# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Working a lunar. was: lunar distance in Wikipedia**

**From:**George Huxtable

**Date:**2007 Sep 19, 16:32 +0100

I've changed the threadname as this discussion is no longer related to Wikipedia. Henry Halboth's contributions are always of interest, and his observations in the past have shown that he still retains a remarkably keen eye, and a steady hand. He wrote- "Regarding the time necessary to calculate the Lunar Distance problem: I recently had the opportunity, with a sea horizon available, to observe a Moon-Jupiter near limb distance, as well as the requisite altitudes for clearing the distance and calculating the Longitude. I used Borda's method, which is pure spheical trigonometry, to clear the distance, and the Longitude Time Sight to subsequently calculate the Longitude. All was done as it would have been done in the early 1800's, employing six place logarithmic tables and all corrections applicable per Norie's Tables. The True Distance at Greenwich was taken from currently published 3-hour interval tables and interpolated by proportional logarithms. The calculation time to complete this work, exclusive of observation time, but including the Longitude work-up was 22-minutes - and I'm a bit rusty. Claims that this problem required hours to solve are simply myths." Response from George- Yes. As Frank has pointed out, it was true that it took hours, before the Nautical Almanac was published, but the precalculation of Lunar distances, from 1767 onwards, bypassed most of that work. For doing the job using precomputed lunar distances, with logs, 22 minutes is pretty good going, and shows that Henry's brain, as well as eye and hand, is as good as ever. But I wonder, did that answer come out right,and within 22 minutes, at his first attempt? Or did he have to make a few tries at it, with some false-starts, as I usually have to do, and correcting arithmetic errors, before eventually getting things right? For mariners of the lunar era, repeating the same calculations day after day, it would all become a matter of routine, involving little thought; but for us today , it's rather different. Before 1767, lunars were indeed taken, and worked out, using the published motion tables of the Moon (by Mayer, and others). But that was a really involved and longwinded matter, and usually called, not for a mariner, but a seagoing astronomer, such as LaCaille, or Maskelyne (see his British Mariners Guide, 1763). Henry continued- When I get the opportunity, it is my intent to publish these observations on the List. However, as the accuracy of Longitude obtained is really astounding, I am double checking everything before giving George and Frank a go at it. ========================= Well, we know already how good Henry's observations can be. But I wish to sound a note of caution here, without wishing to detract in any way from any claim that Henry may make. What follows is pretty self-evident. Whenever you observe some quantity, in which random errors are involved, even after all systematic errors have been corrected out, there will always be some scatter. And the scatter will then centre on the true value of the result. Indeed, the exact true value is the most likely answer. If, just by chance, you happen to hit a result which you know, by other means, to be the exactly correct one, that doesn't tell you much about the errors involved. You can only tell that from a number of different measurements, and from the scatter in those measurements. Perhaps Henry has taken several observations, and found that they average out to the exact longitude he expects. That's creditable, and worthwhile, but it's the scatter in the individual values that tells us more about the precision of the method. Contrarily, however, if instead of an agreement, there's a discrepancy with the known value, that tells us more; it tells us that the inherent errors in the method are likely to be at least of the same order as that discrepancy. I look forward to Henry's account of his astounding accuracy with great interest. By the way, observing lunars with Jupiter is a bit different to observing stars, in that Jupiter has an observable disc when seen through the telescope, with a semidiameter of around 20 arc-seconds. I've never tried a lunar using Jupiter. Presumably, the way to do it is to put the centre of Jupiter's disc against the edge of the Moon, not align them edge-to-edge. In that way, no allowance needs to be made for Jupiter's semidiameter. Is that the way Henry did it, and is that what the lunar distance tables that he used presume? What were those tables, by the way? 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. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---