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Paul Hirose wrote:

> Marcel Tschudin wrote:
>> Why not measuring the sun's horizontal diameter? This measure is
>> (almost?) not affected by refraction.
>
> I discovered the "almost" a few days ago, when programming a function to
> compute refracted semidiameter. The values at the 12 and 6 o'clock
> position angles (in the horizontal frame) were exactly what I expected
> for a Sun-sized object, but at 3 and 9 o'clock they were about .2" too
> small.
>
> After some debugging, I realized there was nothing wrong with my code.
> Refraction reduces the semidiameter in every direction. When the left
> and right limbs refract up, they move on great circles which converge at
> the zenith. With a body of 16' semidiameter, and sea level refraction,
> this decreases the semidiameter about .27". I tested this at several
> altitudes from 10 degrees to 80, and it didn't vary more than .01".
>
> In this case, the algorithm was smarter than its designer.
>
> Later I remembered that Jean Meeus ("Astronomical Algorithms", chapter
> on refraction) says the Sun's horizontal diameter is reduced slightly by
> refraction. His figure (credited to Danjon) is .6", practically
> independent of altitude.

I have to smile--in the sense of someone seeing an old friend. Many years
ago (and I wish I could recall the thread name[s]) the above topic was
broached in many parts.  I am certain most of contributors are still active
on the list and hopefully will refresh my memory.

Regarding refraction:

1. Consider the sun's sphere as a square with its corners at 12, 3, 6 and 9
o'clock. Especially near the horizon, the 6 o'clock corner will be raised to
a greater extent than the 12 o'clock corner.  The 3 & 9 o'clock will be
raised to an extent less than the 6 o'clock corner but greater than the 12
o'clock corner.  It is diamond, but not a perfect diamond.

2. I suggested with two bodies (to the left and right of my line of sight)
being raise by refraction from the horizon to the zenith, I would see two
curves arcing to the zenith.  A more learned contributor suggested that was
not so, each body would be raised from the horizon to zenith on a straight
line.  We finally agreed my point of view might correct if we took a "camera
eye" view.

3.  Here my recollections are very fuzzy.  It strikes me some interesting
relationships exist between two bodies of equal elevation as they rise from
the horizon to zenith.

The above ramblings aside, the list is devoted to "traditional" navigation.
Can you please tell me how to measure 0.x" with a marine sextant? 

Bill B.




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