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On 5 May, Gary Harkins commented on the inconsistencies Robert Eno was
finding in his measurements of Sun diameter-

>One mistake you are making is subtracting out the index correction.  Since
>you are joining opposite limbs of the sun, it is the total distance that
>you are
>concerned about.  For example, if you had a really terrible index correction
>of -1 degree you would have both limb sightings on the arc.  For determining
>semi-diameter this would not matter as it is the total difference divided by
>four.  Applying the index correction when you are trying to figure out the
>semi-diameter not proper.  For normal sights it is of course necessary.

Bill Noyce had earlier that day offered a similar analysis

I think both of them were correct.

The content of Gary's posting has attracted no attention, which has all
been diverted to the controversial "signature" below it. There may be a
lesson there.

Another message from Gary has escaped comment, in the same way, and perhaps
for the same reason. In a later posting he said-

>Another source of error could be refraction.  When calculating the
>semi-diameter you should do it holding the sextant horizontal.  This
>eliminates the
>refraction difference between the upper and lower limbs.

In principle, what he says here is correct. But Robert Eno made it clear
that he was referring to Sun altitudes above 30deg. At 30deg, the
difference in normal-refraction between the upper and lower limbs of the
Sun is no more than about 1.5 arc-seconds, and gets rapidly less as the
altitude increases. So differential refraction is unlikely to have been a
significant factor in Robert's problem.

But we need to be careful about this matter, particularly at lower
altitudes. We are all familiar with a setting Sun that is obviously
distorted, bulging at the sides, flattened at the bottom. On such
(frequent) occasions, layers of air at different temperatures are giving
rise to such gross distortions. Attempts to allow for such local
distortions using the temperature and pressure corrections in the almanac
are quite futile. Often, similar effects can occur that are less severe and
not apparent to the eye, but atill enough to upset sensitive altitude
observations. Altitudes of 30deg and above will usually be immune from such
effects.

Gary's suggestion, to use a sextant flat-on rather than upright for
measuring Sun semidiameter, is good advice.

George.


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