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Alex wrote-

>From what I read, error in the dip due to refraction
>is usually much less less than 1', that is not totally
>negligible but small.

Response from George-

Yes, that USUALLY is the case. But occasionally, the dip is significantly
greater, as in Blish's own report of observations off Southern California,
and you have no way of knowing that it's so. So, presumably, the prudent
navigator has to allow for a possible error in his observations up to the
maximun possible (or likely) value of anomalous dip, whatever he considers
that might be. Unless he has a dipmeter to check it, he is working in the
dark, as far as dip is concerned.

He can null out the effects of anomalous dip by taking a round of
star-sights at dusk that include stars at many different azimuths, or,
better, star-pairs in nearly opposite directions.

Referring to Schufeldt's refraction corrections, Alex mentions-

>He uses some Russian tables of refraction for such altitudes
>published in Moscow in 1930.

Yes, the astronomers at Pulkova Observatory (near Leningrad, I think) took
a special interest in refraction, and there has been a series of
publications on the topic from Pulkova over many years. They have long been
regarded as the authorities on this matter.

The most recent reference I've found (in Seidelmann's "Explanatory
Supplement") is to "Refraction Tables of Pulkova Observatory, 4th ed.",
(Academy of Sciences Press, Moscow, Leningrad), 1956, though I haven't seen
this publication.

>He says that he took 380 sights at such altitudes, and with very
>careful correction of the sights (including water and air temperature
>and other meteorological info, irradiation etc.) his position
>line was never mostly within 2 miles of his true position.

But Alex has to accept that no matter how good are tables of mean
refraction and any meteorological corrections, the evidence of his own eyes
showing distortions in a low-level Sun-disc proves that occasionally
refraction can deviate locally and substantially from any predictions.

George.

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