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

otn

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

Simple Simon wrote:
That's it but you need tables. You need tables because you measure
zenith and local hour angle.

To find longitude? If so, how's that done? I just put together a little
spreadsheet which seems to find longitude for zenith at any given GMT time.

06:00 gives -90 (90 deg east)
18:00 gives 90W
17:21:50 puts me very close to your meridian - 80d 27.5'W


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

OTN,

Now, with your explanation, Neal may be able to get a Local Noon sight.
Maybe, I'm still not sure he aware of what we are talking about. Hitting
a high noon fix with the Earth traveling over 900 MPH is kind of like
catching a Fly with Chop Sticks.

OTN your method is the correct way to take a Noon Sight. Even the
instruction that came with Neal's (Also mine) Sextant explains the Noon
Sight that way.

Ole Thom

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)

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

That's why I said "apparent velocity." As I said, the apparent velocity
changes dramatically, from being very fast near the horizon, to hanging almost
motionless at noon. Basic stuff, but its enough to prove that you've never done
a noon sight.

See my response to Wally - it has the reference from Bowditch. But its pretty
clear you have no desire to actually learn this stuff; you thought nobody would
call your bluff.



"Simple Simon" wrote in message
news
You're totally stupid. The fact is the Sun moves across the sky
at the very same speed all the time it moves across the sky.

Are you so stupid that you actually think the Sun speeds
up as it nears the horizon? Bwahahahahahahhahahaha!

S.Simon



"Jeff Morris" wrote in message
...
Once again, you're proving you've never actually taken a noon sight. By
claiming its just the same as a sunset you demonstrate your ignorance. At
sunset, the Sun is moving at its fastest apparent velocity - it's dropping
at
over 10 arc-seconds per second, about one arc-minute every 5 seconds. At
local
noon, its "hanging" near its highest point, varying less than one arc second
for
almost two minutes. Anyone that has plotted even one noon sight knows
this.
Even if you had one arc-second accuracy, you could still be off 30 miles,
In
fact, your accuracy is about 100 times worse than that.

The only way to get an even approximate longitude with a noon sight is to
take
many sights, and plot a curve. Ten minutes would not be a long enough
time -
maybe 30 minutes would give you a lone enough time span.

Your claim that the "sun only stays at it's azimuth for one second" is
meaningless. There may be only an instant as the sun makes its meridian
passage, but determining this point can't be done as you suggest.

But you don't have to take my word for it - even though I worked as an
astronomer for 6 years, programming celestial navigation for a $250,000,000
spacecraft. I admit shipboard navigation is a bit different. Perhaps
someone
else can arbitrate.



"Simple Simon" wrote in message
...

"Jeff Morris" wrote in message
...
Once again Neal has blown a chance to impress. If he had made a
reasonable
claim, it might have been believable. But by claiming the impossible,
he
proves
that he really knows nothing of celestial.

comments interspersed ...

"Simple Simon" wrote in message
...
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!

Hey, dummy! Celestial is based heavily on dead reckoning. Longitude can
be pretty closely gleaned by dead reckoning using only one noon sight
per day. The more accurate your dead reckoning the less time you have
to sit on the house bringing old sol down to the horizon and checking if
your angle is getting larger (patience), has stabilized (fix) or is
getting
smaller (you missed it) Local noon is local noon is local noon. One needs
only an accurate watch to time when the sun is at noon. The tables then
tell you your longitude when this angle and time occurs and the offset
from GMT. It seems to me that YOU are the one lacking in working
knowledge of noon sights.



Poor Ole Thom who claims to have used a sextant seems to
not understand this simple fact.

1) the angle of the sun at local noon gives you your latitude.

2) the time differential of the sun's azimuth from Greenwich
mean time gives you the longitude.

True, but its a very tedious process for even an approximate position.
It
can't
be done with "one sight."

Yes it can. That's the beauty of the noon sight. One sight does it all.


Expensive chronographs are no longer necessary.

This has been true for almost a century.

GPS time is plenty accurate enough

Well, duh!

as well as a good quartz watch set to radio signals from WWV.

Even a cheap quartz watch will do.


I can take one noon sight anywhere in the world covered by the
tables I have on board, correct it for errors and come up with
my position within ten miles easily every time provided it is not
overcast, raining or some such.

Nonsense. The "zenith distance" of the Sun varies less than one second
for
almost two minutes at local noon. During this time the Earth moves
about 30
miles at the Equator. And that's with one second accuracy! One minute
accuracy is considered pretty good - that might leave you 100 miles
away.
Celestaire says Neal's plastic sextant typically has a 5 minute error.

And Neal is claiming he can do this with one sight! While it is
possible to
do
this for latitude, it is completely impossible for longitude.

You are completely wrong. One accurate local noon angle of the sun
is ALL that's needed. The sun only stays at it's azimuth for one second.
A sight can be taken with a one or two second accuracy which is good
enough to place one in the ten-mile range which is all a sailor of a small
sailboat really needs.

Look at a sunset sometime. Tell me you cannot mark the exact second
the bottom limb touches the horizon. I can so you should be able to.
I don't know about your sextant buy mine allows me to create a sunset
at noon as I adjust the vernier. It is easy to tell when the sun touches
the horizon and then goes the other way. PUTZ! I can tell you are one
of these arrogant people who wish to muddle up the ease of a noon
sight so you can claim you do something magic with your sextant. It
simply ain't so. Any old fool can take a noon sight and ascertain their
position with great accuracy.

S.Simon

Hey Pieman,

You've really turned yourself into "Mincemeat" this time.

Sorry but had to call you on your 5 minute Noon Sight. It is the easiest
sight to make but it does take time; and it should. It is the best sight
of the day. Don't screw around with it, for the novices.

You have been beaten down to your, "Waterline" don't say anything more
or you'll drown.

If you want to further demonstrate you're ignorance start talking "star
sights"

Ole Thom

Wrong. You need the Almanac. For a noon sight, you need to know
declination to compute latitude. As for longitude, using the method you
are referring to, you need to convert LMT at time of sight ( reason for
error) to GMT, which you convert to GHA and then longitude. Again, if
you had a clue as to what you were talking about, you would realize that
this method was not all that accurate except in certain cases. LHA is
measured from you WEST, to the body. Go back to reading.

otn

Simple Simon wrote:
Sight Reduction Tables of course. I use HO249 for speed and simplicity.

You must have local hour angle because the sight is always a combination
of local hour angle and zenith. The tables help you to turn the local hour
angle into latitude since the sun moves daily because of the tilt of the
earth as the seasons progress.

S.Simon



"otnmbrd" wrote in message ink.net...

Pleas explain which "tables" you are talking about and what
calculations you are making for sextant altitude and time. You sure you
want LHA?

otn

Simple Simon wrote:

That's it but you need tables. You need tables because you measure
zenith and local hour angle.

S.Simon

You don't have a clue as to what I'm saying or talking about, do you?
Of course my sextant shows the entire disc of the sun, but it has
nothing to do with my sextant and everything to do with my latitude in
relation to the declination of the sun.

ton

Simple Simian wrote:
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