# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding

**Re: Lunar trouble, need help**

**From:**George Huxtable

**Date:**2008 Jun 22, 23:21 +0100

Thanks to Kent Nordstrom for spelling out the details of his lunar calculations. I think that between us we can now reconcile many of our differences. He wrote- "Firstly I hope we agree that it is the mean time for the LD observation that has to be used as reference for calculating altitudes of (in this case) the moon and the sun. The mean time for LD observation was 06-22-59 (type of time still not completely clear to me)." Yes, we agree. And the "type of time" is simply UT, which Jeremy took from his chronometer, for which we are imagining that the lunar distance is providing a cross-check. Because the lunar-distance almanac information that Kent used was based on UT, then the time derived from it is also UT. I think I can see why Kent and I disagree by a few minutes about our calculated Moon altitude. There's a transcription error crept in somehow. Jeremy's first Moon altitude was stated by him to be timed at 06h 20m 44s, but Kent has based his sums on a time of "06-20-13". If Kent recalculates, I suspect he will come much closer to my own figure. And then he writes, about the Moon corrections- "Refr -29,5m (George -0,083=4m 58,8s) It is not clear to me how George�s refraction has been calculated." Kent's refraction at an altitude of nearly 61�, is far too high, and I suggest it should perhaps have been that number of arc-seconds rather than arc-minutes. And the figure I gave for refraction at that angle wasn't the -.083� quoted by Kent, but -.0083�, which is about right for that altitude. Putting those matters right, I wonder if Kent and I can now agree about all the numbers that apply to the Moon. ========================= About Sun corrections, Kent wrote- "For the sun it seems that we have reached the same altitude, anyway I don�t understand George�s data on refraction (seems to be the same as for the moon). My refraction for the sun is -1m 21,02s." Indeed, we agree completely about that, and I'm sorry to have made a transcription error that made the Sun refraction appear so confusing. I had actually done the calculation on another sheet of paper, in which my Sun refraction worked out to be about the same as Kent's value, as can be seen by subtracting the relevant numbers- "32.9564 alt above true horizon -.0083 refraction -------- 32.9314" the difference being 0.025�, or 1.5 arc-minutes, not .0083�. But then, transcribing the value from that sheet into my email page, in error I copied the Moon's refraction rather than the Sun's, which is why the email wrongly read "-.0083 refraction" for the Sun. At an altitude of 32.9564�, my Almanac table gives me a value of -1.5', which is what I actually used in the correction, and this explains why we agree over the final result for the Sun. I don't quite see where Kent gets his refraction of -1m 21.02s from, but that isn't a big disagreement, between us. Sorry about adding to the confusion, though. ============================= Having done all that, we can arrive at a corrected lunar distance, and from that the UT at which the observation was made. Kent used a value of observed lunar distance that differed by more than 30' from mine, because he allowed for the Sun semidiameter by adding it, rather than subtracting, as the peculiar circumstances of Jeremy's observation seemed to call for. As a result, he arrived at a GMT that was over an hour late on the chronometer . He commented- "GMT 07-26-52,9 (George 06-24-53). This difference depends of course on how the LD�s were measured. However the method for measurement by �overlapping� edges does not convince me. It seems to be a construction afterwards." Yes, of course. It's a pragmatic attempt to allow, retrospectively, for how the observation must have been made. What alternative explanation does Kent have to offer? Of course, if Kent won't make that allowance, then an enormous error of over an hour in GMT will arise, and a longitude error of 15� or so; the very discrepancy that prompted Jeremy's call for help. ============================== But next, the bit that really puzzles me about Kent's treatment is how he manages to invoke sidereal time. That would come in only if he was getting his astronomical positions from an Astronomer's almanac that gave sky-positions in terms of right-ascension, rather than hour-angle. Once the clock has been set to a GMT that's derived from the lunar (and only differs by a couple of minutes from the on-board chronometer) then it's a simple matter to get the sky position at the relevant moment, from the Almanac, for Sun and/or Moon, then work out and plot the relevant intercept from an assumed position. Each body will then provide a good position line for longitude, one being roughly due East, the other being roughly due West, but very poor information about latitude. It's a standard procedure of latitude-by-chronometer, and has little to to with the lunar distance measurement itself. Because of the discrepancy, between the chronometer and the lunar observation, of just less than 2 minutes of time, there will be a difference of just less than 30' between the longitudes calculated by the two methods. George. contact George Huxtable at george---.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---