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Re: Real accuracy of the method of lunar distances
From: George Huxtable
Date: 2004 Jan 12, 10:46 +0000
From: George Huxtable
Date: 2004 Jan 12, 10:46 +0000
Fred Hebard wrote- >Having been prodded by Frank Reed, I previously wrote that I would >"poke around in Chauvenet to see whether I can find reference to the >value of taking lunars in both directions." > >This is the subject of article 256 of the 5th Edition, in the section >on determining longitude by lunar distances. Doing so cancels out the >the index error. If the distances are similar it also cancels out >errors due to eccentricity in the arc. > >Chauvenet is presuming here that either the horizon mirror or the index >mirror will always be pointed toward the moon, usually the index >mirror, so that the direction of the sextant will be reversed between >the two classes of distances. > >He said regarding series of sun distances that it is best to average >the leading from the trailing ones separately. I no not differ at all from the main thrust of Fred's posting, just with his third paragraph- >Chauvenet is presuming here that either the horizon mirror or the index >mirror will always be pointed toward the moon, usually the index >mirror, so that the direction of the sextant will be reversed between >the two classes of distances. Chauvenet doesn't specifically say that, and why should he presume it? Does it matter? Surely, reversing a sextant shouldn't affect its reading at all, nor its index correction. Well, there may perhaps be some slight gravity-distortion of the framing, I suppose, and it may be helpful to use exactly the same combination of shades. Isn't the aim to keep the angular errors exactly the same in amount, when observing a lunar distance to the East of the Moon, and another to the West, so that their net effect on the averaged longitude cancels? George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================