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    Irradiation
    From: Brad Morris
    Date: 2010 Feb 27, 08:10 -0800

    Irradiation was well studied by the USNO as well as by the British Authorities.

    My copy of Dutton, 1973, states:

    2225. Irradiation is the name applied to the optical illusion that causes the apparent size of a bright or light colored object in juxtaposition with a darker one to appear larger than it actually is; conversely, the dark one appears smaller.

    Some twenty years ago, the US Naval Observatory decided to obtain an empirical check on the value of the refraction corrections for low altitude observations presently listed in the Nautical Almanac. Accordingly several hundred observations of the sun's lower and upper limbs at altitudes between 0 degrees and 5 degrees were made by a considerable number of observers. In analyzing these observations, it was found that the upper limb sights were consistently too high while no such error was evident in the lower limb observations. This was caused by the irradiation effect, which tended to increase the sun's apparent diameter, as seen against the sky. At the same time, irradiation tended to depress the horizon slightly, as the sky is usually brighter than the water. For upper limb sights, the two effects were therefore additive, whereas for the lower limb they tended to cancel on another.

    The size of the error caused by irradiation varies with the magnification of the sextant telescope; the greater the magnification, the smaller the error. For telescopes haveing a magnification of 3 powers, the combined irradiation effect for upper limb observations was found to average (+) 1'.2, while for a six power telescope, it averaged about half this amount.

    It was assumed that the error caused by irradiation would remain constant for upper limb sun observations at all altitudes, and the 'Altitude Correction Tables for the Sun' were accordingly prepared for use in the Nautical Almanac, incorporating corrections for mean refraction, average semidiameter for a six month period, parallax and in the case of upper limb sights, for irradiation. The quantity 1'.2 was used for the latter correction, as the sextants in general used at that time were fitted with 3X telescopes.

    With the past few years the British undertook a study of the irradiation effect on upper limb observations of the sun at normal working altitudes. An analysis of several thousand such observations showed the irradiation effect to average 0'.1. Concurrently, the effect of irradiation at altitudes below 10 degrees was studied,and for these observations, the effect was found to average +0'.8.

    It was therefore decided to drop the sun altitude correction tables in their present form from the Nautical Almanac after the 1969 edition.

    The correction for irradiation to observations of the sun's upper limb is subtractive. Quantitiatively, it decreases with increased telescope magnification and with increased altitude.

    Until the average value for irradiation corrections at various altitudes can be determined with some precision, its ommission is recommended in correcting upper limb sights, except at altitudes below 10 degrees, for which the corrections given in this article may prove helpful

    END QUOTE.

    So if I was to use the upper limb of the sun to the lower limb of the sun to set or check my index error, at normal working altitudes (above 10 degrees, then it will be a bright object compared to a bright object. The effect of irradiation should therefore be 0.0, as the article states that alitudes of the upper limb sun at normal working altitudes is only 0'.1. These numbers are consistent with the earlier thread name of "Reliable INDEX ERROR corrections to a Tenth of a Minute". Note that the thread was NOT named "reliable ALTITUDE measurements to a Tenth of a Minute".

    Quite by accident, I came across a reference in the 1906 Bowditch, which indicated that if you want your index error to be zero, then set the vernier to zero and adjust the mirrors. Without knowing that the practice was codified, I was doing it that way anyway. That would be "Reliable Index Error set to Zero!"

    Best Regards
    Brad


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