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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 
>To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
>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
>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.
>
>================================================================
>contact George Huxtable by email at george@huxtable.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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