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    Re: Semi-diameter in the Nautical Almanac
    From: Gary LaPook
    Date: 2009 Dec 22, 04:36 -0800

    Frank wrote:
    
    
    "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."
    
    ----------------------------------------------------------------------------------------------------------------------------------------------
    
    I like that method of checking index error because you get repeatable
    results since the variations in the horizon go away. But you don't need
    to know the S.D. to do it and it really doesn't provide a "sanity check"
    all it does is give you a measure of the accuracy of the scale between
    plus and minus about half a degree. This type of check can be done with
    any far away object such as a building or telephone pole, the actual
    width of the object makes no difference in finding the index error.
    
    BTW, I have a different way of doing the calculation which I think is
    easier.The normal way is to touch the edges of the sun together both
    above and below and you get readings of, say, 36' on the arc and then a
    reading on the micrometer of 32' with the main index below zero. The
    normal way to handle these readings is to subtract the micrometer
    reading of the below zero reading from 60' to determine how far below
    the zero mark this reading is, in this case it is 28' below zero. You
    then subtract the below zero reading (the "off the arc"reading) from the
    "on the arc" reading and divide the result by two to determine the index
    error. So. using this example, 36 - 28 = 8 divided by 2 gives an index
    error of 4' on the arc.
    
    The method I use is slightly different. I simply treat the off the arc
    reading much like the characteristic of a logarithm. In the example, the
    below zero reading is 32 with an understood -60 behind it. So I add the
    two readings from the micrometer, 36 + 32 = 68 minus the understood 60 =
    8 divided by 2 = 4, the same answer but easier to do in your head.
    
    gl
    
    
    
    
    
    
    frankreed{at}HistoricalAtlas.com wrote:
    >
    > 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).
    >
    > -FER
    >
    > --
    > NavList message boards: www.fer3.com/arc
    > Or post by email to: NavList@fer3.com
    > To unsubscribe, email NavList+unsubscribe@fer3.com
    
    --
    NavList message boards: www.fer3.com/arc
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