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In principle using celestial navigation at sea one is looking for an accuracy of
around half a mile but one mile is usually sufficient.

In practice it is a question of getting comfortable with your sextant and your
method of recording the results.   For the latter an assistant with a pencil and
a deck watch is ideal.   For the former much depends on the conditions at the
time of observation but one has the feeling that certain sights are good and
others a bit doubtful.   Provided you mix in plenty of star observations (where
you can get several position lines in one go) you should get a working routine
in about two days.

On a longer passage each day is a near repeat of the day before so one soon gets
into the routine of identification and setting up the sextant for rapid
observation.

The book implies that you should not use the moon's horizon for celestial
observations.   Directly under the moon there can be a false horizon.   In
practice one can sweep from side to side and the position of the true horizon
becomes apparent.    On many occasions when I have missed the sights at twilight
I have obtained a good position usin the moon's horizon.

There are methods by which you can pre-compute the altitudes of the celestial
bodies to be observed and thereby verify the accuracy of your sights as you
proceed.   However I usually look for an accuracy of 0.1', I am happy with an
accuracy of 0.5' and accept that sometimes I do get errors of more than 1.0'.

Philip Ouvry

   

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