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Dear Bill,

> Understood. Still, assuming you are doing things correctly and the
> instrument isn't at fault, about the only thing that could throw a beach
> shot off to that extent is dip.

That's what I am inclined to believe too.
There was strong glare of the sea under the Sun.
Having no horizon filter, I could misjudge what the real
horizon was. Two observations using the shoreline seem
to confirm this. But still, taking the shoreline
instead of the horizon, I would expect an OVERSHOT rather than
and UNDERSHOT.

Yesterday I made a tour of Hamburg shops.
There are many interesting things there.
For the first time I could handle a real reflection circle
(German, by Pistor and Martens, late XIX century).

Unfortunately , German antique dealers heavily polish all
sextants before they go on display. So practically on all of them
the silver scale becomes unreadable:-(

The prices are enormous. A pocket sextant of the type I have
by Troughton and Simms costs 900 Euros.
(I bought my one for $200 on e-bay).
An average early XX century sextant is about 1000 EU
(polished and thus unusable). An early XIX century chronometer
is 4000 EU, working.

A real XIX century backstaff (all of wood, a really rare thing)
goes for 5000 EU. Same for a "boat mortar" of the type described
in Forester's "Admiral Hornblower in the W Indies".
(I suppose transportation of this mortar overseas would
cost about the same. See Forester's beautiful description of
transportation of such mortar few miles up a river:-)

Also found a place where they resilver mirrors and test
sextants. Of the modern sextants have seen only Freibergers,
at 1200 EU full size, and 700 Euros "yacht".

> The upshot was that Frank was using the height of Chicago Buildings (and
> there differences) to calculate distance. His calculations did not match
> actual measurements.
> Possible reasons included the refraction index(s) used
> in Bowditch formulas and hefty anomalous dip (thermal inversions).

He probably measured their height above the lake level.
While actually they are not standing on the lake level,
and I don't see how one can measure the actual height of
the building from the ground level from a distance.

> "I've just received an offprint of a new article by Andrew T Young, of
> the Astronomy Deparment, San Diego State University, "Understanding
> Astronomical Refraction", which has recently appeared in the journal
> "The Observatory"(Vol. 126, no. 1191, pp. 82-115, 2006 April.)"

Have you seen the paper? Is it available on the web?

Alex.

   

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