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    Re: Irradiation
    From: Robert Gainer
    Date: 2004 Nov 29, 00:59 +0000

    D�j� vu all over again. When I was on the sextant group today I was sure I
    had read something about irradiation recently. Now it pops up on this group.
    The thread that started this discussion on the sextant list was in
    connection with a member�s comment that he waited for a full moon to
    illuminate the horizon before he would take a shot of the stares after
    twilight. My thought was that irradiation would spoil his plan. I don�t use
    the moon after twilight because I thought the horizon was in error because
    of this illusion. Now I guess that I am doing the right thing but for the
    wrong reason. Although this leaves me with a question now, can I shoot the
    stares with a horizon illuminated only by the moon?
    All the best,
    Robert Gainer
    >From: George Huxtable 
    >Reply-To: Navigation Mailing List 
    >Subject: Re: Irradiation
    >Date: Mon, 29 Nov 2004 00:24:57 +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
    > >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
    > >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
    > >how far away from the moon must you be to see the true horizon to shot
    > >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
    >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
    >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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