# NavList:

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

**Re: Silicon Sea: Leg 71**

**From:**Dan Hogan

**Date:**2001 May 12, 21:25 EDT

On 12 May 2001, at 14:24, Michael Wescott wrote: > > Sight Reduction and > > Dead-Reckoning problem Leg 71 050801 Test your knowledge! > > ---------------------- ------ ------ -------------------- > > > At 18:27:00ZT {at}XXX.XXX6'N > > 61d 30.1'W) as we passed between the Lessor Antilles islands of St, Lucia > > and St. Vincent. We are bound for, Mid Ocean Point(MOP1)(12d 30.0'N 70d > > 00.0'W), in water of more than 1000 mtrs North of Aruba. This is to avoid > > unfavorable currents along the coast. Dev 1.5dE, Var 12dW. Speed is 8.7 Kts, > > Current Drift 0.7Kts, Set 270.0d. TC 263.7d. > > > 1) At the log reading of 89.6Nmi, what is the DR position? > > -- ------------------------------------------------------- > Using mid-Latitude computation (since Lat is low and course is almost > due west, error due to choice of middle latitude is low as well). > > dLat = 89.6 * cos 263.7 = -9.8 > dLo = (89.6 * sin 263.7)/cos 13d 20.0 = -91.5 > > Lat = 13d 25.6'N - 09.8 = 13d 15.8N > Lon = -61d 30.1' - 91.5 = 63d 01.6W OK > > 2) At the DR position what is the celestial phenomena that is starting > > at this DR Zone Time(ZT)? > > -- ------------------------------------------------------------------- > > Time enroute = 89.6/8.7 = 10h 18m > ZT = 18:27:00 + 10:18:00 = 04:45:00 30/05/2001 > OK, BUT *MORNING* Nautical Twilight. > > > NOTE: Sextant Index correction is on the arc 1.5d. > > > We then take the following sights: > > > Body UT Hs > > > Fomalhaut 08:50:10 40d 54.7' > > Polaris 08:50:55 13d 17.3' > > Deneb 08:51:05 56d 56.3' > > Mars 08:52:03 25d 35.0' > > Venus 08:53:22 28d 13.8' > > Vega 08:53:45 45d 55.6' > > > 3) What is the FIX position? > > -- ------------------------- > > Polaris is almost due north, and Venus due east. From the Polaris table in the > back of the almanac we find a Lat of 13d 01.5'N given an AP of 13d 15.8N and > 63d 01.6'W. For Venus we get: > > UTC 08:53:22 > > Hs 28 13.8 > Ic -1.5 > Dip -2.9 > ========= > Ha 28 09.4 > R -1.8 > Add Cor 0.2 > ========= > Ho 28 07.8 > > GHA-hr 345 25.3 > GHA-ms 13 20.5 > v 0.3 > AP-Lon -63 01.6 > ========= > LHA 295 44.5 > > Dec-hr 7 16.5 > d 0.5 > ======== > Dec 7 17.0 > > AP-Lat 13 15.8 > > Hc 26 38.5 > Delta 89.3 T > > Z 88.4 > > Inspection shows that a closer AP would be about 90 nmi to the east: > > 13d 00.0N 61d 30.0'W > > Working the sights based on new AP: > > Fomalhaut Polaris Deneb Mars Venus Vega > > UTC 08:50:10 08:50:55 08:51:05 08:52:03 08:53:22 08:53:45 > > Hs 40 54.7 13 17.3 56 56.3 25 35.0 28 13.8 45 55.6 > Ic -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 > Dip -2.9 -2.9 -2.9 -2.9 -2.9 -2.9 > ========= ======== ======== ======== ======== ======== > Ha 40 50.3 13 12.9 56 51.9 25 30.6 28 09.4 45 51.2 > R -1.1 -4.1 -0.6 -2.0 -1.8 -0.9 > Add Cor 0.3 0.2 > ========= ======== ======== ======== ======== ======== > Ho 40 49.2 13 08.8 56 51.3 25 28.9 28 07.8 45 50.3 > > > > GHA-hr 7 54.0 7 54.0 7 54.0 101 22.5 345 25.3 7 54.0 > GHA-ms 12 34.6 12 45.8 12 48.3 13 00.8 13 20.5 13 28.5 > v 2.7 0.3 > SHA 15 34.4 49 37.7 80 45.0 > AP-Lon -61 30.0 -61 30.0 -61 30.0 -61 30.0 -61 30.0 -61 30.0 > ========= ======== ======== ======== ======== ======== > LHA 334 33.0 319 09.8 8 50.0 52 56.0 297 16.1 40 37.5 > > > > Dec-hr -29 36.9 45 16.8 -25 47.4 7 16.5 38 47.0 > d - 0.1 0.5 > ========= ======== ======== ======== ======== ======== > Dec -29 36.9 45 16.8 -25 47.5 7 17.0 38 47.0 > > > AP-Lat 13 00.0 13 00.0 13 00.0 13 00.0 13 00.0 > > > Hc 40 49.6 13 00.3* 56 51.5 25 31.5 28 07.2 45 50.3 > Delta .4 A .2 A 2.6 A .6 T 0.0 > > Z 150.4 348.6 232.8 88.6 313.2 > > *Lat from polaris table > > Plotting this out gives a fix 13d 00.3 N 61d 29.4 W 09:00:00 30/05/2001 OK. All that work reminds me of when I was using H.O. 208. > > The navigator has been examining nautical chart INT 400. It shows an area > > labeled the Aruba Gap (13d 50.0'N 70d 00.0'W) with a favorable current of > > 1.15 Kts. He decides to make for this waypoint. Var 12dW, Dev 1dE. Current > > Set 275d Drift 0.8 Kts. Speed 9.3 Kts. > > > 4) What is the TC and Dist from our FIX to the Aruba Gap > > (13d 50.0'N 70d 00.0'W) > > -- ------------------------------------------------------ > > dLo = -70d 00.0 - -61d 29.4 = -510.6' > dLat = 13d 50.0 - 13d 00.3 = 49.7' > dep = 510.6 * cos 13d 25' = 496.7 > TC = atan2(-496.7, 49.7) = 275.7 > Dist = sqrt(49.7^2 + 496.7^2) = 499.2 nmi > > TC = 275.7 D = 499.2 OK > > with current directly behind us there's no need to adjust for drift > > > 5) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap? > > -- ------------------------------------------------------------- > > CC = 275.7 + 12 - 1 = 286.7 OK. But the following current will boost Speed Made Good(SMG) which will affect time enroute.. > > 6) What is the time enroute to Aruba Gap? > > -- -------------------------------------- > > T = D/S = 499.2/(9.3 + 0.8) = 499.2/10.1 = 49.5 hr > > > 7) What is the anticipated Speed Made Good(SMG) to Aruba Gap? > > -- ---------------------------------------------------------- > > Speed is 9.3 with drift 0.8 in the same direction so SMG = 9.3 + 0.8 > or 10.1 knots OK Dan Hogan WA6PBY Catalina 27 "GACHA" dhhogan{at}XXX.XXX Navigation-L List Owner http://www.wa6pby.com