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Alex asked-

>Dear list participants,
>Can anyone give a good reference
>for irradiation?
>I remember the issue was raised few
>times on this list, but using the search
>engine I could not find much.
>CelNav books I have don't even mention this word.
>(Neither my Encyclopaedia Britannica (1960!) does).
>I am even not sure whether this phenomenon
>belongs to optics or to psychology/perception/physiology:-)
>Typing "irradiation" in the Google also
>returns millions of sights irrelevant to optics
>and perception:-)
>
>On the other hand, my measurements of the index correction
>from the Sun, sometimes give some error which I cannot explain.

==========================

It's odd, but both the "sextants" mailing list and Nav-l are dealing with
the same topic of irradiation at the same time.

I sent a posting to the sextants list earlier today under the threadname
"Irradiation", and copy it here in case it's useful.

========================= quote from sextants list posting follows-

Bob Gainer wrote-

>I understand from some reading
>that the moonlight has a distorting effect on the horizon directly below the
>moon diminishing as you change your azimuth relative to the moon. I think
>this was called irradiation, a bright light near a dark mass moves the
>apparent mass because of an optical illusion. Have you heard of this effect?
>Its one of the reasons that I will not shoot the moon after dark, I am
>afraid that the horizon is not real. I wonder if that?s true and if it is,
>how far away from the moon must you be to see the true horizon to shot the
>stars?

=================

I think the effect Bob is talking about is not "irradiation", but is due to
the reflection from ripples in the water surface.

Irradiation is an effect that occurs in the observer's eye, a defect in
everyone's perception of vision. Where there is a sharp change in
brightness between two areas seen in the eye, such as at the edge of the
Sun, the retina always seems to perceive that boundary as shifted toward
the darker area. So, it makes the Sun (even when seen through a shade)
appear somewhat larger than it really is. It makes the boundary of a bright
horizon above a darker sea appear to be lower than it really is.

You can demonstrate the effect for yourself, rather convincingly. Hold your
finger and thumb up a few inches in front of an eye (the distance isn't
critical), such that there's a bright diffuse background behind: a white
cloud, a lit lampshade, even a bright computer screen. Now bring finger and
thumb together. Just as they meet, you see a dark shadow jump across the
narrow gap between them. Part them, ever so slightly, and that shadow
suddenly vanishes. I haven't met anyone who is immune to this effect. It
surprises all who notice it, and they find it hard to explain.

What's happening, it seems to me, is this. As long as there is the
slightest sliver of light, illuminating the narrow isthmus between finger
and thumb, the effect of irradiation makes it look wider than it actually
is (by an arc-minute or so, perhaps). Only when the gap closes completely,
so there's no light shining through that isthmus at all, does that bright
sliver disappear. That's why it appears to vanish so suddenly. Try it for
yourself.

Because irradiation is an effect that occurs within the eye, it can be
minimised by using a high-magnification scope with the sextant.

At one time, the Nautical Almanac allowed for the effects of irradiation as
follows-

For lower-limb measurements of the Sun, it presumed that irradiation
depressed the perceived horizon, and the perceived lower-limb of the Sun,
by about the same amount, so no correction for irradiation applied.

For upper-limb Sun observations, things were different. As before,
irradiation made the horizon look lower than it really was. But now.
irradiation caused the Sun's upper limb to appear to be a bit higher. So,
the argument ran, there was a double effect of irradiation in measuring the
angle between the two, as is done on Sun upper-limb observations.

For that reason, correction tables for the Sun's upper-limb (but not the
lower) were adjusted by 1.2' to allow for irradiation, since 1953.

However, second thoughts prevailed, described in a paper by W A Scott and D
H Sadler, NAO technical note no. 12, Jan 1967, "Corrections, for
Irradiation, to the Observed Altitude of the Sun", published by HM Nautical
Almanac Office, whose present address is-
Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.

The end result of the many observations described in that paper, which
showed the irradiation error varying between observers, with Sun altitude,
and with observing conditions, was this- "... it would be more in keeping
with our lack of knowledge to omit the effect of irradiation from the
altitude correction tables in the Nautical Almanac". That occurred shortly
afterwards, but I don't know at what date.

Although it is so hard to predict numerically, there remains little doubt
that irradiation is a real effect, presenting problems to observers who
wish to observe with ultimate accuracy, such as in measuring lunar
distances.

George.

====================end of quote from sextants posting.

The NAO are usually helpful and are likely to offer a copy of that
technical note, but they like to be asked by letter.

Otherwise, I have a copy, which I could quote from or lend to Alex if he wishes.

The study is of "a quantity d, representing the differential irradiation
coorrection between the two limbs of the Sun; the currently adopted value
of this quantity is +1.2'." [It was, then, back in 1967, but is no longer -
George]

From page 8, I quote- "There is a sound physical explanation of why the
values of d (whether due solely to irradiation or not) at very low
altitudes might differ from those at higher altitudes; this is the effect
of the reflection of sunlight from the water surface. It is less clear why
the effect should differ between the two limbs. However, there is no
evidence that the large values of d found for low altitudes are applicable
to higheraltitudes; the many NAO observations indicate conclusively that,
for altitudes greater than about 10 degrees, the currently adopted vaue of
+1.2', deduced from the low-level observations, cannot be sustained..."

I'm not sure that I follow the reasoning there, but the evidence is based
on nearly 8000 observations taken for the purpose (from the shore).

George.

================================================================
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