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Recent correspondence has focussed on the difficulties some members find in
achieving acceptably small intercepts, using Sun or star to horizon or Sun
to Sun with an artificial horizon. I'm also aware of attempts to measure
lunar distances by seasoned observers, some from a firm footing on-land,
using good equipment, which have surprised them by being 1 arc-minute, or
perhaps slightly more, out from the expected value.

And yet I see little discussion of the phenomenon of "irradiation". This is
a real effect that occurs in the human eye and affects everyone's eye to
some extent. It causes bright objects to appear to be slightly bigger than
they are by shifting slightly the apparent interface between brighter and
darker, toward the darker side.

I have described in an earlier message how the effect of irradiation can be
demonstrated. If you touch your first finger and thumb together in front of
a bright backgrond, and then slowly separate them and bring them together
again, you will see a shadow "jump" between them when the gap is very
small. I haven't met anyone who fails to observe that effect, to some
extent.

The Nautical Almanac used, a few years ago, to make an allowance for
irradiation for Sun upper-limb observations, but have since dropped it on
the grounds that it wasn't predictable between one observer and another.

How it works out in a sextant sight must depend on the relative
brightnesses of the bodies, and the horizon, being observed, and on the
shades that are used. Perhaps part of the ability of old mariners to
achieve consistent and accurate sights was an inbuilt allowance, perhaps
unconscious, for the effect of irradiation on bright images, to give good
answers. Perhaps some Nav-L observers, with long experience, get better
results than others for that reason. Of course, that couldn't explain a
SCATTER in altitudes.

Anyway, it seems to me that irradiation is a reak effect that ought at
least to be considered. When observers find consistent errors in their
sextant readings, are they in a direction that's compatible with
irradiation?

George.




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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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