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    Re: Semi-diameter in the Nautical Almanac
    From: Frank Reed
    Date: 2009 Dec 15, 19:43 -0800

    Gary, you wrote:
    "We are all familiar with the semi-diameter of the sun and of the moon being tabulated on each of the daily pages of the Nautical Almanac. My question is "why?" There is no place where you use this bit of information when using the N.A. "

    Because it's not perfect? :-) I'm serious. I think it's just that simple. The changes made during the 1950s resulting in the modern Nautical Almanac in 1958, very nearly unchanged right through the present, were mostly excellent, but they missed a few things, and there are some features which are either unnecessary or less than optimally presented. There's probably no point in changing it now, except in private substitute almanacs, since celestial navigation is essentially dead from a practical standpoint.

    There is one very small practical use for the lunar and solar semi-diameters. If you use the Sun and Moon to get an index correction by bringing alternate limbs together, you also simultaneously measure the diameter of the object (but only if side error has been rigorously eliminated which is otherwise fairly pointless). By comparing the measured SD with the tabulated SD, we get a sanity check on the index correction. This little trick was even known back in the 19th century.

    you also wrote:
    "I am also curious why the sun correction table has only two tabulations allowing for only two S.D. values when the S.D. of the sun includes six different values during the year from15.8 to 16.3 minutes. This unnecessarily limits the accuracy of the sun corrections. "

    Yeah, this one has puzzled me, too. Clearly, whatever committee decided back in the 1950s that those two columns were sufficient felt that this was "good enough" for the accuracy of celestial navigation as practiced at that time. It seems like it would have been "nice" to include a monthly or bi-monthly table for the Sun, or, as you suggest, to provide an alternate method allowing the calculation by separate steps. This sort of table, as published in the N.A., combining refraction, parallax, and semi-diameter was seen as a great improvement back in the middle of the 20th century. This calculation had "normally" been done in three separate steps for the previous fifty to one hundred years. Even earlier, it was not uncommon to use an altitude correction that combined dip with semi-diameter: +12' for a lower limb Moon or Sun sight, -20' for an upper limb sight. You can find these corrections in most older navigation manuals (especially for correcting the altitudes in lunar observations since a few minutes of arc error in altitudes for clearing lunars doesn't matter).


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