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    Re: Technique question
    From: George Huxtable
    Date: 2003 May 20, 00:51 +0100

    Some comments and questions for Doug Royer et al.
    
    I don't think Philip Ouvry's contibution, about false horizons due to
    shimmer on the water below the Moon, applies in this case. Doug seemed
    quite happy with his altitudes, it was just his lunar distances (which
    don't involve the horizon) that were in question.
    
    I take it that the zone time applying to California at this time of year
    must be +7 hours, daylight saving being in operation. Things fit together
    if that assumption is made, but not otherwise.
    
    Doug admits that he was making errors by the time he got to Spica, and
    indeed his observations seem to have got very ragged then, if one looks at
    the sequence of the last four lunar distances from 22:36:54 zt. Not only
    that, his timing sequence has a 24 minute gap between the altitude
    measurement at 22:10:40 zt and the first lunar at 22:34:02 zt. For these
    altitudes to be usable the time gaps MUST be kept short. After that time
    gap, it would have been better to start again, remeasuring the two
    altitudes. It's clear he is capable of doing much better, to judge by the
    Jupiter observations.
    
    A minor point: I think it's better to use a measurement sequence as follows-
    Other-body alt., Moon alt., several lunar distances, Moon alt., other-body alt.
    This is because the Moon alt. usually introduces a sizeable correction, due
    mostly to it's great parallax. Other-body altitude correction is MUCH
    smaller, due mainly to refraction. So it makes sense to keep the Moon alt.
    measurement as close as possible, it time, to the lunar distances.
    
    It's good to see that Doug has learned all the tricks. How many of us have
    tried observing a back-altitude with the sextant, by facing the opposite
    way and observing a body behind the back of your head, because the horizon
    below the body is obscured by nearby land? Very few, I guess. Full marks to
    Doug! Did he remember that the dip correction has to be treated differently
    in that situation?
    
    Because he knows the actual time from his watch, Doug was able to obtain
    and cross pairs of position lines, obtained from calculated intercepts for
    the two bodies. Though this had nothing to do with lunar distances, it was
    a sensible test to make. Presumably the first position, at "2000" was
    obtained by crossing position lines for Moon and Jupiter, and at "2200",
    similar lines for Moon and Jupiter. Which pairs of altitudes did he use for
    that analysis, taken at what time? Looking at the numbers, it seems to me
    that the Spica altitude at 22:10:40 zt would give a very close fit to his
    GPS position, but the 22:41:13 zt measurement would give a position a
    couple of miles off. (still not too bad)
    
    For each altitude measurement, it would be interesting to know the values
    that were obtained for ha, the apparent height of the centres above the
    true horizon (that is, after correcting for index error, dip, and
    semidiameter) and the values for hc (the same altitudes after correcting
    for parallax and refraction).
    
    As for Doug's questions about the effect of the haziness, only he can judge
    how accurately it was possible to align the body with the moon, and no
    doubt has much more experience than most of us in using a sextant in hazy
    conditions (certainly a lot more than me). The aim is, of course, to brush
    the centre of the (fuzzy) image of the star or planet, against the (fuzzy)
    edge of the Moon, to the utmost possible precision. The fuzziness will
    certainly not help.
    
    I'm puxxled when he says- "The horizon was uneffected by the fog as it was
    >high in the atmosphere and when the bodies were brought down to it sharp in
    >the scope." Even though the horizon mirror was looking horizontally at a
    >clear horizon, the index mirror was viewing the body up in the sky, so
    >bringing its image down to the horizon should not affect its fuzziness.
    >Was Doug's eye being affected by the brightness of the large adjacent
    >Moon-disc in his 6x telescope, I wonder? Would it help to put a weak
    >filter into the Moon light-path?
    
    Fred's suggestion of testing accuracy by comparing a measured Moon
    semidiameter with its almanac value is a good one. The Moon's semidiameter
    is quoted each day in the almanac, but its horizontal parallax is given
    each hour, and as semidiameter is .2725 x HP, it can be obtained more
    precisely that way.
    
    George Huxtable
    
    ===================
    
    Here's a copy of Doug's questions.
    
    >I have a couple of technique questions about Lunar observations for you
    >guys.Mr. Hebard wrote something that got me wondering.Let me give you the
    >observation conditions of the last 2 attempts.On 05-11 and 05-17 there was
    >an atmospheric condition (high,thin layer of fog) that fogged the images of
    >the bodies under observation.The edges of the Moon were clear but had a
    >corona type of fuzziness about it.The images of the planets and stars
    >observed were the same but because of the smallness of the bodies not as
    >pronounced.I used the 6x scope on the sextant and it looked as if the images
    >were in coincidence.However,maybe they weren't after reading the posts this
    >morning.The altitude observations taken at these same times appear to be
    >close to what they should be.The horizon was uneffected by the fog as it was
    >high in the atmosphere and when the bodies were brought down to it sharp in
    >the scope.I have confidence in the altitudes because I am used to doing them
    >and feel there is little observer error in them.However,the sights of the
    >distances may have an observer error due to inexperiance in useing the
    >sextant in the positions needed to take the angles of the distances.My
    >question is mainly about the images under the conditions and if that could
    >lead to further reduction of accuaracy not including the observer
    >error?Under what atmospheric conditions do you guys usually attempt Lunar
    >observations?
    >I have data from 05-17 but will try clearing it before posting it to compare
    >the final output with what you guys will come up with.I also have to clear
    >the data from last week and post my times for critique.
    
    ================================================================
    contact George Huxtable by email at george---.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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