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Re: Irradiation
From: Alexandre Eremenko
Date: 2004 Nov 28, 15:55 -0500
From: Alexandre Eremenko
Date: 2004 Nov 28, 15:55 -0500
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. >