NavList:

As to the declared purpose, the preface to the almanac says explicitly that the NA is produced "to the general requirements of the Royal Navy and the United States Navy...In times when global navigation satellite systems may be unavailable, due to natural interference, artificial interference, or a failure or receivers, The Nautical Almanac remains an independent, primary backup for determining the navigator's position at sea"

As for why not more than one year at a time, I can only speculate, although it does say in the almanac that it can be used in the following year with a few corrections (p261 of the 2017 govt edition). Maybe Navy expects vessels to make it to a US port at least once every year or two years to pick up newer almanacs if needed.

Maybe in the old days, the process of calculating, verifying, proofing, and printing took so long it was only practical one year at a time. In contemporary times, it could be they expect the underlying models they use to change occasionally (e.g. switching JPL models of the solar system) and they want to be able to make edits to other portions as well, such as when the NAO/NASR sight reduction tables were added. 

On Thu, Sep 14, 2017 at 9:08 AM, Herbert Prinz <NoReply_HerbertPrinz@fer3.com> wrote:

I agree with Alexandre. His answer is very clever, because it depends on the definition of "predictable".

Frank's puzzle raises the question for whom the NA is designed and made. Obviously not for the small boat cruiser, whom it may take two or three years to complete a circumnavigation, but, who also does not need (leap-) second precision. So, what is its declared purpose? This question is my puzzle.

Herbert