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    Re: 18 june 2013 lunar distance
    From: Frank Reed
    Date: 2013 Jul 6, 06:37 -0700

    Paul Hirose wrote:
    "Strictly with regard to navigation by lunar distances, I believe you're
    right. Nobody (a dangerous word) has yet accounted for the limb profile."

    No one has done so, in the sense of making a "product" out of it (something available for widespread use). I HAVE done it myself using an earlier limb profile for my own interest, but it wasn't worth cleaning up and converting it into a product.

    And you wrote:
    "But in practice is there anything worthwhile to be gained, given the
    accuracy of sextant observations? I doubt it."

    I agree that the gains would be very small, even trivial. It's a correction potentially as large as two arcseconds. So correcting for it in sextant observations would only lead to a small change, detectable at best in detailed statistics. For example, suppose I can get a standard deviation of 0.1' error in cleared lunars by "true" corrected distances implying that just about two-thirds of observations are within 6 arcseconds. If I'm neglecting a correction of one to two arcseconds in the limb, then it may be that my actual observations are within 6 arcseconds 75% of the time. Is that "worthwhile"? Not much! --which is why it's way down on my "to do" list. But the limb correction IS the LARGEST neglected detail in clearing lunars. Fussing over anything smaller than that (like, e.g. a tenth of a second in Delta-T) is even further "out there" in terms of relevance to "actual" lunar observations.

    And Paul, you wrote:
    "But if that little correction is to be pursued, I suggest looking at the
    software of the occultation people. The limb profile is routine business
    to them."

    On that score, I would say that your information is a few years out-of-date. Though the IOTA folks spent literally decades collecting limb profile data, that's all in the dustbin of history now, thanks to recent lunar spacecraft. As I mentioned in an earlier message, this is a big change in available data. In fact, it's a revolutionary change --the Kaguya (Japan), LRO (US), and Chang'e (China) data are the biggest development in lunar science since the Apollo era. Last I heard, some of the IOTA calculation crowd were busy building new versions of their software, completely junking their old profile model and replacing it with the detailed topographic models. But that's a big project for volunteers. Note, too, that their problem is different. For lunars with a sextant, where we're swinging the other body past the nearest part of the Moon's limb, you need to know broad details across a section of the lunar limb spanning perhaps ten degrees of position angle as measured from the center of the Moon's disk (how large a span of the limb? ...that's probably the only interesting question in all of this). By contrast, for stellar occultations, very fine details, down to the smallest mountain valleys of the Moon, can measurably affect the timing of an occultation. Also, for the IOTA occultation observations to remain relevant, they have to switch gears completely. The lunar limb problem is over and done with. There may be some relevance in new observations in the stars themselves.


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