A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2020 Nov 15, 08:43 -0800
I said: You might be lucky in daytime with the Moon or perhaps Venus if the Sun isn’t too high, otherwise you’re looking for suitable stars or planets during twilight.
I was a bit worried when I suggested observing Venus at meridian passage in daylight, but if you get far enough north in winter it’s possible. E.g. this morning at 09.45 local time above about 67N Venus would have been due South while the Sun was just rising. 67N is ice free in some parts of the World in winter. In the 1950s my dad used to take his iron ore carrier to collect Swedish ore from Narvik in Northern Norway in winter when the Gulf of Bothnia was frozen, and he couldn’t collect it from Sweden.
E.g. At 09.00 UTC on Sunday 15th November 2020, DR Position 67.05N 010.00E Captain Birdseye observes Venus and measures 15°59.0'. He observes Venus again at 0905UTC and measures 16°00.0', and he observes Venus again at 09.10 and measures 15°59.0'. In his younger year,s he might have taken a few more readings before and after, but it’s cold out there and he keeps getting icicles in his beard and moustache, so he decides instead that Meridian Passage was at 0905UTC when Hs was 16°00.0' and LHA Venus must have been close enough to 360. A long time ago he read, he can’t remember where, someone told him not to fiddle with his sextant, so he’s carrying an index error 2.5’ on the arc. He assesses his height of eye as about 30’ so he looks up dip as -5.3’. He’s not really into temperature and pressure corrections so he’s happy to accept refraction as -3.4’. He calculates Ho as:
Index error -2.5' 15°57.5'
Dip -5.3' 15°52.2'
Refraction -3.4' 15°48.8' =Ho
From his almanac he calculates declination Venus at 0905UTC as 7°03.5'S
Scratching his head he sketches a rough diagram on the back of an envelope and comes up with:
Zenith Distance or (90 – Ho) = Latitude + Declination of Venus South
Therefore Latitude = 90-Ho-Dec Venus S = 90-15.48.8 - 7.03.5 = 90 – 22.52.3 = 67° 07.7'N
Confident in his latitude at least, he sits back and enjoys a well-deserved mug of cocoa. DaveP