NavList:

Gary LaPook, you wrote:
"The semiduration graphs have been in the Air almanac for ever"

Some years ago HMNAO ceased publication of their version of the complete Air Almanac while the US version continued. There was a truncated version of the UK "Air Almanac" which was released by HMNAO staff as a sort of "public service" that they believed in and were willing to support off the books. This truncated version was made available as a pdf download for a few years, and occasionally NavList readers got the impression that this was the actual Air Almanac, complete with ephemeris data. Instead it included only visibility and planning data, with graphs like the ones you've posted, right? Now that the US Air Almanac (the only remaining one with ephemeris data?) is made available as a free download, perhaps this has made it easier for HMNAO to include the former data and text from their truncated "Air Almanac" directly in the Nautical Almanac? How do the explanation pages compare? Are the explanations in the truncated UK "Air Almanac" (the one which was published online for some years) comparable to the printed explanations in the new section in the printed Nautical Almanac?

Regardless of the source, it's a smart business decision for them since it helps stop old almanacs from competing with new publication almanacs. For many years navigators have understood that the permanent pages are sufficiently permanent, despite small (and irrelevant in all practical application) modifications to refraction tables, that they can be used for education and training and even in practice on a short voyage as long as an old almanac is supplemented by a few pages copied/printed from a current year's almanac. If HMNAO adds more new content, the new almanacs become more desirable.

Frank Reed