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Gary,

Yes you are right that 16' is good enough but since the tenths are
available I use them. The only other reason to need the S.D. is if the
star refraction table is summed with the S.D. as navigators
preference.

Greg

On Dec 22, 1:11�pm, Gary LaPook  wrote:
> Greg Rudzinski wrote:
>
> "I use the Nautical Almanac S.D. figure for two practical applications.
> The first is for an initial adjustment of a horizon mirror that is way
> out of calibration or newly installed.
>
> Step 1. The micrometer drum is preset to double the Nautical Almanac
> S.D. figure on the arc (Generally this would be around 0 32.0').
> Step 2. The Sun's limbs are lined up with the upper limb touching the
> lower limb by using the adjustment tool on the horizon mirror.
>
> This technique usually gets my index error within plus or minus two
> minutes of arc on the first try."
>
> ------------------------------------------------------------------------
>
> But just using the constant of 16' for S.D. would get you that close so no 
need for the daily listing of S.D. to a tenth of a minute.
>
> --------------------------------------------------------------------------- ----------------------
>
> Greg Rudzinski also wrote:
>
> "The second practical application is for correcting my bubble sextant
> reading to read as a lower or upper limb reading for entry into my
> Palm PDA CN application which is formated for horizon reference."
>
> --------------------------------------------------------------------------- --
>
> Again, considering the level of accuracy from a bubble sextant, using the 
constant of 16' is sufficiently accurate.
>
> gl
>
> Greg Rudzinski wrote:
> > I use the Nautical Almanac S.D. figure for two practical applications.
> > The first is for an initial adjustment of a horizon mirror that is way
> > out of calibration or newly installed.
>
> > Step 1. The micrometer drum is preset to double the Nautical Almanac
> > S.D. figure on the arc (Generally this would be around 0 32.0').
> > Step 2. The Sun's limbs are lined up with the upper limb touching the
> > lower limb by using the adjustment tool on the horizon mirror.
>
> > This technique usually gets my index error within plus or minus two
> > minutes of arc on the first try.
>
> > The second practical application is for correcting my bubble sextant
> > reading to read as a lower or upper limb reading for entry into my
> > Palm PDA CN application which is formated for horizon reference.
>
> > On Dec 22, 4:36 am, Gary LaPook  wrote:
>
> >> 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
>
> >> frankr...@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 , email NavList+@fer3.com

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