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    Re: What is the Longest Lunar Possible?
    From: George Brandenburg
    Date: 2013 Jan 22, 08:06 -0800

    I only recently learned of the term "parallactic retardation" coined by our departed friend George H, and I thought it was rather clever. It nicely describes the fact that the motion of the moon across the sky is visually slower due to the retrograde effect of parallax. As we know the moon moves relative to the fixed stars at a rate of about 30 arc minutes per hour due to its rotation around the earth. However, parallax makes the moon appear to move in the opposite direction by as much as 10 arc minutes per hour. So if we look at the moon relative to a fixed object (not Jupiter!), it appears to move slower than 30'/hr.

    But as Frank points out below, this visual retardation has no effect on the accuracy of a lunar. Even though the measured lunar distances will change more slowly with time, this is offset by the fact that the parallax correction is changing at the same time. So once the lunar is cleared and parallax is removed, the corrected lunar distance should change at the expected rate of of 30'/hr (or less if the distance measured is not along the moon's path).

    George B

    [NavList] Re: What is the Longest Lunar Possible?
    From: Frank Reed
    Date: 21 Jan 2013 15:24
    You added:
    "However we should include the "retardation" effect earlier mentioned by George Huxtable (peace be upon him), which - in my memories at least - is a diurnal effect."

    You probably missed George's retraction on this one. You began posting on the NavList message boards in August of 2009 which was many years after George first talked about this concept. He had long before disowned it, just a few weeks after bringing it up, and hoped to stamp it out. His original reasons for believing that this was a significant issue were, in fact, incorrect. Jan Kalivoda persuaded him of this just weeks after he originally invented the concept and named it "parallactic retardation". Unfortunately, George's postings on the topic still convince many newcomers that it's an important issue. There is a secondary issue, which George never got around to, where this issue does matter, but it's no direct concern to anything we've been discussing here.

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