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    Re: A basic sight reduction question
    From: Stan K
    Date: 2012 Apr 4, 19:10 -0400
    Gary,

    You are the second person who suggested not mentioning the situation to the students, which is sounding like a pretty good idea, but the issue is not accuracy, just procedure.  On the off chance that someone asks, I would like to know what is the "correct" procedure.  I have a message of to the "contact us" at the UK Hydrographic Office.  Let's see what they suggest.

    Stan


    -----Original Message-----
    From: Gary LaPook <garylapook@pacbell.net>
    To: NavList <NavList@fer3.com>
    Sent: Wed, Apr 4, 2012 4:54 pm
    Subject: [NavList] Re: A basic sight reduction question

    My Nautical Almanac has 276 pages of data and explanation and 30 pages of "increments and corrections." If you want perfect, good to the second, I&C tables then they will have to be 1800 pages long instead of the current 30 pages. All of life is an approximation and any concern you have about these corrections not being "exact" is lost in the noise of the rest of the celestial navigation process. A thorough study of thousands of sextant observations taken by professional navigators showed a standard deviation of about 1.5 minutes and if you are using H.O. 229 it states that the accuracy is usually within 0.2' with some weird cases producing errors up to 3.9' in the Hc so don't get too upset about the possible inaccuracy introduced by the I&C tables. Don't mention it too your students, they won't notice and why make it even more complicated for them.


    276 30

    --- On Wed, 4/4/12, slk1000@aol.com <slk1000@aol.com> wrote:

    From: slk1000@aol.com <slk1000@aol.com>
    Subject: [NavList] Re: A basic sight reduction question
    To: NavList@fer3.com
    Cc: slk1000@aol.com
    Date: Wednesday, April 4, 2012, 10:25 AM

    Gary,

    Yes, it seems like nobody ever noticed that situation!

    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.

    I've looked for an e-mail address for someone at the UK Hydrographic Office.  Know of any?

    Stan


    -----Original Message-----
    From: Gary LaPook <garylapook@pacbell.net>
    To: NavList <NavList@fer3.com>
    Sent: Wed, Apr 4, 2012 12:46 pm
    Subject: [NavList] Re: A basic sight reduction question

    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@aol.com <slk1000@aol.com> wrote:

    From: slk1000@aol.com <slk1000@aol.com>
    Subject: [NavList] A basic sight reduction question
    To: navlist@fer3.com
    Cc: slk1000@aol.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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