# NavList:

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

**Re: Rusty!!!!**

**From:**Eric Haberfellner

**Date:**2001 Dec 03, 7:47 PM

I am also confused about what method to use for solving question 1. Here is a copy of a note that I sent to Dan asking for help. Perhaps some of the other listers can give me a hand. -----Original Message----- From Eric Haberfellner [mailto:ehaberfe@rogers.com] Sent: Sunday, December 02, 2001 10:15 PM To: dhhogan@VERIOMAIL.COM Subject: RE: Silicon Sea: Leg 82 OK, I am already having a problem on part 1 of this problem. The initial course that I calculate for the great cricle course is 282.7466d, not the given answer. How I got there is this. The formula for calculating the great circle initial course angle given in Bowditch is: Where L1 is lat. of departure L2 is lat. of dest. DLo is difference in long. C = arctan ( sin DLo / ( (cos L1 * tan L2)-(sin L1 * cos DLo) ) So we calculate: DLo = -22.7466d (decimal degrees) L1 = 11.4967d L2 = 19.5000d We plug these valuse into the equation and we get True Course = -66.7735 = 293.2265d. Now we uncorrect for deviation and variation. Timid Vrigins Make Dull Companions and Uncorrecting east is least, so: Magnetic course = True course - 10.0d E. Var. - 1.0d E. Dev. = 282.2265d. Now for the current, The Annapolis Book of Seamanship gives the formula for correcting magnetic course for a current as: Where RL is magnetic Rhumb-line course = 282.2265 drift is speed of current in knots = .7 knots set is direction of current in magnetic degrees = 285d - 10.0d E. var. = 275d M boatspeed is in knots = 9.7 knots Mag course = RL - arcsin ((drift * sin(set - RL))/boatspeed) Plugging in the values we get magnetic course 282.7466d. Where the heck did I go wrong??