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Jan Kalivoda said-

>George is fully right in my opinion. For neutralizing the errors in both
>altitudes owing to irregular dip, only their azimuths matter and the
>nearer they are, the better.
>
>But for neutralizing the effects of irregular refraction on both
>altitudes, the altitudes are more important, although the azimuths can be
>relevant as well.
>
>And of course, the time of both observations should be the same, otherwise
>a change of atmospheric conditions can wreck the result completely.
>
>Therefore the advice of old authors seems logical - the same time,
>neighbouring azimuths, neighbouring altitudes of both bodies; and both
>should stay near the prime vertical, so that both LHA's obtained might be
>as reliable as possible. These are the cases 1) and 2) in George's reply.

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I'm going to quibble a bit further here.

It can only be when the azimuth difference between the two bodies is small,
that there's any advantage at all to be gained in arranging the two
altitudes to be similar.

And unlike the refraction-component of dip, refraction at higher altitudes
(above 10 deg, say) is well understood and well predicted, shows little
variation with local atmospheric conditions (and those variations are
readily calculated). Therefore, I think that the "advice of old authors"
(to arrange that the altitudes of the two bodies should be similar) is
without any firm foundation. At such altitudes, I doubt that "irregular
refraction" is of any importance: but I am ready to be convinced otherwise.

George Huxtable

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