# NavList:

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

**Re: Silicon Sea: Leg 80**

**From:**Dan Hogan

**Date:**2001 Oct 16, 7:32 AM

On 16 Oct 2001, at 7:36, Noyce, Bill wrote: > Better late than never, I hope. > > >The weather has been overcast for the past several days. Seas lumpy, > >and wind variable. On 03/07/2001 at 01:37:00ZT and a distance of 754.0 Nmi > >on a TC of 267.7d from our last position FIX at (03d 30.0' 097d 30.0'W) > >30/06/2001 @ 11:53:00UT the navigator orders a turn to a TC of 307d. > > > > 1) What is the DR position at the time of the turn? > > -- ------------------------------------------------ > > TC 267.7d = W 2.3d S Dist 754.0 nmi > dLat = 30' S => DR Lat = 3d 0' N > dep = 754 nmi => dLo = 753' W => DR Lon = 110d 3' W OK > > > At 09:27:06ZT that same morning we manage a Sun Lower Limb (Sun LL) > sight of: > > Hs 42d 01.5' > > > > TC 307d, Speed 10.4 Kts. > > > > 2) What is the DR position at the time of the Sun sight? > > What is the UT/GMT time of the sight? > > -- ------------------------------------------------------ > > > TC 307d = W 37d N time = 7h 50m 06s => dist = 81 nmi > dLat = 48' N => DR Lat = 3d 48' N > dep = 65 nmi => dLo = 65' W => DR Lon = 111d 8' W > Zone is 7 hours west of Greenwich => UT = 16:27:06 OK > Working the sight using the DR position as the Assumed position gives: > GHA 178d 56.1' dec 22d 54.8' N Ho 42d 10.6' Hc 42d 12.2' > Intercept: 1.6 nmi Away from 062.3d > > > > At a DR position of (04d 10.1'N 111d 32.5'W) at 12:30:22ZT we manage > > another Sun sight. > > > > Hs Sun LL: 71d 05.6' > > > > 3) What is 12:30:22ZT Sun sight called? > > -- ------------------------------------ > > It's obviously close enough to local noon to give a LOP oriented > mostly east/west, so could be called a "Latitude sight". > A bit of work with the Almanac shows the sun's GHA is within > a fraction of a minute of our estimated longitude, so this is > definitely a "LAN sight" or a "Noon sight". As such, it gives > Latitude directly with little calculation: > > Hs 71d 5.6' decl 22d 54.1' N > -IC 2.5' -ZD 18d 44.5' > -Dip 3.0' -------------- > -Refr 0.3' Lat 4d 9.6' N > +SD 15.7' =============== > ---------- > Ho 71d 15.5' > ZD 18d 44.5' OK. > > > 4) What is the Running FIX(RFIX) position? > > -- --------------------------------------- > > For the RFIX, we must advance the earlier LOP to account for the > motion from 9:27 to 12:30. We do this by advancing the Assumed > Position from which the LOP is drawn. One way is by advancing > the position by the given course, speed, and time. This gives > a new Assumed Position of 4d 5' N, 11d 34' W. Since the earlier > sight was worked with Apos = DR pos, we can simply take the new > DR position as the place from which to draw the LOP. Which is > better depends on why the new DR pos differs from the one I > assumed for the earlier sight. > > Using 4d 5' N, 111d 34' W => RFIX = 4d 9.6' N, 111d 38.5' W > Using 4d 10.1' N, 111d 32.5' W => RFIX = 4d 9.6' N, 111d 34' W > ============================ > > For the remainder of the problem, I'll use the second solution > (111d 34' W). OK > > 5) What is the True Course(TC) and distance to the Hawaii landfall > > point at (19d 30.0'N 154d 45.0'W)? > > -- --------------------------------------------------------------- > > It's a long way! dLat = 15d 20.4, dLon = 43d 11' > Even though it's not appropriate, I tried a Mid-Lat solution just to > see what would happen, and got dist = 2710 nmi, TC = 289.7d. > > Using Great Circle, I get dist = 2687.4 nmi, TC = 296.7 (initial) > ============================= > > > The GC course is closer to the pole (expected), but I was surprised > that the distances came out so close to equal. OK, but Great Circle is not of much use since on a small sailboat it is just about impossible to sail it. But you have an idea of how many days to go. Try using Mercator/Rhumbline calculations, the distance is a bit far for accuracy from Mid-Latitude. > > The sky remains a heavy overcast. Winds are variable and indicate a > slow backing. We maintain at True Course of 289.9d. > > > > At the Running FIX position(RFIX) 03/07/2001 @ 19:30:22UT, Current > Drift 0.7 Kts, Set 095d. Var 9dE, Dev 1.5dE. Speed 10.0 Kts. > > > > 6) What is the Compass Course(CC)/Course-to-Steer? > > -- ----------------------------------------------- > > Not sure whether you're asking for TC 289.9d to be translated to a CCTS, > or for my Hawaii course (TC 296.7d) to be translated to a CCTS. > I did the latter, and came up with > Steer 1.4d further West, and SOG = 9.3 kts > TCTS = 295.3 => TVMDC => CCTS = 284.8d With Silicon Sea always assume that you are in the cockpit navigating using traditional navigation methods and equipment. ============= OK. You would think that after all this time I would have learned how to write a question. > > We travel for a distance of 646.6 Nmi on a TC of 289.9D. > > > > 7) What is the Dead Reckoning position at this point? > > What is the Zone Time(ZT) and Zone Date(ZD) of the DR > > position? > > -- ------------------------------------------------------- > > Using the given distance and TC directly for DR: > 289.9d = W 19.9d N > dLat = 230' => DR Lat = 7d 59.6' N > dep = 606 nmi => dLo = 604' => DR Lon = 120d 38' W > ===================== > I assumed the distance was distance through the water (measured > by a log), not distance over the ground, so the time is found > by dividing by 10.0 kts. 64.66 hrs = 2days 16h 40m > + July 3 19:30:22 UT > -------------------- > We're now 8 hours west July 6 12:10:22 UT > of Greenwich ==========> July 6 04:10:22 ZT > ================== OK > > 8) What is the UT/GMT time and date at the DR position? > > -- ---------------------------------------------------- > > July 6 12:10:22 UT OK Dan Hogan dhhogan@verionmail.com