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We are all familiar with the semi-diameter of the sun and of the moon
being tabulated on each of the daily pages of the Nautical Almanac. My
question is "why?" There is no place where you use this bit of
information when using the N.A. This information would be useful if the
sextant corrections were made separately but both the moon and sun
correction tables in the N.A. combine  semi-diameter with the refraction
and parallax in altitude corrections in just one table so there is no
place to input the S.D. There has not always been a listing of S.D. for
the sun, see the attached page from the 1937 N.A.

I am also curious why the sun correction table has only two tabulations
allowing for only two S.D. values when the S.D. of the sun includes six
different values during the year from15.8 to 16.3 minutes. This
unnecessarily limits the accuracy of the sun corrections. The horizontal
parallax of the sun is 8.8 seconds or just slightly less than .15
minutes. Looking at the sextant correction table for the sun for an
altitude of zero degrees for October to March we find a value for the
lower limb of -18.2'. This includes the refraction correction of -34.5'
and the horizontal parallax of .15'. So by backing out these value we
can see that the semi-diameter used for this correction table is 16.15'
while the S.D during this period includes four values from 16.0' to
16.3' building in up to a .15' error. For the period of April to
September the S.D. varies form 15.8' to 16.0' but the S.D. used in the
correction table is 15.95' also building in an error up to .15'.

So using the sun's actual S.D. with the refraction table for stars
should produce better accuracy but then you have to account for the
sun's parallax in altitude. A separate table for the sun's parallax in
altitude would be quite simple. To an accuracy of .1' the sun's
horizontal parallax of 8.8 seconds rounds down to .1'. Since the
parallax in altitude varies with the cosine of the altitude the parallax
in altitude table, to .1' precision, would show .1' for altitudes of
zero to 70 degrees and zero minutes above 70 degrees.

Frank seems to be an expert on the N.A. maybe he can explain  this.

gl

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