NavList:

Stan,

There is no v correction for Sun in the Almanac.
And they explain on p. 255, 4.
"v is zero for Aries and negligible for Sun".

Thus the Sun GHA at x hours 0 minutes 0 seconds is exactly what
the daily page of the almanac says.

There is no d correction for Aries. Aries declination is always 0.
There is d correction for Sun, but it is small, usually less that 1.0.
If you look for the correction for d=1.0 at the Corrections tables
for 0 minutes, you see that the correction is 0.

Thus the data on daily page for x hour 0 minutes 0 seconds is exact,
(or has negiligible error for Sun) and the use of Correction table
is not needed for this case.

The explanation in the Almanac is complete but very brief, indeed, I 
agree.

Alex.

On Wed, 4 Apr 2012, Stan K wrote:

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> Gary,
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> Yes, it seems like nobody ever noticed that situation!
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> I understand all this, but it doesn't help.
The problem is not with the mathematics but with the
proper procedure when doing an "on paper" sight reduction,
i.e. with a typical sight reduction form.
It still seems that if you follow the instructions in the Almanac,
for the 0 minutes case we are discussing,
any time the v or d value is between 6.0' and 17.9',
for example, you would be adding a v or d correction
of 0.1' to the hour value on the daily page, which appears incorrect.

> And when would the v and d data of the 0 minutes column ever be used?  When 
there are 0 minutes but non-zero seconds?  Not according to the Almanac 
instructions.
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> I've looked for an e-mail address for someone at the UK Hydrographic Office.  
Know of any?
>
> Stan
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> -----Original Message-----
> From: Gary LaPook <garylapook---net>
> To: NavList <NavList@fer3.com>
> Sent: Wed, Apr 4, 2012 12:46 pm
> Subject: [NavList] Re: A basic sight reduction question
>
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> I never noticed the  situation that you pointed out. The answer is that the 
"v" and "d" correction tables in the N.A. are simply multiplication tables, 
multiplying the "v" and "d" factors (which are simply how much the body changes 
its position in one hour) by the proportion it will change in a period less than 
a full hour. Since the tables only list correction for whole minutes they are 
actually the average correction for the whole minute tabulation so they are 
actually calculated for m + 30 seconds. They can be considered precise only for 
the +30 sec case and approximate for other times. If you wanted more precision 
then just divide the "v" or "d" by 3600 and multiply by the number of seconds 
after "on the hour." If you do it this way then you will find the correction for 
"on the hour" is zero, zero seconds times any "v" or "d" value will always equal 
zero. But you are trying to make it too hard on yourself.
>
> gl
> --- On Wed, 4/4/12, slk1000---com <slk1000---com> wrote:
>
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> From: slk1000---com <slk1000---com>
> Subject: [NavList] A basic sight reduction question
> To: navlist---org
> Cc: slk1000---com
> Date: Wednesday, April 4, 2012, 6:50 AM
>
>
>
> I have always though that the hour values of GHA and Dec on the daily pages of 
the Nautical Almanac were "exact" (on page 255 of the Nautical Almanac:  "The 
daily pages give the GHA and Dec...for each hour of UT."), and that everything 
else (minute/second GHA increment and v and d values and corrections) concerned 
adjustments for sights not being taken on the hour.  Now I am not so sure.
>
> For the first time, I have had to concern myself with sights taken on the 
whole hour (minutes and seconds are zero), and I am not clear as to how they 
should be handled.  Based on my original thinking, just use the hour value of 
GHA/Dec, and nothing more.  But according to page 256 of the Nautical Almanac:   
"The table of Increments and Correction for the minute of UT is then selected.  
For the GHA, the increment for minutes and seconds is taken from the appropriate 
column opposite the seconds of UT; the v-correction is taken from the second 
part of the same table opposite the value of v as given on the daily pages."  So 
if the minutes value is zero, you go into the 0 minute block (the "same table") 
with the value of v from the daily pages and extract the value of the v 
correction, no?  (This all applies to Dec also.)
>
> If this is wrong, then why show v/d values and corrections for 0 minutes at 
all?  Where would they be used?
>
> And then what if the minutes are zero but the seconds are not?  Is that 
another "special case"?
>
> Practically speaking, this is no big deal, since the v/d correction is either 
0.0', 0.1', or 0.2' (and that only for a v/d value of 18.0') for 0 minutes, but 
using the corrections for whole hours is either right or wrong.  I cannot find 
any examples showing this situation.
>
> Am I way off base here, or does the explanation in the Almanac leave something 
to be desired?
>
> Hard to believe that for all the time I have been doing CN, this is the first 
time I have asked myself these questions.
>
> Stan
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> : http://fer3.com/arc/m2.aspx?i=118693
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