NavList:

Tom K, you asked:
"in the moon bubble corr. with the naut. almanac  after using the u. & l. corr and -15' corr. do you also use the whole main alt correction."

I have to say, I'm not sure what you're asking here. Maybe, could you elaborate?? Some thoughts for now: The table in the back of the N.A. from which you get the UL/LL corrections is the main correction for the Moon. It incorporates refraction, parallax, varying SD (semi-diameter), the works. Of course if we shoot the altitude of the Moon's center, as with a bubble horizon (usually), then we want to "back out" the SD elements, and that's done by what amounts to "averaging" of the UL and LL corrections, and it's what's described in the somewhat "opaque" and "succinct" instructions for the table.

Just to emphasize, the Moon correction tables in the standard Nautical Almanac work just fine for bubble sextant observations. The tables are more accurate than required for bubble sextant sights, including, for example, the "augmentation", but that does no harm. The augmentation is just a portion of the Moon's SD that is altitude-dependent. For bubble sextant sights, the augmentation (amounting to a few tenths of a minute of arc), like the primary SD (on the order of 16 minutes of arc) cancels out when the UL and LL corrections are averaged.

Also, I fully agree with Gary LaPook and others who suggest that the Air Almanac tables are easier to understand and possibly easier to use. The Nautical Almanac Moon correction tables can be a little intimidating, but they do work and quite well, with excellent accuracy. But if you're happy with +/- one minute of arc in the results, the work can be simplified a bit. And it's also worth emphasizing that the whole process can be easily implemented on a calculator. Learning the steps on a calculator and following the computation of refraction, parallax in altitude, and semi-diameter from the base parallax, is a great way to gain confidence in the tables, too.

Frank Reed