A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2015 Dec 3, 00:33 -0500
Official advice in many pubs has been given on this topic. Within 2 degrees of the pole, you use the pole as the AP.
All celestial bodies circle the sky at a constant altitude at the pole, except for slow changes in declination. You substitute Body GHA for azimuth. Azimuth along the meridians which meet at the pole.
Body declination is Hc. Compare Ho to Hc, plot using the pole as AP.
If a body is near the zenith, say Polaris at the North Pole, then the LOP must be plotted as a circle. It's no longer a large circle where a straight line approximates, its a curve that must be accounted for. Near is a relative term, say 2°.
Typical bodies are the sun and moon, which will be low. Some brighter stars may be visible with planning and high power scopes.
Refraction is the biggest variable, as most objects are at low altitude with low atmospheric temperatures.
HO229 Vol 6 (75 to 90°) provides not only the method but also the requisite tables.
Dutton, twelfth edition, paragraph 4213 gives a description, but not the tables.
HO211, HO214 can be used. HO249 can be used.
The article reproduced here lately about Scott's and Amundsen's polar shots uses this exact method! Pole is the AP. Most shots were of the sun around midday. LOPs plotted generally orthagonal to the meridians due to midday shots.
Take a bunch of shots whilst sitting at the pole over the course of 24 hours. The LOPs need no advancing, as you are stationary. The LOPs should neatly box the pole, under the assumption that you know the refraction. (Dip Meter is sounding better and better, isn't it!!!)
The Hc vs declination swap with an AP at the pole set me to my bad habit of sketching nav triangles agian on napkins, placemats, and newspapers. After a bit of contemplation it all seemed to make good sense on a basic level.
As usual it set me to some practical and impractical contemplations.
Practically speaking, what is the cutoff and why would I execute it? If I were sitting on the ice ...what choice and why so to speak.... 2 degrees, 3 degrees, mmmmm. Then I started to wonder what happens to the LHA term with the AP at the pole. After all LHA is measured westward.....to the GP of the body?....mmm
I started thinking about the Cos of 90 degrees... once I go from 89 degrees to 90...the term sort of falls off the sled.....and (cos lat 90) would take the LHA term in the HC equation pretty much with it anyway. But that drop into oblivion starts pretty close to 90.
Anyway, on a more esoteric level, I then contemplated how I would determine I was precisely at the North Pole, even if I couldn't find the handy pivot bearing in the ice like my little globe has.....LOL.... You always read that polar explorer so and so lay on the ice to get a horizon without dip....and concluded with a sight he was at the pole.
I decided I would be doing this madness in the Peak of summer (polar winter, forget it...too severe), so the sun was up 24 hours. I guessed this ambient light might kick out the other stars? and Planets? what about at 180 degree view away from the sun position...sun at my back...dark enough? (never been that far north...prefer palm trees myself)
If the sun were at 180, my AP on the pole.... and my altitude/declination observation is dead on the pole, I assumed I end up with a single line of position, through the pole, with a straight line approximation on the 90 -270 line viewed from overhead. Arc approximation can be added as well. In other word I might be att eh pole or a substantial distance off either way on the LOP.
Now..I need a second line? so they cross. If the moon just happens to be north dec. today and visible...great. I suppose if I just sat and made coffee and waited long enough, a second sun sight gives me a crossing line.
Meridian passage at local mean time (azimuth based) popped into my head, but I thought that might be tricky at the low long sun angles, and long arcs.
I then thought of pelorus compass bearing. Mag North down there around Baffin Island somewhere should give you a compass needle reading at the pole. If you knew the variation at the pole... or the variation where you were vs where you want to be...dammit...sank that one......
So as the Polar explorer proving I made the exact pole, what is the fastest easiest way to prove I got there..mmmm?.
Just practically speaking, why do I invoke the GHA/Zn & Dec/Hc at any specific point vs using a standard Triangle.
If I am a bother......Feel free to ignore my babbling....I just try to work through the curveballs ...If I worry this art form long enough ..I might really begin to understand it.