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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Double Altitudes: Prelude to Sumner's line?
From: Jim Thompson
Date: 2005 Feb 28, 07:34 -0400
From: Jim Thompson
Date: 2005 Feb 28, 07:34 -0400
> -----Original Message----- > > on 2/11/05 7:22 AM, Jim Thompson at jim2@JIMTHOMPSON.NET wrote: > > Sumner opens his Introduction with this glorious single-sentence paragraph: > > "It is not so much the object of this work to present the navigator with a new method of 'Double Latitudes', as to afford him an accurate method of finding, by one Altitude of the Sun taken at any hour of the day, with the Chronometer time, the True Bearing of the Land, the Latitude, &c., being, from any cause, uncertain; and to place him on his guard, when near a dangerous coast..." On re-reading this paragraph, I think that I need to understand the significance of his phrase, "Double Latitudes". If calculating a second solution for a sight based on an assumed latitude was well known practice by 1837 for other purposes, then that would have been the building block that allowed Sumner to creatively apply the technique to his problem, and thus allow him to stumble on the celestial line of position concept when he found himself off Small's Light. I do not understand the concept of "Double Latitudes", however, so I might have that all wrong. I tried a google search, but turned up nothing on that technique. I found lots on "double altitude", but not "double latitude". Does anyone wish to explain the concept? Jim Thompson