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From: Bill (no email)
Date: Wed Jun 14 2006 - 15:49:51 EDT
Bill wrote
>> My impression at this stage of my development (outside of the problems
>> inherent in observing from a medium-displacement small craft, almanac
>> roundings, and blunders) is that accurate dip (both knowing your exact
>> height vs. the horizon and atmospheric anomalies) is the major stumbling
>> block to 0.2' to 0.4' accuracy.
Alex replied
> This time (with a pocket sextant) were are talking of 1' accuracy.
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.
>
>> I reference Frank's beach shots of Chicago from an Indiana shore, and our
>> inability to resolve the discrepancies.
> I don't remember any Frank's beach shots.
> Can you give a reference?
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).
The thread started about Nov 6, 2005 under the subject "Distance off with
Chicago buildings," author Bill, and continued through January 2006.
>
>> I am excited about George's latest
>> find.)
>
> Which one?
"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.)"
Happy sailing
Bill
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