A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Jan 13, 19:42 -0800
Jim, you wrote:
"Error in Lunar: 0.3'
Approximate Error in Longitude 9.6' "
So that's great! And you rounded up to 84°57.8' on your distance in this case, while in your spreadsheet, you listed 57.75 for the minutes of arc plucked off your best-fit line. You could have rounded down to 57.7' and then your results would have been:
Error in Lunar: 0.2'
Approximate Error in Longitude 6.6'
Take the latter result, just for bragging rights! Ha ha. :) I'm going to use the 0.2' error in what follows below...
You then wrote:
"so, I'm not sure how that stacks up as an observation."
You're over-thinking this. The app is telling you --right there-- how it stacks up as an observation: error in lunar 0.2'. Your observed lunar observation was long by a mere 0.2 minutes of arc. Obviously that's very good. The "approx error in longitude" figure is nothing more than 30x the error in minutes of arc in the observed distance. It's just an informational number so that you can ponder how this would have affected your position historically. If your longitude was wrong by 6.6', would that have been good or bad in, let's say, 1799 in the mid-Pacific? Since you can see that far from the top of a rather short mast, clearly it would have been counted as an excellent determination of longitude back then.
And you added:
"And it appears to be telling me that GMT at that time was actually 18:05:46. But I also couldn't really tell someone how that could be.... poor shooting? I know it was 18:00:00 and I know the place. "
Where? How? Er... What the...? From what hat did you pull this number "18:05:46" and was there a rabbit in there, too! Heh. :) I'm kidding, but I'm really puzzled where you got that number 18:05:46. As you learned in the lunars workshop, every tenth of a minute of arc error is equivalent to 12 seconds error in GMT. So if your lunar is long by 0.2' when it should have given you 18:00:00 GMT, then the implied GMT is 18:00:24 (two tenths yields 24 seconds).
"And, more to the point of this whole manual recalculation exercise my LD calc was 84 38.1, Waaayyy off. So, I will start afresh tomorrow and check my work. Plenty of places to screw up decimal places etc. I better get this figured out soon... xyl would like the kitchen table back! Any suggestions or advice from you, Frank, or anyone else who is reading this saga, would be most welcome. "
None of us can help you debug your work unless you tell us what you got at each step. What do you have for the pre-cleared LD?? I don't think you've mentioned that yet. You already listed the altitude corrections, and I agreed that they sounded good. You'll also notice that my web app showed you values for the altitude corrections which were very close to the ones you have previously given. So that's good. There are only two likely places your work went wrong after that. You might have made a keypunch error working up CCm and CCs (the "corner cosines" for the Moon and Sun). What values did you get for those? Since the LD is near 90°, Q is insignificant --nothing to worry about there. The remaining step just multiplies the altitude corrections by the corresponding corner cosines and adds them onto the pre-cleared LD. You could easily swap a positive for a negative sign there, so it might be as simple as double-checking your work.
Oh and don't give up the kitchen table! I sincerely commend you for sticking at it. You're very close. And once you've done one... can you stop? Ya know... you gotta ask: do you really need a kitchen table more than you need a lunarian workstation?? Yes, the body requires sustenance but the mind demands nourishment! ;)