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Thanks to Stacy Hanna for an interesting contribution about US Navy
practice. It's good to know that astronavigation is still being practised.

Here is an extract from Stacy's posting on the Day's Work, which is
relevant to our discussions about noon sights-

>8-12: QMOW compute watch time of LAN (Local Apparent Noon).
>......
>Noon: Observe LAN. Recommend observations be started at 10
>minutes before computed time of LAN, and for a couple of minutes
>after. Reduce sighting and determine ship's latitude.

You will note that there is no mention at all of measuring the MAXIMUM
altitude of the Sun around Noon. What is needed is the altitude AT the
moment of LAN (Local  Apparent Noon), whether that is or isn't the maximum
observed altitude. If the Navy vessel (generally, a fast ship) is making
significant Northing or Southing, then at that moment of LAN the Sun will
still be observed as rising or falling. Only by observing the Sun's
altitude at the predicted moment of LAN will the exact latitude be
obtained. But to predict that moment requires a reasonably good knowledge
of the DR longitude and the ships speed in the E-W direction.

The resulting error varies as the square of the ship's speed, so is
generally negligible for our small sailing craft, but becomes increasingly
important as ship-speeds increase. As there are hydrofoil vessels at sea
nowadays doing 40 knots and more, then if they happen to be observing noon
latitudes (which may be unlikely except for Navy vessels) it will be
important for them to follow Dutton's recommendations, below.

Reading Stacy's comments sent me back to my ageing copy of Dutton's
"Navigation and Piloting", 1969, which was presumably the "bible" of US
Navy practice at that time. In art. 2803 he explains how to predict the
moment of LAN, taking account of the dead-reckoning longitude and the
ship's speed in Easting/Westing.

Dutton says (with my comments in square brackets)-

"Precision in determining LAN is especially necessary when a ship is
steaming  on a generally northerly or southerly course at speed. For a ship
proceeding towards the south, the sun will continue to increase its
altitude for a considerable period AFTER it has actually crossed the ship's
meridian, and an observation made at the moment when it reaches its maximum
altitude will yield a latitude which may be considerably in error."

[For Dutton, usually very precise, this is a bit slapdash. What he
describes above applies only where the vessel is North of the Sun, as it
would always be in US home waters. When South of the Sun, the situation is
reversed.]

[Dutton's paragraph quoted above, however, is a bit inconsistent with what
he says in the next paragraph, as follows-]

"Under most conditions, however, the sun will appear to "hang" for an
appreciable period of time at LAN. That is, it will not change perceptibly
in altitude. To obtain a latitude line at LAN, the navigator usually starts
observing about two minutes before the time of transit, and continues to
obtain sights until the altitude begins to decrease. The average of the
times of the three or four highest altitudes will be used for reduction. On
a northerly or southerly course, several altitudes may be taken before and
after transit, to ensure that an unacceptable random error did not occur in
the sight taken at the instant of transit."

It seems, then, that the navigator is advised to adopt quite different
procedures, depending on his course and speed. In the second procedure
"under most conditions" he can stop taking sights "when the altitude begins
to decrease". But if a significant component of his speed is pointing away
from the Sun, LAN will not occur until several minutes after the moment
when the altitude starts to decrease. So in that case, if he plans to
measure altitude at LAN, he has to stick to his job for some time after
that maximum.

And what is Dutton getting at when he says "The average of the times of the
three or four highest altitudes will be used for reduction"? Note that he
refers to averaging the TIMES, not the ALTITUDES, here: are the altitudes
to be averaged also? Reduction of an altitude at LAN doesn't normally need
any times, in any case; that's the whole point of observing altitude then.
Is he proposing a further correction, using "ex-meridian" tables, such as
tables 29 and 30 in Bowditch (1981)? I am failing to understand this
passage, and wonder if any listmember can explain it.

Dutton's recoomendation, to choose to observe at LAN rather than maximum
altitude, applies, he says to a ship "steering a generally Northerly ot
Southerly course at speed." But he doesn't set any limit on how Northerly
or Southerly that course has to be. Because the Northerly component is
proportional to cos(course), a vessel steering 45 deg off, at say NW, still
retains 70% of her speed as a Northerly component, so it must apply to a
rather wide band either side of North and South.

Can anyone else confirm whether a more recent edition of Dutton's has
altered those comments?

Dutton's is getting at the resulting errors in the latitude deduced from a
near-noon sight, resulting from the time shift between maximum altitude and
LAN. For small slow vessels such as many of us sail, those errors in
latitude are quite negligible. He is not dealing with  errors in longitude
caused by that time-shift, which would be by no means negligible for our
craft. In fact, he doesn't even refer to a noon sight as providing any
useful information other than latitude; and rightly so, in my opinion.

Frank Reed said, on 23 Jan-

>In addition, as has been discussed already on the list, if your vessel is
>moving
>at any significant speed, both the time and max altitude at LAN will be
>thrown off. For emergency navigation, this is probably not a significant
>problem.
>If things are that bad, I think you could afford to slow down for twenty
>minutes around noon! This point would have to be included in the
>instructions for
>the method.

Well, I suggest that it really would be a significant problem, even for
emergency navigation. In the example I quoted, of an observer at my own
latitude of 51deg N, observing a noon Sun at the winter solstice, a speed
of as little as 6 knots towards North or South, would give rise to an error
of nearly 40 minutes in the resulting longitude. Frank's suggestion of
coming to a stop for the duration of a noon-sight would not be very popular
with most skippers.

Indeed, we might as well accept that noon is the VERY WORST moment for
attempting to determine longitude. Particularly as it only involves a wait
of an hour or two, to make a much better job of it.

George.


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