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    Re: Irradiation
    From: George Huxtable
    Date: 2004 Nov 29, 00:24 +0000

    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
    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
    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
    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
    ====================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 -
    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).
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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