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    Re: What is the Longest Lunar Possible?
    From: Frank Reed
    Date: 2013 Jan 21, 15:24 -0800

    Antoine, you wrote:
    "I would think that a distance in the vicinity of 90 degrees is close to ideal for lunars because that configuration seems the one for which closure rate is maximum (as a positive or negative value) for a given set of Moon & Body declinations."

    In a single day, three different people have brought up declination. But really dec has nothing to do with this. Just think in ecliptic coordinates. The Moon is running along close to the ecliptic, bobbing up and down, with an inclination a bit more than 5 degrees, moving about 13 degrees per day in ecliptic longitude (as slow as 11 degrees per day at apogee and as fast as 15 degrees per day near perigee). The Sun is right on the ecliptic moving about 1 degree per day in ecliptic longitude. The planets are near the ecliptic moving with variable speeds, occasionally retrograding but in most cases at rates less than 1 degree per day. The stars appropriate for traditional lunars (lunars for GMT) are near the ecliptic and have zero motion in ecliptic longitude. So the "closure rate" as you've described is not at a maximum for any particular angle. At any angular distance the rate of change of the distance is 10-14 degrees per hour for Sun-Moon lunars and about 11-15 degrees per hour for lunars with the stars and outer planets. There is a small increase in the ability to determine GMT when using the stars since the rate of change is a bit higher, but this is nothing compared to other issues. If one really wanted to worry about variable rates, the biggest concern would be perigee versus apogee. And just so we're clear, no one ever did worry about that! If the rate is 10 degrees per hour, then the ability to measure lunars to +/-0.1' accuracy implies an ability to resolve GMT at a level of +/-15 seconds. That's what you could expect from Sun-Moon lunars with the Moon at apogee. The same +/-0.1' accuracy in the measured distance implies an error in GMT of +/-10 seconds when the rate of change is 15 degrees per hour as it is for star-Moon lunars when the Moon is at perigee.

    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.

    -FER


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