NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Silicon Sea Leg 72
From: Bill Noyce
Date: 2001 May 29, 16:13 EDT
From: Bill Noyce
Date: 2001 May 29, 16:13 EDT
> > > 4) What is the Compass Course(CC)/Course-to-Steer for Aruba Gap > > > from the DR position? > > > -- ------------------------------------------------------------ > > > > 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 > > Mmmm..Barely OK. Try a current vector diagram. I drew a diagram to get the orientation of the pieces right, but I have a hard time measuring the resulting course adjustment, because it's so small. Thus the attempt at a 'digital' method. TC=283.4, Set=265.0, angle between = 18.4 degrees sin 18.4 / speed = sin adj / drift sin 18.4 / 8.5 = sin adj / 0.7 I did this with log tables before; doing it on the computer now says adj = 1.490 degrees. The diagram indicates it needs to be added to the course. Is there something inherently wrong with this approach, or have I made a slip somewhere? > > > > 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. I saw a discussion initiated by Sam Chan about Mercator sailing near 90 or 270, where the problem arises from rounding the course before using dLat/cos(C) to compute the total distance. Is this a similar problem? In Mid-Latitude sailing, where we already have dLat and departure, the total distance can be computed either as dLat/cos(C) or as departure/sin(C), so we don't have to divide by a tiny rounded number. Or are you talking about the error in distance that arises from assuming a spherical earth? What method would be better? I assumed we weren't planning to sail a great-circle course here.