NavList:

ThomasK
With respect to this P in A correction, I might have confused you.  High altitude air navigators tend to fly very fast, perhaps 480kts. Therefore, to complete a fix before we fly past where we’re going, we tend to pre-calculate our celestial well in advance, (sometimes even on the ground before we set off.)  If we intend to fly at around 40,000ft, we can expect to be above cloud, so we can normally expect to shoot on time.  If we must shoot early or late, we move our position lines or assumed positions to allow for the time difference. Therefore, when we pre-calculate we apply some of our corrections with the sign changed, so we only need to compare our calculation with what our sextant reads to plot a line on our chart very quickly.  I'm home again now with my cribs, so to tie in with your original question, I’ve done the calculation again using horizontal parallax from the NavList Nautical Almanac for the date of the shot and the Moon Altitude Correction tables from the 2014 Nautical Almanac (the nearest I have).  You’ll note a slight difference from the Air Almanac value.  The Nautical Almanac value is probably the most accurate.

Incidentally, latest Mks of bubble sextants have the bubble replaced by a pendulous reference, so you could, if you wished, align the lower or upper limb with the horizontal part of the graticule and use the Altitude Correction Table from the NA directly for lower limb or -30’ for upper limb. Good luck DaveP

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