NavList:

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

 

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

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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.