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Celestial navigation as currently practiced relies on two concepts:
1. The celestial line of position, developed by Sumner, ca. 1830
2. The altitude-intercept method, developed by St. Hilaire, ca. 1870
We compute azimuth and intercept for each sight, then plot lines of
position from our DR or assumed position, and where the lines of
position come together is our fix. Sight reduction can be via various
tables or by calculator/computer using the Law of Cosines. We can think
of these methods as "indirect sight reduction": we get our latitude and
longitude indirectly by first computing azimuths and intercepts.
"Direct sight reduction" means we compute latitude and longitude
directly without computing azimuths or intercepts, without using DR or
AP, and without any plotting. A GPS does direct "sight" reduction.
Direct sight reduction has not been widely used to date because it has
been considered "too hard". It appears that this may no longer be the
case.
The newest issue of Ocean Navigator (September/October) has an extremely
interesting article on page 110 about a new direct sight reduction
method being developed by Tony DiCambio and a company called Triangle
Navigation, Inc., of Rye, NY. They call it the Evening Star Astronomic
Ephemeris (ESAE) system of Direct Sight Reduction. The company has
apparently been working on the method for 8 years and intends to sell
computer programs using the method for a profit. So far, they have a
version that works only for the sun, runs on MS-DOS, and sells for
$240. They are working on versions that work for stars and that also run
on Windows and Macintosh. The programs have built-in almanacs.
The idea is to harness the power of a personal computer to grind out the
arithmetic. They claim that you can get an accurate fix from two sun
sights taken 5 minutes apart. Ocean Navigator's tests confirmed that
claim. Applying the method to stars, you would enter two star sights
taken a few minutes apart and get out your lat/lon at the time of the
second sight, without any need to adjust for the difference in time
between the two sights.
The method works by solving 3 celestial triangles in succession:
Triangle	Vertices
--------	--------
1		Star 1, Star 2, and the observer's zenith
2		Star 1, Star 2, and the elevated pole
3		Star 2, the elevated pole, and the observer's zenith
The article (by Andrew Howe) omits some details, which the company deems
"proprietary". Aparently the algorithm has a number of branches
depending on the relative positions of the stars (east or west of the
observer, north or south declination), which adds to the
complexity. Also not explained is how the method allows for the
difference in time and the motion of the observer between observations.
As mentioned, the programs being developed by Triangle Navigation
include built-in almanacs. It would be interesting to see how well the
method could work using look-ups from a paper almanac and doing
calculations with a programmable calculator.
For more information, please see the article in Ocean Navigator, or
contact Triangle Navigation at (914)-381-9210, triangle@XXX.XXX or
http://home.sprynet.com/sprynet/triangle/. Triangle will happily send
you a reprint of the article.
Chuck Taylor
Everett, WA, USA
ctaylor@XXX.XXX
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