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Subject: Re: Sextant Accuracy
From: Fred Hebard (Fred@XXX.XXX)
Date: Mon Mar 17 2003 - 23:12:25 EST
I don't know that I'm not too upset by recent war-precipitating events
to respond well, but here goes. Perhaps we can find rational refuge in
our abstruse meanderings about an _almost_ obsolete technology.
It was very gratifying to hear from our professional sea officer, Doug
Royer. I might imagine that my attempt to get as accurate as possible
is related to his comments about real-world conditions. I also wonder
how close the winner of one of those jackpots generally was to the GPS
position? I recall our correspondent from the arctic saying his shots
were within 0.2', and Bowditch saying that an experienced observer
could shoot to 0.1'. I doubt I'll ever get to the level of experience
to which Bowditch refers, which I would expect comes from taking rounds
of shots five times a day or more.
George Huxtable trotted out his old hobby horse of anomalous dip, which
I was pleased to see. In that regard, I have recently come into
possession of the 1962 edition of Bowditch, which is a delight for us
sextant lovers, as it very much concentrates on navigation by sextant,
being written before electronics had thoroughly permeated our art.
Although I have not had an opportunity to study it at length, I did
peruse the section on anomalous dip. One interesting point was that
measurements of three or four bodies more-or-less equidistant in
azimuth would cancel the deleterious effects of anomalous dip. Another
was that backsights of an object combined with normal sights would
allow one to estimate the magnitude of anomalous dip.
George was also curious about some of my data. I am probably an
optimist when it comes to examination of data, always looking for the
most favorable (to me) interpretation. To let him judge for himself,
below are summaries of my more recent observations; I would provide
more detail on request. Remember that the standard deviations would be
twice the reported size if I were not using an artificial horizon. In
that regard, I remove the glass over my pool of oil when the wind is
not blowing, but have not yet examined the effect of the glass
systematically; it may be important. Note that I also have not yet
systematically trained myself by watching a body pass through a preset
altitude while listening to the time.
Regarding Bruce Stark's comment on irradiation, several of the objects
are Sirius or Jupiter, and they fall within the same range as the sun.
However, irradiation is one phenomenon I ought to examine more closely.
Most of the sights are between the east and southwest, as that is the
most favorable aspect from my house.
In response to Gary Harkins, my house is about 2200' above sea level.
That does not markedly affect parallax, perhaps 0.01' to 0.1' for the
moon (consider that 2200' is not a large fraction of the earth's radius
of about 3400 nautical miles). But elevation does markedly lower
atmospheric pressure (by about 2 inches of mercury here), which is a
main component of refraction. In winter, the other main component of
refraction, air temperature, is low and tends to cancel out the effect
of the reduced air pressure. Now that it's spring, at least as far as
the thermometer is concerned, I'm often adding 0.1' or 0.2' to my
altitudes, per Table A4 of the Nautical Almanac. Since I'm using an
artificial horizon, dip is not a factor.
My sextant only has a 2.5-power telescope. I would very much like to
have a more powerful scope, as that may be what is keeping me from
greater accuracy, especially with star shots. I might add that
although I am new to celestial navigation, I am very used to optical
devices, having used microscopes professionally for many thousands of
hours, as well as binoculars to a lesser extent. So I definitely had a
leg up in that regard compared to many beginning navigators.
I have plotted altitude against time for a few rounds of these sights,
and have been struck by how straight the line was. The graphs have not
been useful tools for detecting bad shots, compared to knowledge of the
true altitude. I'm sure they could be helpful in spotting bad data
points if one did not know one's position. I have settled on three
observations of most objects, so that I don't tire myself out too much
before observing a round of three or more. Perhaps here, however, it
would be better to get more observations per object.
Regards,
Fred Hebard
36o 46.8'N, 81o 50.7'W
Obsvtn Rise Ho - Hc
# Object or Set N Ho dd mean std dev
73 Sun RISING 3 20.615 -0.28 0.29
74 Sun RISING 3 30.537 -0.12 0.49
75 Sun RISING 3 48.384 0.30 0.10
76 Moon RISING 3 24.663 -0.01 0.56
77 Jupiter RISING 3 44.132 -0.69 0.63
78 Sirius RISING 3 34.326 -1.01 0.56
79 Moon setting 3 54.659 0.04 1.46
80 Moon setting 3 22.442 -1.30 1.66
82 Jupiter RISING 3 46.829 -0.58 0.30
83 Sirius RISING 3 35.166 -0.58 0.24
84 Sirius setting 3 26.507 -0.52 0.22
85 Moon setting 3 47.242 -0.58 0.26
86 Sun setting 3 46.959 -0.32 0.21
87 Sun setting 3 17.388 -0.52 0.31
88 Moon RISING 3 52.570 -0.44 0.10
89 Moon RISING 3 56.003 -0.81 0.31
90 Sirius setting 3 29.114 0.18 0.27
91 Saturn setting 3 42.991 0.59 0.29
92 Moon setting 3 61.565 0.70 0.23
93 Moon RISING 3 53.312 0.71 0.93
94 Sun setting 4 51.092 -0.55 0.41
95 Moon RISING 5 19.061 -0.28 0.12
96 Sirius RISING 3 35.827 -1.21 0.32
97 Jupiter RISING 3 52.750 -0.31 0.12
98 Moon RISING 3 58.330 1.01 0.99
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