Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Mike Burkes' observations.
    From: George Huxtable
    Date: 2003 Jul 27, 16:41 +0100

    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.
    
    
    
    
    
    
    
    
    
    ================================================================
    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.
    ================================================================
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site