A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: John D. Howard
Date: 2018 May 25, 04:20 -0700
No one has replied to your post so let me throw in my 2 cents.
"If this diagram is right, is it true navigators using Almanacs prior to 1925 need to be careful of how their dates align with the system they are using. For example, Nautical/Sea Days prior to 1805 would use thursday's ephemeris for ship board Friday sights? I'm sure this could not be, this would be an un-natural act."
It was not un-natural at all - we still do it today. You live in California and today you go sailing and decide to do Cel Nav. At 0800 Friday it is 1600Z Friday. Your noon shot is at 1200 Friday and 2000Z. Now you do an afternoon sun shot at 5:30 PM on Friday - you must go to the NA and look up Saturday, 0130 Z.
If you live in New Zeland and took a morning sun shot Friday at 0800 you would go to the NA and look at data Thursday at 2000 Z
I think your diagram is misleeding because a ship used one system at a time. I the ship's day started at Noon ( local ) and the NA GMT also started at Noon Greenwitch then that was the only system the crew used. If you were in the Atlantic then the days were nearly the same but round the Horn and sailed the Pacific then the days soon differed.
I think what Frank said is the key - they knew the system they were using and it carried over day by day. I f the navigator used Thursday yesterday evening then today he will use Friday. They kept their log every day.
When we look a a single page of the log or worse, a working scrap paper we do not see that yesterday was the 24 th so today is the 25 th. Also, meridian passage on the ship may be on a different day than meridian passage in England.
Just my thoughts. John H.