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Marvin,
Thanks a lot!
Unfortunately, this does not give any references
that really explain the phenomenon, give any quantitative
information, etc.
So I am still looking for this info.

They mention "low altitudes, btw". What does the irradiation
phenomenon have to do with low altitudes?

Alex.

On Sun, 28 Nov 2004, Marvin Sebourn wrote:

> Alex -
>
> This is from "The American Practical Navigator" 1984 edition:
> I didn't check the later electronic edition. Hope this helps.  Probably have
> more elsewhere, will check.
>
> Marvin
> _osugeography@aol.com_ (mailto:osugeography@aol.com)
>
>
>
> (p) 442
> SEXTANT ALTITUDE CORRECTIONS
> (after continuation)
> 1609. Irradiation correction (J).-When a bright surface is observed  adjacent
> to a darker one, a physiological effect in the eye causes the brighter  area
> to appear to be larger than is actually the case; conversely, the darker  area
> appears smaller. This is called irradiation. Thus, since the sun is
> considerably brighter than the sky background, the sun appears larger than it  really
> is; and when the sky is consider­ably brighter than the water, the
> horizon appears slightly depressed. The effects on the horizon and lower limb of
> the sun are in the same direction and tend to cancel each other while the effect
>  on the upper limb of the sun is in the opposite direction to that on the
> horizon  and tends to magnify the effect.
> >From 1958-1970 a correction of 1�.2 was included in the Nautical Almanac data
> for the upper limb of  the sun as an average correction for the effect of
> irradiation. Recent  investigations have not supported that average value and
> have revealed that the  magnitude of the effect depends on the individual
> observer, the size of the  ocular, the altitude of the sun, and other variables. In
> summary, the accuracy  of observations of the limb of the sun at low altitudes
> may be affected  systematically by irradiation, but the size of the correction
> is so dependent  upon the variables enumerated above that it is not feasible
> to include an  average correction in the  tables.
>

   

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