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    Re: Help with Lunar distance.
    From: George Huxtable
    Date: 2010 May 13, 14:28 +0100

    This is an amended version of a posting I had tried to send yesterday,
    which fell foul of the list's rules on attachment size.
    Since then, there have been relevant postings from Paul Jackson and from
    Andres Ruiz, which have led to changes.
    Clearly, Paul Jackson wishes to draw a line under his Jupiter lunar and go
    on to something else. But it looks as though we at Navlist are not going to
    allow that to happen. We're insisting on discovering just what has gone
    wrong, in order to draw lessons from it, for Paul, and for the rest of us.
    Unfortunately, Paul has been particularly reticent about details of his
    observation and calculation. We have had to draw those details out of him,
    a bit of a time, rather like drawing teeth. That is not the way to ask for
    Navlist help. We enjoy a challenge, and thrive on data and facts. If Paul
    really wants our help, he will find us willing and ready to provide it, but
    he must be more forthcoming.
    Only recently has he informed os of the local date and time of his
    observation, which has allowed Andres to deduce a serious error, in the
    stated date of GMT.
    And only recently has he stated- "I used  a calculated Alt in a software
    programme for the Moon & Jupiter". Until now, it has been ambiguous,
    whether his stated altitudes were calculated from the almanac or observed
    with a sextant. But he hasn't yet told us what that program was, or what
    data was fed into it (date, time, position), or what the actual answers
    were. We need to know, please.
    Now that we have a new date for the observation, the discrepancies in his
    stated altitudes have got even greater.
    I attach a plot of the view of Moon and Jupiter as seen from 36º48'S
    174º 46'E at 18h 15m 52s on 8 May 2010, according to my Skymap program.
    As I understand it, this picture should be showing apparent positions in
    the sky, as seen by an observer, and affected by parallax and refraction;
    not the corrected positions. It should correspond with observations, made
    at the time; but clearly, it doesn't, in several respects.
    Paul's listing of his numbers can be found in the attachment to his posting
    of 12 May (copied there 3 times, for some reason)
    Let's deal with the inconsistencies.
    Paul's stated altitudes are far too low, and do not correspond at all with
    those displayed by Skymap, even with the amended Greenwich date. We need to
    know why. More details, please.
    The next problem, we have already touched on. The Moon's travel through the
    stars is always very nearly along its symmetrical bisector; that is, at
    right angle to the line joining the horns. A lunar distance. is trying to
    deduce where it has got to along that path. So ideally, the measurement
    should be to a star or planet, acting as a milepost, which is somewhere
    near the direction of that path (within 25º of it, say). As that picture
    shows, Jupiter at that moment was near conjunction with the Moon, and some
    way off the desired direction, so the lunar distance was changing rather
    more slowly (and what's more, non-linearly with time) than the expected
    amount, which is, if conditions are right, always in the region of 30
    arc-minutes per hour.
    Most of the time, a planet such as Jupiter provides an ideal target for a
    lunar distance, because it's always near the sky-path of the Moon. But not
    near a conjunction because then the angle swings around so much, as the
    Moon passes the planet. That's the very worst time for a lunar, so near to
    conjunction. Wait a couple of days, and all will be well. Or choose another
    sky-object than Jupiter, that night; there are plenty of alternatives.
    But even though it's not in an optimum direction, an observed lunar
    distance observation, such as that shown in the plot, ought to "work", and
    should provide an angle which can be compared with predictions to provide
    some sort of decent result. That seems far from being the case. We still
    need to discover why.
    There seems to be something odd about the way that Moon and Jupiter
    altitudes have been corrected. There are two important factors to this
    correction that affect lunar distance: refraction (for both bodies) and
    parallax (a big correction for the Moon; negligible for Jupiter).
    Refraction always makes a body look higher than it really is, so its
    correction is always to reduce the altitude below its observed value. That
    is what has happened, correctly, with Jupiter. Parallax always makes a body
    look lower than its geocentric position would imply, so to correct for it
    calls for adding a large amount to the observed altitude. This is in the
    opposite direction to the Moon refraction, and much greater, so the
    difference between the two will always be an amount, usually quite a large
    fraction of a degree, by which the observed asltitude has to be increased.
    And yet, Paul's corrected Moon altitude is less than his observed Moon
    altitude. I think something may have gone wrong with the sign here. But I
    am not familiar with the software that is being used, and the
    misunderstanding may be mine.
    Paul has expressed a wish to forget this observation and go on to Sun
    lunars. But that will only affect the second of the three problems that we
    have discussed. The others will remain, unless they are investigated and
    fixed. There's nothing wrong, in itself,  with using a planet or star.
    And to Paul, I say - Take heart! Everyone gets things wrong on their first
    lunar.  Look at your results critically, and offer the next lot to Navlist,
    just as you have done this time.
    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.


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