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Jeremy has properly questioned an assertion I made in [9435] about the moon
data-set he provided in [9359]

I wrote-
"The deduced scatter, of one standard deviation about the fitted trendline,
I now make to be 0.53 arc-min in the case of the "Moon near LAM" set, and
0.38 arc-min in the case of the "Moon away from LAM" data set. That's no
better, and no worse, than one would expect from observations at sea from a
large vessel."

He commented-
"I wonder on what basis this statement is made.  I am not claiming to  be
any better or worse, than any other practiced navigator, but how can we know
"somewhere around 0.5 arc-min is what "...one would expect from observations
at  sea from a large vessel.  Is there some collection of data that would
back  this up?  I am just wondering how we can expect this kind of deviation
from  large ships as opposed to small vessels unless there has been some
sort of study  on ships of various sizes and under different observing
conditions or a review  of a variety of navigational logs.
Can any other navigator on this list give data that would support this kind
of statement, even if the data isn't recent?"

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

That's a fair question. I should explain that I had previously stated, in
[9412], that-

"A least-squares fit to the non-culminating series arrives at a rms scatter
of little more that 0.1 arc-minute, and of the series aroundculmination of
about 0.15 arc-min. ....  they show what can be achieved by a careful
observer on a large vessel in, presumably, benign conditions."

I had indeed been rather surprised by that conclusion; as well I might,
seeing that it had resulted from a silly spreadsheet error, as I pointed out
later (I had put a comma in a formula, where it should have been a colon.).
I expect those low values for rms scatter may well have surprised Jeremy,
also.

However, with that error corrected, the revised scatter became about four
times greater, and my comment about that was really intended to say no more
than- "now, that looks more like a reasonable value". Which it does, as I
suspect Jeremy would agree. Perhaps it would have been wiser to say no more
than that. I can offer no direct evidence of what would be a reasonable
range of horizon scatter, from a bridge over 100ft high in various
conditions. Jeremy's opinion would be more valuable than mine. In what range
would his own estimates lie, in various conditions, I wonder?

I agree with almost every word of Jeremy's statement-

"The reason I bring this up is that as I look through my recent navigational
log, I notice that the most critical aspect of shooting a star is seemingly
never mentioned (at least far less than sea state, large ships, and
anomalies in  dip).  This factor is the quality of the visible horizon!
The horizon varies with time, azimuth, and circumstance every time I  shoot.
I have shot with a crisp horizon in one direction, and a fuzzy  horizon in
another quadrant at essentially the same time.  The  horizon's quality at a
given azimuth, affect my sights far more than any other  shooting condition.
The quality of the horizon greatly changes the  accuracy of my star fixes
which is the only measure I truly care about."

Qualty of horizon is indeed crucial, and often neglected. I remember
suggesting to Jeremy, in one posting some time ago, to improve on a hazy
horizon by following  Lecky's advice and descend from his lofty bridge, with
its 12-mile horizon distance, to a lower deck level, from which the horizon
would get much closer and sharper. Lecky would occasionally observe by going
over the side to the foot of the accommodation ladder, which is rather more
than I would expect Jeremy's Captain to allow.

When we compare altitude observations from big ships and small boats, vessel
size does much more than reducing the amount the observer gets thrown about.
It has two major effects on the scatter of the horizon.

From my own cockpit, the horizon is only 3 miles away, so it's much less
often spoiled by haze than would happen to Jeremy. In that respect, the
small vessel has the advantage.

On the other hand, when surrounded by waves and swell that can be greater
than the observer's height-of-eye, and affected by heave to a similar
extent, a small-boat navigator can find himself looking sometimes up at the
wave-peaks that surround him, sometimes down. That can be minimised, to an
extent, by sighting to surrounding wave-peaks when the craft is at the top
of its heave, but it's still a chancy business in rough conditions. By
contrast, Jeremy will always see a horizon that might have been drawn with a
ruler. Even though his ship may roll, that won't affect altitudes
significantly, and the effect of any heave will be negligible. In rough
conditions, all the advantages lie with the big-ship.

He ended "Given the nearly endless variations of observing circumstances for
any sight using the visible horizon rather than a bubble or other artificial
horizon, I find it hard to justify stating in all but broad terms what
magnitude of scatter we can expect from a given type of vessel."

I agree.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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