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+--- Thread: Moon alt correction with AH (/showthread.php?tid=103)
Moon alt correction with AH - BigBill - 04-05-2017
Can I assume the procedure is the same as the bubble sextant when using an AH?
RE: Moon alt correction with AH - Rdutton - 04-05-2017
BigBill, I think you are correct. However, what has me puzzled is the Moon Altitude correction table tells you to take the average of the upper and lower limb shots and then subtract 0° 15' which I assume is a rough figure for the Moon's semi-diameter.
A good way to test this idea is to take a series of sights of the moon spacing them over a period of a few hours. Then use the Moon Altitude correction tables and apply it to the Hs (plus or minus IE). The result will be Ho as I'm sure you know already. But when comparing the resulting Ho to Hc you'll get a series of plots that, if the above assumptions are correct, will give you a good series of LOPs and hopefully a good fix of a known Longitude and Latitude (your deck). If the LOPs appear to be off by the SD of the moon or the 15 minute subtraction they tell you to use then you'll know, to correct the previous sights, you'll have to subtract the 15'.
Shooting the moon over the ocean is easy. Shooting the Moon using an AH is easy but I've never used a bubble sextant. Regardless, my corrections for a Moon sight Lower Limb are as follows when using an AH;
Ha= Hs -/+ IE
Ho= Ha + Moon Correction
Roland
RE: Moon alt correction with AH - BigBill - 04-05-2017
Thanks Roland,
I was just doing something similar the other day with some old moon sights I took a couple of years back. What was interesting was that I didn't recall the lat/ long where I had taken them so I used two different whole degree intersections for my AP that I thought to be closest. I'd already subtracted the 15' and all the LOPs plotted pretty close, the main error was IC* that I hadn't quite worked out at the time. Once I plugged that in everything fell into place and came quite close to the actual location. Using the 2 different APs meant that some of the LOPs were plotted towards and some away so once I adjusted the amount of error and corrected for it, that moved them all in the right directions.
I don't think I've yet sighted the moon using an AH, that will be on the next clear, warm night with the Davis Mk15.
*When I first got the bubble sextant the mirror arm was loose on the axle. After a number of adjustments it's still off but gives a usable reading.
Bill
RE: Moon alt correction with AH - Rdutton - 04-05-2017
Bill,
What's nice about the St.Hilaire method is that no matter which Ap Longitude or Latitude you use you will get the LOP where it's supposed to be.
Roland
RE: Moon alt correction with AH - CelNav57 - 04-06-2017
Bill,
Bowditch Chapter 20- SIGHT REDUCTION provides information concerning a Moon sight reduction.
Here it is- 2008. Reducing a Moon Sight
CelNav57