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On Feb 28, 2005, at 6:34 AM, Jim Thompson wrote:

>> -----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
>
>

Sumner didn't use St. Hilaire's method, assuming one location and then
finding a line of position based on azimuth and distance from that
position.  Rather, he assumed two latitudes and found the longitudes
corresponding to those latitudes.  Then fitted a line of position
through those two locations.

Having the assumed latitude plus the sight, he determined longitude.
Assuming a second latitude gave him a second longitude, with the same
sight data.

Fred

   

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