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    Lunars and delta-T
    From: Frank Reed
    Date: 2010 Nov 11, 16:34 -0800

    Dave Walden, you wrote:
    "Yes! Please continue to give "TT-UT", and thanks for doing so!"

    Oy vey... Dave, have you considered exactly how different would your understanding of the analysis of any lunars have been with/without specific values of delta-T?? How would you describe delta-T? And I don't mean, how is it formally defined. What is it? What is its significance astronomically? How does it enter into ephemeris calculations for navigation?

    And I'll repeat here, since I've changed the subject, a couple of comments I made to Antoine under the "Jupiter Lunar DSLR" subject:
    "Ya know, folks publish their calculations on lines of position, altitudes, etc. in NavList posts on a regular basis. I can't recall any of our detail-conscious calculators ever including a value for delta-T in those posts. Why is that? Well, of course, it's because that is an irrelevant detail.

    But for lunars, people do become obsessive about irrelevant details... Consider that delta-T is known for any year within the past century to a fraction of a second and any year within the historical period of lunars (back to the 1760s) to within a second or at most two. If you get it wrong by even two seconds of time, what impact does it have on the clearing process? Well, as we know, a tenth of a minute of arc (6" of arc) corresponds to 12 seconds of time. So a two second difference in delta-T would yield a single arc second error in the clearing process. Anyone worrying about ephemeris data at this level for lunars is missing the forest for the trees. You don't need to post values of delta-T except in exotic cases (or cases before about 1760 or hypothetical cases after the present --after 2015 or so)."


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