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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Where on earth are lunar distances the same?
From: Jim Thompson
Date: 2005 Mar 14, 17:34 -0400
From: Jim Thompson
Date: 2005 Mar 14, 17:34 -0400
> -----Original Message----- > From: Ken Muldrew > > On 14 Mar 2005 at 7:09, Jim Thompson wrote: > > ...I cannot picture the set of places on the surface of the earth > > from where a navigator would find the same lunar distance for a given > > Moon-Body pair at a given instant of time. > > To which Ken Muldrew replied in part, > The true distance will be the same for everyone who can see the moon and > the other body (of course) > but the apparent distance will change from > point to point due to changes in refraction and parallax. Thanks Ken. Ken's "of course" makes me realize that I had missed the very obvious (which is why my side of a chess board empties so rapidly). Doh! The lunar-clock-in-the-sky is simpler than it seems: it is simply the same giant clock from anywhere on the earth, as long as it is visible. I wanted to picture the situation for the true (corrected) distance, not the apparent distance. I now see that the moon-body angular distance relative to the surface of the earth for a given body is *the same* at a given instant of time over the whole surface of the earth facing that pair, so that anyone who can see the pair at the same time would obtain the same cleared angle after correcting their observed lunar distance for altitude (parallax and refraction). Bruce Stark once wrote on Nav-L: "Until (1907) the Nautical Almanac gave pre-calculated comparing distances every third hour. Suppose you'd measured and cleared the distance between the moon and Regulus. You'd find, in the Almanac, the two tabulated distances of Regulus from the moon that your observed distance fit between. Then, proportioning change in time to agree with change in distance, you'd find what your watch would have read, had it been keeping Greenwich time, at the moment you measured your distance." In order for lunar distances to work, the cleared observed distance must be unique for an instant of Greenwich time, but it is not unique from anywhere on the surface of the earth. I went back to Frank's online calculator at http://www.clockwk.com/lunars/lunars_pre_v5.htmland and did more careful observations of the results. Clearly, changing lat and long for a given day does not change the lunar distance result. Jim Thompson With a lifeong history of many down pawns, knights, et cetera.