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    Re: Moon corrections: how computation order affects the result
    From: Eric Fernandez
    Date: 2020 Sep 3, 12:34 -0700

    Hi David. Thanks for your time and insights. I understand where you are coming from with the 0.1' descrepancy, this is because the moon altitude correction table in the NA are computed using the central value of each column (aka 82.5 for 80 to 85). But this is not what I am talking about, and what I found out persists, using formulas instead of tables.

    The problem I had was with the parallax calculation, made after or before adding SD and correction. According to Mr. Umland's guide, parallax is calculated before adding SD if not considering the augmentation, but after if using an augmented SD. This leads to a different parallax value, which compensates augmentation in the final Ho computation. However if someone simply added all terms independently calculated from Hs (SD, P and augmentation), then Ho is increased by the value of the augmentation.

    What I mean is that if someone wants to use correction of the order of the tenth of minute, such as augmentation (which increases SD by up to 0.3'), then he should also take care of the order of operations and from which corrected value of H to calculate parallax. The air almanach does not help in that sense, because the table onlz gives the value of HPxcos(H) but does not tell you which H to use. Umland's guide seems unambiguous about it, but this is not the case with other sources. Also the NA (2018) mentions augmentation, but does not provides the formulas to calculate it.

    So to be very precise, one should know if they should add augmented SD then parallax, or the contrary.

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