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Bill and George,

1. I checked the IC from more remote buildings as well.
IC is zero.
2. On anomalous dip. As I said the dip correction was
-2.8 in most cases, in two observations it was -2.2
and -3.5.
Suggestion that the -3.5 error comes from anomalous dip
implies that it was so anomalous that the actual dip
correction was POSOIIVE.
(In other words, all shots were undershots even without
any dip correction. Do you think this sort of anomalous deep is
possible at all?)
3. There was no waves at all visible. Of course I was looking at
the surface of water in the fjord, and few miles away there could be waves
which I did not see.
4. It is still possible
that something was badly wrong with the horizon.
Of course with the Sun low, there was substantial glare on the
horizon under the Sun. (And as I said there is no horizon filter
to surpress it).
I just reduced two observations taken later, when the Sun was already
over the shore. I used the water edge at the remote shore as the horizon,
it was more than 1 mile away, and I was standing on the water edge.
The dip correction applied as if there was no shore.
The observations had errors -1'.3 and -1'.1 respectively,
which is far from -3.'5 of the rest of the observations.

I am going to explore this further, when I sail.
The suggestion that the arc is wrong near zero cannot be excluded,
but then this is my second sextant with the same defect:-)
Am I so unfortunate that all sextants I buy have bad arc near zero?
:-)
Alex

On Wed, 14 Jun 2006, George Huxtable wrote:

>
> Alex wrote, about his observed errors with a box sextant-
>
> | So where could these -3.5 possibly come from?
> | This cannot be any anomalous dip (the whole dip correction
> | was between -2.2 (when I sat on a stone just next to the water edge)
> | and -3.5). It is hard to believe to such large arc irregularity at
> | such moderate angles.
>
> "Cannot" is rather overstating things, It's perfectly possible though
> not very likely, for temperature gradients near the water surface to
> give rise to such anomalous dip.  If, for example, warm air from over
> the adjacent Sun-heated land blows over the surface of a cooler body
> of water, that's a recipe for anomalous values for dip; significantly
> worse than in open-sea conditions. Just because the dip itself is
> small (because near the horizon) does not necessarily imply that such
> refraction errors in the dip must also be small.
>
> If Alex is certain that the weather was calm enough to dismiss Bill's
> suggestion (overlapping waves on the distant horizon toward the head
> of the fjord) then he is left with anomalous dip or a serious error in
> marking (or reading) of the scale near the zero-degree point.  The
> interesting thing would be to test again under very different local
> weather conditions or in the open sea.
>
> George
>
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>

   

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