A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Rod Brown
Date: 2021 Nov 20, 05:20 -0000
I have been addressing the matter of reducing a moon sight making use of the Nautical Almanac and the associated sight reduction tables. The method using a moon limb sight and a natural horizon is understood. Basically, After taking out index error and dip, the apparent altitude and horizontal parallax are used to obtain two further corrections which are added to the apparent altitude. A nominal 30 minutes is subtracted if an upper limb sight is taken. The end result is true altitude and one goes on from there.
Is there a lucid explanation somewhere by some knowledgeable person as to what the two altitude correction components consist of and what rationale is used in determining the split between the two components?
The inputs should include refraction (subtractive), parallax in altitude (additive), HP contraction due to latitude (subtractive), semi-diameter augmentation due to altitude (additive) and semi-diameter (additive). If a nominal 30 minutes is taken off when an upper limb sight is involved then the semi-diameter must comprise 15 minutes and a delta. It seems to me that the first correction includes most, if not all, of the parallax in altitude component and it probably includes a nominal semi-diameter of 15 minutes (but I am not sure on this).
When it comes to reducing a moon sight obtained by using an artificial horizon using this method I haven’t been able to find a worked example. The text box in the moon altitude correction table in the Nautical Almanac advises averaging the lower and upper limb corrections in the HP table and subtracting 15 minutes should a bubble level be used. I have read elsewhere that this should be done when using an artificial horizon. My interpretation then is that once the apparent altitude has been got by halving the measured sextant altitude (corrected beforehand for index error), one extracts the first altitude correction and adds it to the apparent altitude in the same way as before and then adds the averaged correction as the second correction, and deducts 15 minutes. That makes sense to me because parallax in altitude has to be accommodated and this time the sight is concerned with the centre of the moon’s disc. I’d like to confirm if my outline is correct. Has anyone seen a good worked example using the method above? I think authors tend to bypass the moon case I have described. This is possibly because the sun sight case is easier to cover since parallax is ignored and index error and refraction are the only other corrections.
Any advice would be appreciated. I am aware of other techniques but would like to know the answer on this one for completeness. Thanks in anticipation.