NavList:

I am trying to work out a historical problem utilizing the Polaris-table data from 1998.  I understand that if I have a Nautical Almanac for a given year, that the sun's GHA/Dec numbers will be reusable four years later.  That is, if I have an almanac for year 2011, the sun's data should work pretty well for year 2015.  Question #1:  Is this also true of the Polaris tables?  Question #2:  If the answer to question #1 is "no" then does anybody have a scan of the Polaris tables that they can post from 1998 specifically?   Question #3:  If the answer to question #1 is "yes" then does anybody have a scan of the Polaris tables that they can post from any one of the years 1994/1998/2002/2006/2010/2014?  Thanks,  Bob Goethe

I would say it depends upon what you want to use your Polaris shot for and how accurate you expect the result to be.  If you just want a simple latitude, the only time dependent item you need is GHA Aries and hence LHA Aries leading to a Q correction and an azimuth.  Although GHA Aries at 00hr on 1^st March ought to be the same every four years, it isn’t quite.  E.g.  1977 158° 41’, 1981 158° 43’, 1985 158° 44’ (AP3270 Vol1 1980 Epoch Table 4).   Presumably this mainly because the Earth’s orbit around the sun is 365.256 days and not 365.250.  The rate of change of GHA Aries appears to be about 3’ per eight years, so using 2014 GHA instead of 1998 would give a GHA Aries approximately 6’ too large.

Does this matter?  Looking at AP3270 Table 6 Q Correction, a 6’ error in LHA Aries might put Q into the next bracket so there might be a 1’ error in latitude.  Similarly, looking at Table 7 Azimuth of Polaris, a 6’ error in LHA Aries is going to have a negligable effect on an azimuth plotted with a Douglas Protractor and a 2B pencil.  However, we must not forget Q itself changes slightly anually because of P&N and stellar motion, and that’ll require a bit more thought.  Looking at AP3270 Vol1 1980 Epoch Table 5 Correction for Precession and Nutation , the angles appear almost the same for each year off centre.  It’s just the distance that increases with years off.  For maximum lattitude error, we’re looking for maximum P&N distance x cos Az.  This appears to be about 2nm or 2’ latitude in five years, so using P&N figures for 2014 without adjustment  could lead up to a maximum of 6.4nm or 6.4’ error in latitude.  Similar things could be said about azimuth, but my 1943 edition of AP1618 Volume L tells me in Table VII that between 1980 and 2000 the azimuth of Polaris doesn’t go outside 359 and 001, so can probably also be ignored for practical fix plotting.

I’m sure someone will find at least one error in these figures worked out just before midnight, and of course if you wan’t something cleverer than a simple latitude shot, you probably need 1998 values to start off with. Dave