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A basic sight reduction question
From: Stan K
Date: 2012 Apr 4, 09:50 -0400
From: Stan K
Date: 2012 Apr 4, 09:50 -0400
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
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
