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The discussion about averaging data has been very interesting and for me the
math has been quite a challenge to follow. From a practical point of view I
have quite a different take on the whole process. I use the sextant as a
tool to provide needed information. If the weather is good you don�t need
the same degree of accuracy in navigation that you need in times of reduced
visibility or extreme conditions and those are the times that celestial
navigation is at its worst. During the good times you have many other tools
that you can use to confirm your position and landfall. The simplest one is
just looking up from the chart table and seeing land. Under those conditions
even a poor sight will tell you if the harbor is above or below your
landfall.

A landfall under poor conditions is the worst situation that the navigator
will ever find himself in. If you can only get a few quick shots though the
overcast you need to get a feel for the validity of the sight without the
luxury of time and averaging data that you just don�t have. With enough
practice you can develop a feel for if the shot is worth anything and how
far out it might be. You will of course have your taffrail log over the side
to use as a check on the reasonableness of the position you get from the
sight. The currents and leeway are very constant things and easier to allow
for then the unknown of the sight. The angle that the taffrail log line
makes with your course is the leeway that you are making though the water.
The currents offshore are well known and covered in great detail by several
publications. A DR that is maintained with care can be very accurate over a
72-hour period. The last 48 to 72 hours is the most critical time period in
a single-handed trip. If you are off in estimating the landfall and need to
stay awake for more then 72 hours you are almost certainly going to get
yourself into trouble.

A more important point then the accuracy of the data itself is that I think
you need to select the stars so that the LOP will give you the most useful
information and if you select the right pair of stars the errors of the
instrument and most other errors are canceled out. My plan has always been
to take sights so that the LOP�s will give me the distance from the coast
with the least amount of doubt. A LOP that is parallel to my course is not
as useful as one that is parallel to the coastline. An early shot of the sun
will give you a better approximation of your time of landfall then a noon
sight regardless of the errors involved. Of course in better conditions you
can get as many shots as you want, but you might see land before you have
done all the calculations. Navigation to me is not the goal itself but is
the means to an end.

My expectation in using a sextant is to get a position at landfall within 3
to 4 miles, which is close enough to make the transition from offshore
navigation to coastwise navigation. When I am on soundings the sextant is
occasionally used to get a LOP that will give me a position along the
coastline where landmarks are scarce. There the accuracy is not very
important to me because you are just getting a guideline as to the distance
traveled between stops. The water depth will usually provide a better guide
to the distance offshore then the sextant in those situations.
All the best,
Robert Gainer

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