A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2017 Sep 26, 20:30 -0700
RE: Stuart-sep-2017-g40186 and Couëtte-sep-2017-g40189
After a few additional thoughts ... the conditions earlier stated in Couëtte-sep-2017-g40145 do remain sufficient, but the latitude constraint derived from Gacrux being forced to stay circumpolar and 10°above the horizon just needs to be computed without error.
Here is a corrected version of this post:
-  - [Southern Cross] most northern star to be always above the Horizon. If you want a clear view of it, let's define that its height should always be above 10° (but it could somewhat be lower of course in clean and clear air) hence the Observer should always be south of
N17° 43 ° South in order to see Gacrux [declination close to S 57°] with 10° minimum elevation as a circumpolar star around the South Pole.
-  - Sun to be always below the horizon at midnight local solar time, let's say for instance at 9.5 ° minimum below the horizon. If we assume the Sun southernmost Declination to be S 23.5°, then you would like the Observer to be at least 33° away from the south Pole, which tells you that the Observer should remain north of 57 ° South.
I also stated further down: “ We could refine this a bit through checking the Azimuth of the Southern Cross at Midnight local solar time at or around Dec 21 st. ”
- Your own approach in Stuart-sep-2017-g40186 about the time of the Year when the Sun has the same Right Ascension as Acrux [or Gacrux] is better and could maybe help us to slightly extend one of these limits or maybe both of them.
In conclusion, with terrestrial Observers constrained to be in quite a narrow band [43°S - 57°S], we can see that there are not many lands left, and only: Tasmania, barely South New Zealand, the extreme southern tip of South America, and Islas Malvinas / Falkland Islands (yes !) with South Georgia Islands, and also the Kerguelen Islands as well as Crozet Island and a few more windy confetti islands in this south Indian Ocean area.
All Africa has now become excluded, as well as all mainland Australia.
That’s about it !
Hope this time we have found the right answer. Everybody agree?