NavList:

   

Reply

This is a bit odd. Jeremy calls it a LAN (Local Apparent Noon) observation,
but what he describes is a maximum altitude observation.

Those are not the same thing. They do not occur at the same time, and they
do not produce the same result. The LAN observation gives the latitude at
LAN. directly. The maximum altitude is dependent on the North-South
component of the vessel's speed, and needs correcting for it. The faster the
vessel, the bigger the difference. Can Jeremy explain, please?

Later he added- "In today's world, given the accuracy I have found in noon
fixes by the sun, I would certainly consider them more accurate than running
fixes."

To achieve that result, how much time would he devote to such a noon fix?
Clearly, the longer the time that it's spread over, the better it would be.
What if he restricted his observation time to that suggested; ten minutes
before LAN to 2 minutes after? Would that suffice? For this purpose, exclude
the special case of observations in which the Sun passes nearly overhead.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: 
To: 
Cc: 
Sent: Sunday, January 10, 2010 7:32 PM
Subject: [NavList] Re: QMOW Days work in Navigation


| What Byron speaks of is what I was traditionally taught about observing
| LAN.  Multiple altitudes ARE NOT RECORDED, but a frequent observation of
the
| sun was taken and the sextant adjusted to keep the limb on the  horison.
| You start 10 minutes before calculated LAN in order to insure  that you
are
| ready for the observation in the case that the DR or calculations  were in
| error.  We were taught, as I have mentioned before, to watch until  the
sun
| "hung" in the sky (no apparent changes in altitude, and then  the maximum
| altitude was recorded and the Latitude then calculated.  This  Latitude
was
| advanced/retarted along with the AM sunline where a 1200 LT Running  fix
was
| determined.
|
| What is interesting to note, is that the DR plot was not changed at noon,
| but only "reset" when a more reliable star fix was obtained.  DR's in my
| training, were changed only at the two star times.
|
| In today's world, given the accuracy I have found in noon fixes by the
sun,
| I would certainly consider them more accurate than running fixes.  There
| are several cavats to this however: between 35N and 35S Latitudes at
| moderate speed (say <20 Kts), I have found noon curves to be adequately
reliable
| to plot a fix.  Still, this is not the teaching or norm for the US
Merchant
| Service, and they still teach to advance an AM Sunline to noon for the
| running fix.
|
| Jeremy
|
|
| In a message dated 1/9/2010 1:55:15 A.M. Eastern Standard Time,
| pmh099@yahoo.com writes:
|
|
|
| Thank you, this is very educational.
|
|
| Also,  experience has apparently shown that doing a running fix is
| preferable to  getting longitude from the time of LAN. This is very
interesting.
| Does anyone  know how this rule was established? Has anyone tried both
methods
| and compared  their accuracy? It is also conceivable that nobody has
really
| tested this and  the reasons are historical: i.e. the LAN is used to give
| latitude ONLY and  thus the running fix is the next best thing to
establish
| position.
|
|
| Peter Hakel
|
|
|
| ____________________________________
| From:  "byronink@netzero.com"
| To:  NavList@freelists..org
| Sent: Fri, January 8, 2010 7:59:07  AM
| Subject:  [NavList] QMOW Days work in Navigation
|
| [parts deleted by  PH]
|
| Mid-morning Shoot sun to determine LOP. Plot on plotting sheet.
| 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. Advance mid-morning sun LOP to LAN LOP on
| plotting sheet to  obtain running fix. Plot running fix on track chart and
pass
| fix information  to CIC/CDC. Advance sun LOPs to 1200, for 1200 Ship's
| Position  Report.
|
|
|
|
|
|

   

Reply