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From: George Huxtable (no email)
Date: Tue Jun 07 2005 - 10:09:33 EDT
Robert Eno asked, on 5 June-
"But this also begs the question: does this correction really matter for
the surface navigator who is not likely to be moving faster than 7 - 10
knots? Having myself, taken numerous sights at sea, including noon sights
(just in case George questions my experience!) it seems to me that this
factor is not terribly significant. Or is it?"
Here, I understand that Robert is questioning the effects of North-south
speed of the vessel on a noon observation.
And the answer to his question is "No, it doesn't matter (much), and is
usually neglected."
But let's be clear about it. Usually a noon sight is used only for
measuring latitude, and not for longitude at all, for reasons we have
discussed at somewhat painful length.
If you observe for latitude, to make the calculations simple, the Sun's
altitude should be measured at the moment of meridian passage. You can
estimate your longitude, look up the almanac, and work out when that moment
will be, and measure the Sun's altitude then, using your chronometer.
That's the number you need. It won't in general be the Sun's MAXIMUM
altitude, though, because you may be moving toward the Sun, in which case
it will still be rising then, to peak later. And vice versa.
Or you can do it differently, in a way that doesn't need any knowledge of
time. You can simply watch the Sun and measure its MAXIMUM altitude. For
the reasons given above, it won't be the MERIDIAN altitude, which is what
you really need. But the Sun's altitude is changing only slowly around
noon, so for a vessel travelling slowly, as ours all do, it won't be far
off the meridian altitude. That gives rise to a small error of no more than
a few minutes in calculated latitude, which is often simply neglected.
That's the situation that Robert Eno correctly remembers.
But it's quite different when you try to extract longitude, from the TIMING
of the observed maximum altitude. That timing is MUCH more sensitive to
North-South velocity. It's one of the (many) reasons why longitudes at sea
have been measured at any time EXCEPT near noon, at times when the rise and
fall of the Sun, due to its motion in the sky, quite overwhelms any effect
of the speed of the vessel.
George.
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contact George Huxtable by email at , 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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