Yes, its just that simple. You understand perfectly. There's only one problem.
Here's what Bowditch has to say, from the 2002 edition (LAN is Local Apparent
Noon):

Determining the time of meridian passage presents a
problem because the Sun appears to hang for a finite time
at its local maximum altitude. Therefore, noting the time
of maximum sextant altitude is not sufficient for
determining the precise time of LAN. Two methods are
available to obtain LAN with a precision sufficient for
determining longitude: (1) the graphical method and (2)
the calculation method. The graphical method is
discussed first below.
See Figure 2011. For about 30 minutes before the
estimated time of LAN, measure and record several sextant
altitudes and their corresponding times. Continue taking
sights for about 30 minutes after the Sun has descended
from the maximum recorded altitude. Increase the sighting
frequency near the meridian passage. One sight every 20-30
seconds should yield good results near meridian passage;
less frequent sights are required before and after.

Bowditch goes on to describe the process of plotting the curve and estimating
the time. The other approach is similar, except they give a more mathematical
method for averaging all the sights. Either way relies on taking a number of
sights before and after the Meridian Passage.

The exercise of plotting a Noon Sight is one of the first things a novice
learns. Neal's lack of understanding shows he has never actually done this.

Reference section 2011
http://pollux.nss.nima.mil/NAV_PUBS/APN/Chapt-20.pdf


"Wally" wrote in message
...
Jeff Morris wrote:

One sight at local noon yields your latitude and longitude.

Clearly not true - for starters, you can't take "one sight at local
noon" unless you already know your longitude. If Neal had actually
ever taken a Noon Sight, he would know this!

I am but a sweet, innocent, fresh-faced n00b, and wonder if my reasoning is
correct...

I have a sextant and an accurate chronometer set to GMT. I am somewhere on
the surface of the earth, but know not where. I awaken from my slumbers one
morning and find myself wondering what my longitude is, so I watch the sun's
upward climb across the sky. As it approaches zenith - local noon - I note
the time on my chronometer and then crunch time into degrees to determine my
longitude.


--
Wally
www.art-gallery.myby.co.uk
Latest work: The Langlois Bridge (after Van Gogh)

Jeff Morris wrote:
Yes, its just that simple. You understand perfectly.

Good-o.


There's only
one problem. Here's what Bowditch has to say, from the 2002 edition
(LAN is Local Apparent Noon):

snip

Yup, I'm aware of the difficulty in taking a sight precisely at local noon
due to the sun apparently hanging in the sky, and that some method of
extrapolation from sights either side of zenith is needed.


The exercise of plotting a Noon Sight is one of the first things a
novice learns. Neal's lack of understanding shows he has never
actually done this.

Neal seems to be contending that it's possible to line up a limb of the sun
with the horizon, such that there's a perceptible difference between limb
and horizon being 'in contact' and being separated. While this may well be
possible when everything is nice and stable, I can't help feeling that it
would be rather difficult in a bobbing boat.


--
Wally
www.art-gallery.myby.co.uk
Latest work: The Langlois Bridge (after Van Gogh)

"Wally" wrote in message
...
The exercise of plotting a Noon Sight is one of the first things a
novice learns. Neal's lack of understanding shows he has never
actually done this.

Neal seems to be contending that it's possible to line up a limb of the sun
with the horizon, such that there's a perceptible difference between limb
and horizon being 'in contact' and being separated. While this may well be
possible when everything is nice and stable, I can't help feeling that it
would be rather difficult in a bobbing boat.


Its really more difficult than that - as I said, the altitude of the Sun varies
less than 1 arc-second for roughly a minute before and after LAN; a few
arc-seconds for the minutes before and after that. The diameter of the Sun is
about 30 minutes, or 1800 arc-seconds, so we're talking a teeny, tiny, itsy,
bitsy amount here. If the Sun appears to be about the size of a pea at arm's
length, then this distance is about a tenth the thickness of a human hair (if my
napkin math is still good).

If that weren't enough, Neal claimed he just presets his sextant to the right
altitude, but even a good sextant can't be set better than about 12 arc-seconds,
most have errors much greater than that. Neal's plastic probably has completely
random errors more like 5 arc-minutes. And then there's things like the dip
correction, where a change in the height of the viewer of just a few inches
throws it off by more than an arc-second. On top of that, knowing the altitude
to preset means knowing one's Latitude - in this case to about 100 feet for each
arc-second. If my dead-reckoning were this good I wouldn't have need for a
sextant!

And remember, the Earth is moving 15 miles per minute (at the Equator) so there
isn't much room for error when making this determination. Neal claims he can
get 10 mile accuracy; its probably more like 100 miles with this method. In
reality, assuming Neal actually owns a sextant and has taken it out of the box,
what he has probably done is preset (as best he could) based on his GPS
position, and then verified that local noon occurred as predicted, within a few
minutes.

-jeff