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Thankyou George Huxtable for your comments.

I think you have put your finger on the nub of my question. The refraction
from the horizon is considerable and accounts for about 20% or so of the
height-of-eye or dip correction. So while the height-of-eye may be greater
on a supertanker than for a yacht bobbing about in the waves, leading to a
flatter horizon, the corollary of this is that the refraction from the
horizon is greater for the supertanker, leading to a greater potential for
error due to abnormal refraction from the horizon.

For small vessels then, the principle error may be the uncertainty of the
horizon due to the fact that it is not flat and that the height-of-eye may
not be constant (due to bobbing about in the waves!) For large vessels,
height-of-eye is not a problem, but as the horizon is now further away,
problems of abnormal refraction could start to dominate, particularly where
there is a large difference between the water temperature and the air
temperature, and/or where the air is still. Too, the horizon may be
indistinct for any number of reasons.

I should come clean and say that I have very little personal experience of
using a sextant for navigation at sea. My original question, regarding the
uncertainty you would place on an altitude reading using a marine sextant
at sea, was really aimed at the old sea salts who have used a sextant to
keep themselves found at sea and came to know from experience just how much
precision they could rely on from their sightings.

So, the sextant is a Plath and it comes with a certificate saying the arc
is good to 6" of arc. You take what seems to be a perfect sighting of a
body on a clear day and make the necessary correction for dip and
refraction and come up with an observed altitude of, say 55 degrees 26.3
minutes. Do you say, "Right, that is what it is." Or, as an old sea salt do
you say, "That's all very well, but in my experience, the actual altitude
may be up to x' either side of that, so I will just keep that in mind when
plotting my position."

The question to the old sea salts is, how big is x?

Thanks again,

Geoffrey Kolbe

   

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