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In the thread "Materials from Navigation Weekend Talk",

Jim Wilson asked -

"I understand that the purpose of the double altitude sight is to get time
of meridian passage. Is there another use?"

and Geoffrey Kolbe replied-

"To be specific, it is the method of equal altitudes that give the time of
the meridian passage rather easily, from which you can derive longitude. In
fact, before the advent of GPS technology, it was probably the most
accurate way of deriving longitude available - giving very similar
precision. This alone justifies its continued presence in the navigator's
"kit of useful methods".  One great advantage of this method (if not the
great advantage) is that corrections for refraction etc are not required.
The so called "Equation of Equal Altitudes" can used to correct for
declination changes if the sun is being used as the celestial object."

I think some of that needs to be questioned, or qualified.

When Geoffrey says "giving very similar precision", then I ask- similar to
what?

Given accurate time, it can certainly be a precise procedure, when used for
position-finding from on land, given a good spread between observation
times, before and after noon.

Not, however, if attempts are made to telescope the procedure into a short
time-span, such as the proposals for measuring longitude-around-noon over
an hour or so, as have featured so strongly in recent pages of Navlist.

And for navigation rather than geography, the motion of the vessel, over
the period between observations, has to be taken into account, and is a
crucial factor in the reckoning. Just as it is in the double-altitude
procedure.

Geoffrey points out that corrections for refraction are not required, which
is true. But that's a minor issue anyway: above a limiting altitude of 5
degrees or so, refraction is small and well-predicted, and doesn't in
practice vary significantly from the predicted value. Though such
corrections may still be called for, if morning temperature differs greatly
from afternoon temperature, as can happen. The equal-altitude procedure,
for longitude, will also be immune from any calibration errors of the
sextant. And as a by-product, the latitude, also, will be determined.

But it won't do significantly better, in sea practice, than by working up a
position, by crossing position lines using the St Hilaire method, from any
pair of Sun, or other-body, altitudes, that are well spaced in azimuth.

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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