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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Real accuracy of the method of lunar distances
From: Fred Hebard
Date: 2004 Jan 12, 00:09 -0500
From: Fred Hebard
Date: 2004 Jan 12, 00:09 -0500
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.
