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Hello Guy

I don't know the book you are taking your exercises
from, and so can only guess about the repeated
systematic error...

If these were real sights, there could indeed be a
sextant index error, and/or an operator sextant drum
reading error caused by using the wrong end of the
vernier scale as the zero mark. However most current
sextants have a vernier scale length of 4 minutes not
5 minutes.

As exercise sights, it may be that the exercise was
constructed to give better results than you achieved
at your first attempt. One way a repeated 5 minute
error could occur in the sight reductions was if the
exercise gave an index error of 2.5 minutes, but you
applied the index error correction in the wrong
direction - an oft repeated slip.

Sound navigation is more than just position fixing,
and uses self checking methods. The whole point of
using pairs of sights in opposition to each other is
to show up possible slips and/or systematic errors.
Similarly, the point of using three bearings rather
than two for a hand bearing compass 'fix' is to allow
the navigator to assess the size of some ( not all)
the possible systematic errors in the bearings.

We do not need fabulous accuracy, just a self checking
basis for our navigational decisions. That's exactly
what Geoffrey's analysis shows so well. I strongly
support the suggestion elsewhere that you get a copy
of Mary Blewitt's Celestial Navigation book - it's
small, cheap, timeless, succinct. Mary explicitly uses
the concept of the most probable position. Some books
do, many others don't. You seldom see any advice as to
what symbol to use for the most probable position. In
the UK tradition we use a triangle to surrond the dot
for a plotted estimated position: I personally use an
inverted triangle for 'most probable' astro positions
when plotting. It helps to remind me and others on the
yacht that nothing is certain out on the water.

Michael Bradley 55 01.2N 001 27W


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