The "Q" table is not quite as accurate at the Polaris correction table in the Nautical Almanac but probably makes no difference in practical navigation.The first factor in the NA table is equivalent to the "Q" table with a different tabulation method. Factor 2 has a variation of + and - 3.5 minutes as does factor 3. Since factor 2 and 3 are not included in the "Q" table, the worst case is that the Polaris derived latitude using the "Q" table might vary from one derived by use or the Nautical Almanac by at most 0.7' and usually is much less, and this slight difference is lost in the noise of sextant observations.
gl
gl
--- On Sat, 5/4/13, Gary LaPook <garylapook@pacbell.net> wrote:
From: Gary LaPook
<garylapook@pacbell.net>
Subject: [NavList] Re: Polaris correction tables
To: garylapook@pacbell.net
Date: Saturday, May 4, 2013, 11:39 PM
The "Q" table in the current Air Almanac is based on the coordinates of Polaris in 2013 while the "Q" table in H.O. 249 is based on the epoch of volume 1 (currently 2010) Polaris coordinates, (I have attached copies of each.) Compare them and you will see the slight differences due to the change in the coordinates of Polaris. I have also attached the Precession and Nutation correction table from H.O. 249, volume 1 (2010). Work a couple of examples and you will see that if you apply the P&N correction to the H.O. 249 "Q" values that you get the values from the 2013 Air Almanac "Q" values. For example, LHA Aries = 5°; lat = 40°; P&N
correction = 2.6 NM in direction 061°. "Q" from 2013 = -32' so an Ho of 40° results in a latitude of 39° 28' N. "Q" from 2010 = - 33'. Apply the P&N correction to the north-south axis (cos 61° x 2.6= 1.3' which
changes the 2010 "Q" correction to -31.7' making the computed latitude 39° 28.3', close enough?
So which one should you use? If you are working star sights with volume 1 of H.O. 249 then use the "Q" table from H.O. 249 because when you apply the P&N correction it will move the Polaris line along with the other lines the proper distance so that the fix will be accurate. If you are combining a Polaris sight with planets or moon or working stars with volume 2 or 3 then use the "Q" table from the Air Almanac otherwise you will need to apply the P&N correction to only the Polaris line, additional work.
gl
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Attached File: 123868.polaris page from pub249vol1-2.pdf (no preview
available)
Attached File: 123868.precession page from pub249vol1.pdf (no preview
available)
Attached File: 123868.polaris q
table.pdf (no preview available)
: http://fer3.com/arc/m2.aspx?i=123868
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