# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Double Altitudes: Prelude to Sumner's line?
From: Fred Hebard
Date: 2005 Feb 28, 17:08 -0500

```On Feb 28, 2005, at 4:24 PM, Jim Thompson wrote:

> Thanks Fred.  Herbert Prinz emailed me offline to suggest that perhaps
> mis-typed "latitude" for "altitude".  He is correct: Sumner's original
> wording was, "Double Altitudes".
>
> I did find some google postings about "double altitudes", but I remain
> unclear about the purpose of that method, and how it might have given
> Sumner
> the idea to rework a sight using a second, theoretical latitude, which
> eventually caused him to realize that such points lay on a line of
> equal
> altitudes.  So my question now is, what is (was, more correctly), the
> meaning of "double altitudes"?
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> --------------------
>

Jim,

I don't know that double altitudes would have given Sumner the idea,
other than he was hampered in only having that single sight.  The
introduction to his work gives his explanation of how he discovered the
line of position.  The new method of double altitudes in the text _may_
refer to taking two sights of the sun at different times and getting a
resultant running fix using two lines of position, and he is
contrasting that to using a single sight to get a line of position,
trying to emphasize the line of position.  Here's the text for
reference:

>> "It is not so much the object of this work to present the navigator
>> with
a new method of 'Double Altitudes', 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..."

I don't know what the old method of 'double altitudes' may have been.
Is it referred to in an early Bowditch or some such?

```
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