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Nicolas asked, about Frank's comment

"frankreed@HistoricalAtlas.com wrote:
> If you want a really good value for IC, shoot the Sun or the Moon
> limb-to-limb ... it's usually more effective than the standard sea horizon
> test (but the standard test is usually quite sufficient and it's the one
> that should always be taught)."

"could you please explain why the limb-to-limb method is more effective?
And can you explain how this is done? What I mean is: are you
superimposing opposite limbs onto each other (so calibrating the sextant
at the sun's/moon's diameter), or the same limbs (so calibrating for
zero at two spots in your field of view)? I take it it is the latter
method. Do you have any data supporting the effectiveness?"

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

The main difficulty with using the sea horizon to check index error is that
the horizon is often hazy or disturbed (especially when seen from a small
craft).

A standard method for checking using the Sun is to put the reflected image
of the Sun above the direct image so that they just kiss,
tangent-to-tangent; then below it, and split the difference. That is, using
opposite limbs, not similar limbs.

It calls for, ideally, a very-dark-glass cap to fit to the telescope
eyepiece, as was once a standard accessory, to make both Sun images
viewable. (That carries a possible danger that the undiminished focussed
heat from the Sun can crack it; an acknowledged eye-hazard with some
astronomical telescopes, but I've never heard of it actually happening with
the smaller telescope that's found on a sextant.) Alternatively, it calls
for a very dark shade in the direct-view, as is always available for the
reflected-view, and not all sextants are so fitted. And any difference in
refraction between those shades can skew the result.

It's quite hard to do it the other way, by superimposing precisely two
images of the Sun, one exactly on top of the other, because when making the
final adjustment, it's difficult to be sure which image-edge is which. Doing
the job by averaging two displaced images, because it combines two
independent observations, has its errors reduced by root-2 because of that
averaging.

But it doesn't provide any useful calibration of the sextant by taking the
difference between the two readings,  about 1 degree apart, as Nicolas
suggests. The extrapolation is too great, combined with the sensitivity to
the observer's judgment of where the Sun-edge happens to lie.

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

The Moon is another matter, and I don't see how it is possible to determine
index error precisely by aligning opposite limbs of the Moon , because of
its partial lighting, except at full Moon. It would call for tilting the
scope into the plane given by the line between the horns, which in itself is
no real problem. The difficulty comes in that those horns are just at the
boundary between sharp-edge and shadowed-edge, and I doubt whether those
opposite limbs would provide a pair of sufficiently-sharp targets to do the
job well, though to be honest I have never tried the Moon for that purpose
in real life. Perhaps Frank has done so, and will explain.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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