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    Re: Silicon Sea Leg 72
    From: Dan Hogan
    Date: 2001 May 29, 13:38 EDT

    On 29 May 2001, at 10:27, Noyce, Bill wrote:
    > > 1)  What is the DR position at the end of the Tack 4?
    > > --  -------------------------------------------------
    >
    >          Tack 1
    > TC       322.0d       231.5d       252.5d       297.0d
    > dist      30.0 nm      93.5 nm      40.0 nm      90.0 nm
    > dLat    N 23.6'      S 58.2'      S 12.0'      N 40.2'
    >  Lat   N 13d 23.7'  N 12d 25.5'  N 12d 13.5'  N 12d 53.7'
    > dep     W 18.5 nm    W 73.2 nm    W 38.1 nm    W 80.2 nm
    > dLon    W 19.0'      W 75.1'      W 39.2'      W 82.3'
    >  Lon   W 61d 49.0'  W 63d  4.1'  W 63d 43.3'  W 65d  5.6'
    >
    > DR = 12d 53.7 N  65d 5.6' W
    OK
    > > 2)  What is the DR position local Zone Time(ZT) and Date?
    > >     What is the DR position GMT/UT time and Date?
    > > --  -----------------------------------------------------
    >
    > 33 hrs = 1 day 9 hrs:
    >     UT 31 May 17:50:10
    >     ZT 31 May 13:50:10
    OK
    > > 3)  From the DR position, what is the TC to Aruba Gap
    > >     (13d 50.0'N  70d 00.0'W)?
    > > --  -------------------------------------------------
    >
    >   TC = 283.4d  dist = 291.7 nm
    Mmmm..OK
    > > 4)  What is the Compass Course(CC)/Course-to-Steer for Aruba Gap
    > >     from the DR position?
    > > --  ------------------------------------------------------------
    >
    > Is current set given in True, Magnetic, or Compass direction?
    > Not that it makes much difference here.  I assumed True.
    > Using Law of Sines, I find I need to adjust my course North
    > about 1.5 degrees.
    >   CC = 283.4 + 1.5 + 9 = 293.9d
    Current, wind, etc. are ALWAYS in TRUE.
    Mmmm..Barely OK. Try a current vector diagram.
    > > 5)  What is the FIX position?
    > > --  -------------------------
    >
    > Roughly estimating progress westward for 16 hrs, I picked
    >   AP = 13d 20' N  67d 30' W
    > It wasn't until checking my work that I noticed this puts
    > us right on the border of a time zone, but if I get the
    > conversion ZT->UT wrong the 15-degree error in position
    > will be obvious.
    >
    > Body     Mars         Vega         Venus
    > GHA    128d 45.8'   115d 58.2'    11d 32.5'
    > Decl  S 25d 54.5'  N 38d 47.0'  N  7d 49.7'
    > Hc      18d 41.3'    40d 19.9'    34d 47.7'
    > Az      236.4d       310.0d        89.6d
    > Int      35.3' A      16.3' A      31.2' T
    >
    > These give a position at about 13d 35' N  66d 57' W.
    > This is about 30 miles east of my rough estimate.
    > Perhaps we were running only about 12 hours instead
    > of 16, due to UT/ZT confusion?
    The time zones are 7.5 degrees either side of the principle zone.
     Zone Description(ZD) = Lon / 15
     If it's 4.4 still in previous zone, if its 4.5 you are in the next zone.
    >
    > > 6)  What are the names of the two unknown bodies?
    > > --  ---------------------------------------------
    >
    > Running a signt reduction but exchanging Ho<->decl
    > and LHA<->Bearing, I get
    >
    > Compass bearing     349.7d       327.3
    > True bearing        340.7d       318.3
    > computed decl     N 71d 17.2'  N 29d 51.2'
    > computed LHA        87d 48.0'    33d 44.7'
    > GHA                154d 45.0'   100d 41.7'
    > SHA                119d 23.2'    65d 44.7'
    >
    > None of the planets are near the resulting position, and
    > none of the 58 'standard' stars are very near.  I concluded
    > that the northern one was in the bowl of the Little Dipper,
    > but it's not Kochab; the second looked like it might be the
    > head of the Swan, opposite Deneb in Cygnus.  Now that I see
    > the second correction, perhaps I'll go back and redo them...
    I screwed this up as much as any one. The simplest method is to use the
    Almanac and find a body with a close GHA and dec. Other wise use yout SR
    Table and solve for:
            In place of dec use Ho
            In place of LHA use Obs Azimuth
            In place of Hc find Dec
        In place of Z find LHA
    > > 7)  What is the TC and Distance to the Aruba Gap (13d 50.0'N  70d
    > 00.0'W)
    > >     waypoint from our FIX?
    > > --
    > ---------------------------------------------------------------------
    >
    > Mid-latitude using 13d 42'
    > dLat = 15' N  dLon = 183d W  dep = 177.8 W
    > TC = 274.6d  dist = 178.3 nm
    OK. Be aware that above 500 miles at 90d/270d Mid-Latitude can give an error.
    Above 1200 mils it gets unreliable. When I used Mid-Latitude (Hawaii to Long
    Beah, CA)I broke the passage into 600 Nmi legs, it fits the Pilot Charts.
    Dan Hogan WA6PBY
    dhhogan{at}XXX.XXX
    Nav-L Web Page: http://www.wa6pby.com
    Catalina 27 "GACHA"
    

       
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