"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

Wally,

Read OTN's method of determine the Azimuth. You can't get a accurate
instant sight due to the speed of the earths rotation, Now, that leaves
the question; how accurate do you want to be? Your choice. There are
many way to get close but it's for you to decide. I like to use a shadow
facing true north & time but OTN's way is the exact way, if your using
your Sextant

Ole Thom

Thom Stewart wrote:

Read OTN's method of determine the Azimuth. You can't get a accurate
instant sight due to the speed of the earths rotation, Now, that
leaves the question; how accurate do you want to be? Your choice.
There are many way to get close but it's for you to decide. I like to
use a shadow facing true north & time but OTN's way is the exact way,
if your using your Sextant

I'll have a look at his post...


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

Right on Jeff,

You sprung the time trap on Longitude that I'd set up for Neal. It
didn't take you anytime at all. I raise my glass to you! You even picked
up the error of Mr Davis. They recommend using the plastic model for
practise only.

I'll only mention that a ten mile error is over the horizon and out of
sight. This in real voyaging could make DED course selection rather
iffy. I guess tied to a mooring overcomes that problem.

Good Job Jeff!

Ole Thom

So, how do we split the point and assist? Or do we each get half a sack?

The funny thing is that every beginner plots a noon sight and has the moment of
realization that the Sun "hangs" at an essentially constant altitude for several
minutes. Clearly Neal has never done this. Its a perfect example of someone
that reads a book once but refuses to learn from anyone - its just like his
understanding of the rules.

--
-jeff
"Constant Vigilance!" - Frances W. Wright


"Thom Stewart" wrote in message
...
Right on Jeff,

You sprung the time trap on Longitude that I'd set up for Neal. It
didn't take you anytime at all. I raise my glass to you! You even picked
up the error of Mr Davis. They recommend using the plastic model for
practise only.

I'll only mention that a ten mile error is over the horizon and out of
sight. This in real voyaging could make DED course selection rather
iffy. I guess tied to a mooring overcomes that problem.

Good Job Jeff!

Ole Thom

Sorry Neal, I never used that method to take LAN ..... too hard to judge
the right moment .... if you're going to use this method, best to take a
sight 10+ minutes prior to LAN and note the time and sextant altitude,
the take LAN (no close time really needed) then a third sight and time
when the sun returns to the altitude of your first sight, and work time
for LAN from that.

otn

That's a good way to do it when the seas are rough and
the horizon hard to judge. We call that extrapolation.

S.Simon


"otnmbrd" wrote in message ink.net...
Sorry Neal, I never used that method to take LAN ..... too hard to judge
the right moment .... if you're going to use this method, best to take a
sight 10+ minutes prior to LAN and note the time and sextant altitude,
the take LAN (no close time really needed) then a third sight and time
when the sun returns to the altitude of your first sight, and work time
for LAN from that.

otn

The higher the sun is a LAN, the more accurate you may get, using your
method. However, the lower the sun at LAN, the less accurate.
It has less to do with rough seas and poor horizons.
Biggest problem with your method is getting an accurate time for LAN.

otn

Simple Simon wrote:
That's a good way to do it when the seas are rough and
the horizon hard to judge. We call that extrapolation.

S.Simon

Does your sextant show the entire disk of the sun in the mirror?

Mine does. There's a fancy name for this trick that I forget
what they call it but it makes it pretty easy to tell when the
Sun stops climbing and starts descending in the heavens.

S.Simon


"otnmbrd" wrote in message ink.net...
The higher the sun is a LAN, the more accurate you may get, using your
method. However, the lower the sun at LAN, the less accurate.
It has less to do with rough seas and poor horizons.
Biggest problem with your method is getting an accurate time for LAN.

otn

Simple Simon wrote:
That's a good way to do it when the seas are rough and
the horizon hard to judge. We call that extrapolation.

S.Simon

ROFL Shame I have to go out tonight. I'll answer you tomorrow Neal.
Gawd, what a frawd.

otn