NavList:

Bruce, I think you may have missed my reply to your message yesterday.

I didn't see where you were getting "30 minutes" from yesterday, but it's clear from your latest post. You're following the directions in the table in the back of the Nautical Almanac. So that's good!

You report an error of:
"1.5 nm when I used my latitude to calculate longitude via a "time sight". I expected to be closer."

Yes, you should expect much better results so, as you suspect, something has probably gone wrong, though it's not a huge problem.

You wondered:
"I guess I should have subtracted 2 times the SD in the nautical almanac or 2x32.1 on June 20,2013? Question: Why does the NA say subtract 30 minutes when the SD for moon is always larger than 15.0? What am I missing here?"

No, that "30 minutes" is not related to the SD on the date. Rather that's a tabulation trick --always exactly 30.00 minutes of arc. If we tabulated a correction for the Moon's Upper Limb directly, it would range from about +40' (depending on the exact value of the HP and the refraction) to about -17'. But it's a nuisance to have positive and negative values in one table. So they "fix" it and tabulate all the UL numbers with 30 added in. Then when you've looked up the right correction in the two parts of the table for a UL sight, you subtract 30, which is easy in arithmetic terms, to get rid of the "fix". The table itself is accurate today or six months from now or ten years from now, if you use it correctly.

I don't know how you were working this all up as a "time sight". Could you elaborate? There's a simple way to check your work as I described in my reply yesterday. Assuming no blunder with your theodolite, you certainly should be able to get position data from that Moon sight accurate to within a small fraction of a nautical mile, certainly less than a quarter of a nautical mile.

-FER

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