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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Ritchie
Date: 2022 Aug 11, 14:35 -0700
Lars / Antoine,
My software gracefully gives a warning that the angle between Procyon and the Moon’s background motion (43°) was over its limit of 40°. If I relax that to 50° I get the following results for an assumed position of N47° 12’ E009° 30’ and the stated values of IC 0, HoE 0, Moon LL 40° 29.0’, Procyon 10° 19.0, LDs 30° 9.8’.
A: Assumed altitude MSL, Temp 10C, Pressure 1010 hPA, I see a UT of 03:38:54.
B: Assumed ‘lowland’ altitude of Vaduz on the Rhine river as 450m and ambient ISA pre-dawn conditions at that altitude of 12C and 961 hPa, I see 03:39:31.
So bad guessing of the unspecified altitude, temperature and pressure could be responsible for up to 35 secs error.
Changing the Moon altitude plus/minus 5’ changes the time minus/plus 13 seconds, whilst the same plus/minus change for Procyon changes it plus/minus 33 seconds. So, in the worst case, this could contribute 46 secs error.
The reduced closure rate (about 73% of the value if the bodies were aligned) must reduce the observation accuracy somewhat. The quoted (super) sextant accuracy of 0.05’ still allows another 7 seconds variation.
I attach a plot of the Lunar line of position for the result (B:) above. If I assume that the actual position was where that LLoP crosses the Southern shore of the Walensee (N47° 06.8’ E009° 16.6’) and check it all with Frank’s Lunars Analyzer, I get a LD error of 0.06’, 1.2nm. (The fact that this position is outside the border of Lichtenstein infers that my guess B: was bad.)
Bill Ritchie
