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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Silicon Sea: Leg 81
From: Dan Hogan
Date: 2001 Oct 20, 7:46 PM

On 19 Oct 2001, at 17:21, Noyce, Bill wrote:

> > At 14:00:00UT @ 08/07/2001, by log meter, we have traveled a distance
> of
> > 439.5 Nmi since our last DR position. True Course(TC) 289.9d.
> >
> > 1)  What is the current DR position?
> > --  --------------------------------
>
> By Mercator sailing:  10d 19.8' N, 128d 34.8' W
> (Mid-lat gives                     128d 38.1' W)

OK

> > 2)  What is the approximate distance and direction to the storm
> >     (019d 45.0'W  120d 30.0'N) from our DR position at 14:00:00UT @
> >     08/07/2001?
> > --  ----------------------------------------------------------------
>
> How approximate is "approximate"?
> dLat = 9d 25', dLon = 8d 5'
> 9*9 + 8*8 = 145 ==> storm is about 60*12 = 720 miles away,
> somewhat north of northeast.  I assume this is enough info
> to estimate the storm's effect on us...
>
> More carefully,
> Mercator:     760.0 nmi, 039.6d
> Great Circle: 733.6 nmi, 038.7d

If you are out there you need to plot the the storm direction its position at
a given time. There is  a need to know where in the semi-circle of the storm
you are and what wind and wave strenght can be expexted.
>
>
> > 3)  What is the DR position at the end of this time?
> > --  ------------------------------------------------
>
>   TC    Dist    Lat          Lon
>               10d 19.8 N  128d 34.8 W
> 289.9d  57.0  10d 39.2 N  129d 29.3 W
> 278.0d  74.4  10d 49.6 N  130d 44.3 W
> 275.0d  24.75 10d 51.7 N  131d  9.4 W
> 270.0d  46.8  10d 51.7 N  131d 57.0 W
>               =======================

OK

>
> > 4)  What is the UT/GMT date and Time?
> >     What is the Zone Time(ZT) and day?
> > --  ----------------------------------
>
> Time adds up to 2 days 11.5 hours.
> Time is 11 July 01:30 UT, zone is 9 hrs west,
>         10 July 16:30 ZT

OK

> > 5)  What is the TC & Distance from the DR to our Hawaii landfall point
> at (19d 30.0'N  154d 45.0'W)
> > --  ---------------------------------------------------------------------
>
> by Mercator (Meridional Parts):
> From- Lat= 10d 51.7 N  MP=  651.4  Lon= 131d 57 W
> To:   Lat= 19d 30.0 N  MP= 1185.8  Lon= 154d 45 W
>       dLat    518.3'   dMP  534.4  dLon 1368'
>
> TC = atan(1368/534) = N 68.7d W = 291.3d
> Dist = 518.3/cos(68.7) =         1424.7 nmi
>                                  ==========

OK
>
>
> > 6)  What is the unknown body?
> > --  -------------------------
>
>   Vega.

OK

> > 7)  What is the FIX position at the time of the unknown body?
> > --  ---------------------------------------------------------
>
> DR position at 03:40 was about 11d N, 132d 20' W.
> The FIX draws out to about 11d 33' N, 132d 25' W.

OK

> I'm not very happy with my work on this fix.  I originally had
> the wrong DR, and all my intercepts were over 60 miles long.
> But at least Antares and Mars, with nearly the same azimuth,
> also had nearly the same intercept.  (I remember showing the
> Cub Scouts at camp last July how the Red Planet was right next
> to the Red Star -- at least I thought it was cool!).  Found
> the error in the DR, and recalculated the sights from there
> -- but now my Antares and Mars lines are nearly 5 miles apart
> and still nearly parallel.
>
>         Antares    Mars
> GHA   98d 17.3     90d  1.2
> dec   26d 26.2 S   26d 51.1 S
> LHA   34d  2.7 E   42d 18.8 E
> K     30d 57.8 S   34d 23.6 S
> Hc    40d  2.7     34d  9.6
> Ho    39d 33.3     33d 44.2
> Int       29.4 A       25.3 A
> Z        139.1d       133.5d

Th problem was set up to have more than 30' intercepts on some bodies.

Your Almanac data is within +/- 0.5'. The MARS Hc is less than my calculated.
BUT that depends on the Sight Reduction method used. The accuracy is
determined by plotting the results as a FIX.

>
> I've checked it by hand -- time to ask the computer where I
> screwed up, I guess.
>         -- Bill

Dan Hogan WA6PBY
C27 "Gacha"
dhhogan@verimail.com
Nav-L Page: http://www.wa6pby.com

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