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It is, unfortunately, a very common mis-conception that there is no effect of
refraction between stars of equal altitude. This is not so. If you clear the
distance between a few such pairs of stars with one of the rigorous methods, you
will easily see this. You can visualize the effect if you consider that atmospheric
refraction pulls images of objects in the sky towards the zenith: As they go up,
they converge.

In September 1996, the Ocean Navigator (a popular American sailing magazine that
likes to maintain the fiction that it is more serious then the rest of them)
published an article about sextant calibration. The author "verified" the excellent
calibration of his newly acquired antique sextant via stellar distances.
Unfortunately, he made exactly the same mistake. I pointed out in a letter to the
editor that this sextant must in fact have had a non-linear error of as much as
0.8',  not the 0.2' that he thought he had - the difference being due to neglect of
refraction. It goes without saying that my letter was never printed. Too much of
this "scientific" stuff would not interest the reader!

I believe to remember that, back then,  I had worked out a rigorous proof that the
effect of refraction on distance between two given bodies becomes a minimum when
both bodies are at the same altitude. I can't retrieve it now, but it seems very
reasonable. Nevertheless, refraction correction is always of an order of magnitude
that you do not want to ignore in a lunar distance.

Herbert Prinz (from 1368950/-4603950/4182550 ECEF)


Chuck Griffiths wrote:

> First, the easiest way to deal with
> refraction when measuring interstellar distances is to use two stars at equal
> altitudes and not correct for refraction,

   

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