NavList:

I wrote before:

"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."


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Now, giving it more thought, it would make more sense to have a table combining just refraction and parallax in altitude for the sun as such a table would avoid the slight inaccuracy of rounding the P in A correction to the .1' level. Such a table would look just like the star refraction table but the cutoff altitudes between the correction values would be different.

The sextant correction tables in the Air Almanac treat each correction factor separately (which helps in teaching this since a student can see where the corrections come from.) Using a bubble sextant and the A.A. correction tables you apply only the refraction correction which is the same for every body. If using a marine sextant you would also apply dip  and, for the sun and the moon, semi-diameter which is tabulated for the sun and moon on each daily page.  To the level of accuracy in the A.A. you don't need to allow for P in A of the sun. See attached excerpts from the A.A.

For the moon the A.A. also treats each correction separately. The S.D. is given for each day (which varies from 15' to 17') and there is a P in A table on each daily page based on the horizontal parallax for the moon for that day. The H.P of the moon varies from 54' to 61' so there are only eight such tables needed. For a bubble sextant observation of the moon you apply the same refraction correction you apply for every other body and also the P in A correction taken from the table on each daily page. If using a marine sextant you would also apply dip and S.D. Doing the corrections this way provides the accuracy commensurate with using the positional data given in the A.A. Keeping the corrections separate makes it easier to teach what is going on.

For the moon, the N.A. combines refraction, S.D. and P in A into one complicated table. The S.D. and P in A corrections can be put in one table because S.D. and H. P. move in lockstep with each other based on the distance from the earth to the moon. Since S.D. is included in this one complex table there is nowhere to input the S.D. values from the daily pages of the N.A. so why do they even give the S.D. for the moon on the daily pages?




gl



Gary LaPook wrote:

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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