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    Re: Can you work a Polaris problem just like any other star?
    From: Geoffrey Kolbe
    Date: 2014 Jan 5, 07:43 +0000



    On Sun, Jan 5, 2014 at 4:02 AM, Frank Reed <FrankReed{at}historicalatlas.com> wrote:

    Greg, you wrote:
    "am I wrong in thinking that can you work a Polaris problem just like any other star?"

    Frank wrote 

    Yes, certainly. All sights are all the same from the perspective of modern celestial navigation. The coordinates of Polaris changes more rapidly due to precession, but the significance of this can be exaggerated. Given all this, there's also no particular reason to prefer Polaris over any other second magnitude star. We don't need Polaris.

    Well, Polaris is still very useful in surveying to fix azimuth and as such, Polaris (which can be seen during the day with a good theodolite) and the Sun are two bodies that surveyors use almost exclusively - if they are not using GPS. In particular, archaeoastronomers who cannot afford the fancy differential GPS do-dads can get good results using Polaris. 

    Too, I have found Polaris very useful to fix a position of a camp site (in the desert) when I was using a bubble octant (A-12) of whose index error I was unsure. Basically, the method is to create a 'box' of LOPs using the setting Sun in the West in the evening, the rising Sun in the East next morning, Polaris in the North - because it is always in the North - and perhaps Sirius or Arcturus in the South. The box should be square if index error dominates, which is the advantage of this method over the usual cocked hat using three sightings. If the box is square and I consequently get a nice warm fuzzy feeling the sightings are good, my position will be in the middle of the box.

    And Greg wrote:
    "I know that the USNO Nautical Almanac has a set of tables [...]"

    And Frank wrote 

    These are traditional tables, and they're kept in the almanac primarily because of tradition. There are other ways of handling this that are just as good (among many, see for example the attached tables). Note that the calculation for latitude by Polaris is usually SHORTER than a complete sight reduction by hand so it does have that benefit.

    In my little offering "Long Term Almanac 2000-2050" (which has attracted some very favourable comments recently - my grateful thanks), Polaris is just one of the 38 listed stars and (it is intended that) it should be treated as any other star or body when it comes to reducing a sighting of it to an LOP. Frankly, it is quicker in the end to jump through the hoops you know with your regular sight reduction method, than to turn to unfamiliar Polaris tables and figure out what is going on and what you need to do to get a latitude.

    By the way, the "Long Term Almanac 2000-2050" is now available in hard-back from StarPath Publications. So, if you store your navigation charts, books and other equipment in the bilges of your boat (as seems to be the habit of at least one member of this list!) then you can avoid your LTA being reduced to a sodden pulp too quickly now by buying in hard-back...
    --
    Dr Geoffrey Kolbe, Riccarton Farm, Newcastleton, Scotland, TD9 0SN
       
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