error are that imprecise, there's no
further point in examining the fine-detail of his altitude observations
either, as errors in one will lead to similar errors in the other.

Thanks to Alan for sharing his observations with us.

George.

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.
----- Original Message -----
From: "Alan" 
To: 
Sent: Wednesday, January 26, 2011 10:24 PM
Subject: [NavList] Re: Making an artificial horizon, and leveling thereof


George:

If the weather around here ever breaks, clear sky, no snow, less cold than
it has been, and I survive a really rotten cold I somehow acquired, I will
go out and shoot some more with my AH, concentrating on a particular roof
line visible from my apartment complex, though the relatively short
distance to it, from my sighting position will have to be lived with. It's
difficult to move a house, epecially one that is occupied.

As to my proceedure, which possibly I faild to describe clearly, note the
following.

1. Micrometer is "zeroed"
2. Same for index arm.
3. Sighting on, locally, the afore mentioned roof line, turn the micrometer
until an unbroken lin
4. Read IE from micrometer. If reading is between zero and 10, as you
mentioned the error is "on the arc", and I subtract the reading from Hs. If
the reading is between zero and 50, say 56, I subtract 56 from 60, leaving
4, which is the IE, being "off the arc" I add to Hs. Height of eye is not
considered. Sun shields are rotated out of line of view.I hope this
clarifies the maddness of my method.

As to inconstiencies in IE, there are several possibilities that come to
mind. One is "old eyes" they being 78 years of age, and not as sharp a once
they were. Another is the possibility of my misreading the roof line,
perhaps making an IE correction where none is needed. I do not however
think that having obtained a reading, that I'm misunderstanding or
misconstruing it, taking what is actually "on the arc" for "off the arc".

George, you had asked for a reading of actual sea horizon IE's. The
following are some obtained last fall in October 2010, at Emerald Isle,
North Carolina, coordinates  34D 39.4M North x 77D 03.6 M West. I can send
more if needed.

9 October around 0900 IE @ start 1.0 On, IE at end of shooting 2.2 on
9 October around 1400       "    2.0 On          "            1.6 On
6 October around 1500       "    0.6 On          "            0.0
6 October around 1100       "    0.6 On          "            1.4 On
5 October around 1500       "    1.6 On          "            1.4 On
5 October around 0900       "    0.4 On          "            0.4 On

for whatevrer it might be wrth, I had had some correspondence earlier on
with a gentleman, who said he was familiar with Emerald Isle, and the
general area, having done sextant shots from a couple of miles away from my
location. He made mention of the "sea horizon" there not being as good as
one might think this due to low handing clouds tending to obscure the sea
horizon viewed from the beach, one would be looking generally south. The
referenced clouds caused by The Gulf Stream. For all I kniow, he was
entrirely right, but I suspect low hanging clouds or not, the sea horizon
at the beach is better than what one might have to work with in and around
Pittsburgh.
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