# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Moon altitude corrections and artificial horizons
From: Tibor Miseta
Date: 2021 Nov 20, 15:08 -0800

Rod, I try to clear some points.

The NA Moon correction table has two parts to be added: the first part is calculated with a choosen fix HP, and the second part contains all the HP-related corrections, to be more precise the deviations from the choosen fixed HP corrections. The HP contraction due to latitude is not included in this table (there is no Latitude argument), the whole table is calculated for an assumed fix latitude (around Lat 40° if I remember well, should be checked in the explanatory pages). All other corrections that you have listed are included.

The LL corrections in the second part are always positive (because SD is to be added, and it is big enough to keep the sum of all other corrections in the positive range). But for UL the corrections in the second part would be negative. It would be very inconvenient to subtract from the first part, so they simply added 30' to them. This 30' has nothing to do with the SD, it is just a convenient correction to avoid negative numbers! You add the UL correction and subtract the 30'. I used to subtract the 30' from the first part before adding the UL correction from the second part, it's more convenient.

With bubble artificial horizon (bubble sextant) one usually measures the center of the Moon. For the center, the average of the LL and UL corrections should be used. Considering the above: (LL + UL -30)/2 = (LL+UL)/2-15. This 15 again has nothing to do with the semidiameter, it is just the halve of the 30! (Very sily and inconvenient, for bubble sextant use the correction table from the Air Almanac instead.)

With mirror artificial horizon one measures the limb (either upper or lower), so the table should be used as normal. Happ=(Hs+IC)/2, ignore dip, then use the table as you would use it with a sea horizon. Don't average the UL and LL corrections: averaging is only for observing the center!

Happy full Moon! :-D

Tibor

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