NavList:

----- Original Message -----

From: Anabasis75@aol.com

To: NavList@fer3.com

Sent: Sunday, June 01, 2008 1:13 AM

Subject: [NavList 5230] Re: Exercise #7 LOP's

 

 

First off, thanks for the practice in working sights using East longitude - it had been long enough that I'd forgotten that for that case you subtract AP longitude minutes from 60, then add that to GHA to get an even value of LHA. My first attempts were ending up with really long intercepts and I couldn't figure out why until I ran it through the Navigator software and the light bulb came on (sometimes we forget how "easy' we have it here in the West longitudes with just subtracting to get LHA).  :-)

JCA

Yes, it was always rough for me in exams when they threw the East Longitude in there, especially right after a West Longitude problem.  Fortunately, I will be in the East Longitude for the considerable future, so you can have lots of practice.  I was taught to take the GHA and add whatever minutes it takes to make it a full degree.  The biggest error I used to make was forgetting to carry the 1 to the degrees of LHA.

 

 

So, here's what I get for sight reductions using Nautical Almanac and Pub. 249 (and Navigator agrees with me within a few tenths of decimal points):

JCA

    Are you using pub 249 or 229?  I have only used one Vol of pub 249, the one for selected stars.  I tend to use Pub 229 and sometimes 214 for AP reductions on paper.

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1) PM Sun line

 

AP Lat: 15 deg N

 

GHA: 276 deg 29.7'

AP Long: 145 deg 30.3'

LHA: 62 deg

 

Dec: N 21 deg 41.1

 

Hc: 31 deg 08'

Ho: 31 deg 04.4'

Intercept: 3.6 NM Away

Zn: 286 deg

 

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2) Early-morning Jupiter sight

 

AP Lat: 15 deg N

 

GHA: 243 deg 46.5'

AP Long: 145 deg 13.5'

LHA: 29 deg

 

Dec: S 21 deg 46.0

 

Hc: 43 deg 31'

Ho: 43 deg 41.4'

Intercept: 10.4 NM Towards

Zn: 219 deg

 

-JCA.  To be honest, I haven't worked the assumed position reductions on these yet.  I will post my results with both the AP and DR reductions when I get the AP reduced.

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I'm assuming this is supposed to be worked as a  running fix - and the vessel appears to have moved 27.5 NM on TC 133 deg between fixes (which is right at 11 hours and gives an average speed of 2.5 Kts. - are we just drifting with the current, or was there not much wind that day?).

JCA  Actually, no these are independent sights, not meant to be run although there is no harm trying.  The ship was moving at about 14.6 kts during these 2 sights, but we did a lot of turning and strange courses during those hours.  The Jupiter sight is actually part of a AM star fix which I will post in a different exercise, so don't lose it lol.

 

With the LOPs plotted out it looked easier to retire LOP #1 to the DR #2 position rather than the other way around (the Jupiter LOP almost paralleled the vessel's course, and I figured a better "cut" would be had that way).

 

So if I plotted this right (and it's been a good long while since I plotted fixes manually), I show a position of 14 deg 50' N / 145 deg 09.8' E at 19h 22m 31s on 28 May 2008. Please tell me I'm at least in the ballpark...  ;-)

JCA- Wait for the starfix Greg and you should get pretty close indeed :-)

--

GregR

 

P.S. Navigator's solution is 14 deg 53.4' N / 145 deg 05.1' E, and I'm guessing that's a lot more accurate than my manual plotting work.

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