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From: George Huxtable (no email)
Date: Sun Jul 27 2003 - 11:41:12 EDT
Mike Burkes wrote (under another heading)
My "fix":34 24.8N, 119 01.5W vs
>www.Geocode.com:34 28' 25.36"N,118 40' 05.23"w. Bodies were Sun,Moon,Vega.
>Data: All dates and times G reenwhich,IC =0,Dip not applicable because of
>artificial horizon, HO 249 for sight reduction. Sun LL:2Hs=97d 48.3m, July
>04,2003, GMT 17-00-09, assumed L=34dN,aLong=118d 56.7W. Moon LL no.1:
>2Hs=59d 48.9, July 05, GMT 04-23-10, aL=34dN,aLong=118d 38.1W. Moon LL no.2:
>2Hs=92 d49.8m, July 07, GMT 03-27-54 aL=34dN.aLong=119d 21.3W. Vega: 2Hs:
>103d 40.1m,July 07, GMT 04-23-04,aL=34dN,aLong=119d00.0W. My resulting
>fix:34d 24.8N, 119d 01.5W. Sun is Lunar Dist. body. Data: Four obs: GMT
>20-53-15, 20-58-35, 21-02-40, 21-03-30, corresponding Ds: 61d 32.2m, 61
>34.3, 61 36.0, 61 36.3. AP for Hc"s: 34d 24.0N, 119d 02.0W. Sun Hc=74d
>36.5m,Moon Hc= 37d 30..5m. C.E.Fast 12m 25s,July 04, cleared dist"D" as per
>Stark book=61d 23.0m resulting in GMT error of 00m 07s.
=======================
Response from George.
I have put Mike's lunar aside for the time being and concentrated on his
four altitude observations, one for the Sun, two for the Moon on different
days, and one for Vega. These are spread over several days, but that
doesn't matter for a stationary observer on land. My astronomical positions
come from a different source than Mike's, so my check is an independent
one. It should correspond well to his figures, although he may have needed
to make some small corrections to refraction, if he happens to be well
above sea level.
And it's just as Mike says. The four position lines all cross quite
closely: Mike has identified a point at N 34deg 24'.8, W 119deg 01'.5 as
his best-estimate or fix, and all of the four position lines pass within
one-and-a-half miles of that point. Nothing to complain about there.
Measuring from on land using a liquid horizon eliminates all the errors
caused by anomalous dip and the uneven sea surface, so a seasoned observer
might hope for a bit more precision than that, but still, Mike's fix is a
pretty good one.
The only problem is that Mike knows where he is on the map,and can check
the known coordinates with his fix. He quotes his position from
www.Geocode.com: N 34deg 28.42', W 118deg 40'.09. (I've changed, slightly,
the way that this position is expressed). So his fix is about 3.6
arc-minutes too far South. This is not badly incompatible with his observed
position lines; I would put it as well within acceptable limits of error.
However, his fix is all of 20.6 arc-minutes, or 17 miles, too far West.
This is a more serious discrepancy, as Mike has observed.
Mike wonders if this is related to errors caused by mixed-liquids in his
artificial horizon, which will be discussed below. To me, that seems
unlikely, because any such altitude errors would tend to shift the position
lines in various directions depending on observed azimuth. That scatter
would tend to give rise to a big four-way "cocked hat", which doesn't seem
to be the case here. It's possible that such varying errors, if large
enough, could by chance combine to give a small crossover 20 miles too far
West, but it seems unlikely.
Presumably Mike is quite confident that he has identified his observation
point precisely on some map which relates to the stated Geocode website
position (a matter about which I am ignorant). Has he cross-checked,
roughly, his position from a local map? My atlas puts him near Camarillo,
California.
If questions of high precision are involved, it's often necessary to be
very careful about conflicts between differing map datums, but a 20
arc-minute discrepancy in longitude is too great to be due to any such
datum shift. What might possibly have caused it?
If all the longitudes are in error by a similar amount, thoughts turn to a
possible common error in time, by about 80 seconds. Several different
time-readings were taken from the watch over periods of several days, so
there's unlikely to be any common error in reading all those times. But
what about an error in relating the watch to GMT? Could it conceivably be,
perhaps, that a watch difference from a Greenwich time signal of 40 seconds
was noted, but then was applied in the wrong direction, to give an
80-second error when referring every watch reading to GMT? Just a
suggestion... Such things are possible.
======================
Now for Mike's horizon-pool, which he made up by adding bath oil to baby
oil because he didn't have enough baby oil. I am worried about his
observation that "Reflections at certain points of tray were distorted."
Certainly, there should be no distortion visible to the eye, in any
horizon-pool, except right at the very edge of the liquid due to surface
tension. What could cause the distortion Mike saw? Well,do those two
liquids mix properly? If not, do they remain in globs, like badly-mixed
French dressing? If any such interface between globs can occur at the
liquid surface, then surface-tension effects can distort it from being
exactly plane and level. In which case, that mixture would be a most
unsuitable choice.
Alternatively, is Mike observing, as well as the reflection he wants to see
on the upper surface, another view caused by light passing down through the
transparent liquid, and reflecting from the bottom of the tray? It was to
avoid such confusion that black engine oil, black tea, or a levelled
black-glass reflector, have been used for the job. It may by worth
experimenting with, say, syrup dosed with ink or inkjet refill fluid.
Keeping flies out of the syrup may present a problem, though.
However, as noted above, it seems unlikely that any cause of random upset
to the reflected altitudes could give rise to a consistent longitude error
without greatly enlarging the cocked hat.
Mike was right to bring his observation to our attention as an interesting
puzzle. I wonder whether others can provide more plausible suggestions than
mine.
In a previous mailing on this topic, I professed my innocence of baby-oil,
but this isn't quite true. I emptied my old Sestrel Minor compass of oil a
few years ago, cleaned it out, and refilled with Johnson's baby-oil. Since
then, it's been sparklingly clear and bubble-free, with (in my view) just
the right amount of damping. To me, it seems the perfect material for the
job. Don't use it to top-up a spirit-filled compass, though.
George Huxtable.
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contact George Huxtable by email at , 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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