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Alex Eremenko asked about the precision of sextants and sextant measurements.

This is a matter which surfaces from time to time on Nav-L, but hasn't come
up for a little while. And now we have several new and active members, so
perhaps it's a good time to revive it, and exchange some prejudices around
the list, which may have been rehearsed before. Here are my own views, with
apologies to the list members who have seen them before; they can turn away
now.

I speak, not as an ocean navigator, but as a small-boat sailor who is never
out of sight of land for more than a day or two at a time; so my life has
never depended on my astro observations, though I have made quite a few of
them in my time.

Sextant observations could be divided into two categories; those that
involve the horizon, and those that don't.

=================

Let's consider first those that don't. They comprise measurement of lunar
distances, and altitudes measured on-land using an artificial horizon.
Because the sea-horizon, with all its defects, isn't involved, the
observation is capable of high accuracy. For a lunar, in particular, high
accuracy is called for, because the resulting error in longitude, in
minutes, is roughly 30 times the error in measuring lunar distance, in
minutes. That is why the brass Vernier sextant was developed, in the
mid-1700s, and later the micrometer sextant, in the 1900s. In capable
hands, it became an instrument capable of reading to about 0.1', and
calibrated to a corresponding precision.

These were superb instruments, many of them; the ultimate expression of the
instrument-maker's art and skill. They were cossetted, treasured, and
prized; rightly so. They became the "badge of office" of the real
navigator; a totem of his craft.

============

For altitudes above the sea-horizon, the resulting error in position is
only one mile for every minute of error in altitude. Few mariners ask for,
or really need, better accuracy than a few miles: an exception would be for
marine surveyors. For ordinary navigation, the wooden "Hadley" quadrant was
quite adequate, when fitted with a vernier, as long as its index error was
checked at regular intervals. If lunar distances had never appeared on the
scene, I suggest that such simple instruments might have been acceptable,
even to this day.

The problem with all altitude measurements, up from the horizon, is the
quality of the horizon itself as an indicator of the horizontal, especially
on a small boat.

To start with, the horizon, except under calm conditions, consists of a
series of wave-peaks or swell-peaks. When observed from a small vessel,
heaving up and down on those same waves, the best it's possible to do is to
time an observation when the boat is on the top of a similar wave, so as to
cancel these elevations to some extent, but it's a chancy business. Who can
estimate the effective distance, and the angle, of the horizon in such
conditions?

Even in still conditions, there's the problem of anomalous dip. That's the
effect caused by refraction due to temperature gradients within a few feet
of the sea, which causes the dip to deviate (one way or the other) from its
nominal predicted value, by (commonly) 1', (occasionally) 2', (rarely) 3'
or more, depending on local atmospherics. There's no way of knowing whether
anomalous dip is perturbing the horizon, without special instruments.
Applying corrections to the refraction for non-standard atmosphere doesn't
help in this case.

So (for those that don't take lunars) is there any point in using an
instrument capable of measuring to 0.1', for measuring an altitude, above a
horizon that's only determined to 1' or so?

And yet, on this list, the finer points of calibration of these expensive
and exotic instruments, ancient and modern, are discussed in detail, by
many navigators who will never use them for anything but an altitude above
the horizon. There's another list which is devoted to sextants and nothing
else. I suspect that many of these instruments are treated as trophies, as
possessions to cherish, or even as investments, rather than as tools to do
a job.

I use a plastic sextant, which has, for me, acceptable accuracy appropriate
to the rough measurements I can make on my 26-foot boat. If I drop it, it
bounces. It has defects, particularly in the optics, which I put up with.
It wouldn't do for lunars, though, not by a long way.

Don't get me wrong; I would love to own a precise brass sextant (preferably
Vernier type). But if I had one, it would live on my mantelpiece, and the
plastic one would remain in use on my boat.

If Alex is planning to measure lunar distances, then he is asking all the
right questions. But if he plans to navigate a small boat by measuring
altitudes, then any old sextant will do that job as well as any other.

Those are some of my prejudices, anyway.

I hope Alex will go ahead with his checks on sextant accuracy, and inform
us of his results.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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