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Thanks to all who answered about irradiation.
(I rarely look in the "sextant" list; I like
this one much more, for several reasons:-)


On Sun, 28 Nov 2004, Trevor J. Kenchington wrote:

> The better demonstration of irradiation, for members of this list,
> might
> be to measure the semi-diameter of the Sun, under various condition

Actually, this was the main resaon for my questions:

Nov 25 16:00 LT (Sun very low, just before the sunset)
Sextant vertical, Sun SD from the almanac 16.2; 4SD=64.8

Up       Low      Sum   "IC"
31.1     32.5     64.6  -0.2

Nov 25 16:25 LT (Few minutes before sunset)
Sextant vertical:
31.8     32.3     64.1  -0.25
31.9     31.9     63.8   0
31.9     31.9     63.8   0
31.9     32.      63.9  -0.05
Average 4SD: 63.9 (The vertical SD of sun compressed by refraction).

Sextant horizontal (handle down):
33.0     32.9     65.9   0.05
32.8     32.8     65.6   0.
Average 4SD: 65.75 (Irradiation??)

Later, same night: index correction from
stars 0.0 both upright and horizontal and upside down.
An a good lunar the same night.

Nov. 28 LT 15:39
Sextant vertical:
32.1     32.5     64.6  -0.2
32.3     32.1     64.4   0.1
32.5     32.6     65.1  -0.05
Average 4SD: 64.7 (Very close to the truth)

Sextant horizontal:
32.5     32.6     65.1  -0.05
32.8     32.4     65.2   0.2
32.2     32.6     64.8  -0.2
Average 4SD:  65.0 (Irradiation?)

Sextant upside down:
32.7     32.2     64.9   0.25
32.1     32.3     64.4  -0.15
32.6     32.4     64.7   0.1
Average 4SD: 64.7 (Close to the truth again).

Conclusions:
a) It seems that the sextant is quite rigid.
b) In horizontal position I always get somewhat higher
numbers for 4SD than the almanac predicts.
Maybe because of irradiation?
c) In vertical position this seems to be compensated
or partially compensated by refraction.

Alex.

   

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