NavList:

At the risk of embarrassing myself, I'm determined to understand how the various 19th century time conventions work and how almanac's dates/times align with them.

Frank is probably right - navigators simply 'knew' what to do to get the work right. And once they got it right, they were set for the rest of the voyage. Another point, frequently made, is there is a big difference between historic navigation text books and what navigators actually did. The same applies for decrees about new standards and when they are put into practice.

All this aside, it is instructive to see how the various time conventions compare and how almanacs align.

The attached diagram tries to compare four daily time conventions: two versions of Nautical/Sea Days, Civil Time and Astronomical Time.  Each share the same Meridian Passages (Mer Pass) as a reference.

The top two time-lines show Nautical/Sea Days pre- and post-1805. Both conventions begin 12 hours earlier than Civil Time for the same day. But in 1805, the BRN ordered logs to use Civil Day naming convention, thus, if I got it right, Saturday in the prior system is now split half-day Friday and half-day Saturday in the new post-1805 convention which is now aligned with Civil Day names.

The middle time-line is the conventional Civil Time convention.

The bottom time-line is Astronomical Time. Like Nautical Days, its day begins at Mer Pass but it's 12 hours later than Civil Time and a full 24 hours later than the pre-1805 Nautical/Sea convention.

All four time lines share the same vertical Meridian Passage references (vertical dashed lines). All four emphasize day-by-day boundaries and same days between systems have diagonal dashed lines. Day names are arbitray but correspond to the comments in the Nautical Day thread about Cook's voyages (see links below). Their only use here is to note how named days align in each system. You can see that Friday in the Astronomical covention could be Saturday in the pre-1805 Nautical system or Friday-Saturday in post-1805  Nautical system.  Gosh, could this be right?

Each of the four systems use a 2400 clock and AM PM marked though this is doubling up and unnecessary. You don't need both conventions to be clear.

There are two important footnotes from Frank's history of Nautical Almanacs.  The goal here is to know which Almanac time convention was in use and how day names and ephemeris times correspond to each of the above conventions.

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.

This is the first time I have tried to graphically see how these conventions work. There are likely big mistakes so I appreciate any comments you might have.

Ed Popko

A very useful thread on the Nautical Day for those interested. The links in chrono order:

Nautical Day thread, Feb 2004

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