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It used to be applied to Sun upper-limb observations, but not to lower limb
observations.

The argument was that the effect on the image of the brighter shaded sun
with respect to the darker sky was in the same direction, and of similar
amount, to that of the brighter sky with respect to the darker sea. So when
the lower limb was observed the effects of irradiation largely cancelled.
But when the upper limb was observed, the effects added.

A series of observations made by the staff of the almanac office, made many
years ago when they were based at Hurstmonceux, concluded that although
there was a definite effect of irradiation, it was so variable from one
individual to another that there was no sense in including it in the almanac
tabulations. After some hesitation, the American side agreed, and it was
dropped. Thare was an internal report on the subject produced by the almanac
office, which may still be available, if they are asked nicely.

But the effects were nowhere near the alarmist values quoted by Douglas
Denny.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Douglas Denny" 
To: 
Sent: Saturday, February 27, 2010 3:27 PM
Subject: [NavList] Re: Reliable Index Correction to a Tenth Minute of Arc


Hello Hewitt,

I cannot answer your question immediately as I do not know if irradiation
correction is still applied. I was quoting from the Ames paper of 1969.

It might well have been dropped as you say, and I have some vague
recollection in the recesses of my mind that you are right.  No doubt others
better informed will give us the definitive version here in due course.

Regards,

Douglas Denny.
Chichester. England.
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