🔍 Key Concepts

• Mercator sailing relationships between difference of longitude, meridional parts, and course • Converting latitude and longitude to difference of latitude (D.Lat) and difference of longitude (D.Long) with correct E/W and N/S signs • Using meridional parts tables (or formula) to get difference of meridional parts (D.M.P.) and then finding course from \( \tan C = \frac{D.Long}{D.M.P.} \)

💭 Think About

• First decide: from your position to the distress vessel, are you going generally NE, SE, SW, or NW? Eliminate any course options that don’t fit that general direction. • Work out the numerical values of D.Lat and D.Long between the two positions, being careful with south latitudes and west longitudes. Which one is larger in absolute value, and what does that suggest about whether the course is closer to due north–south or due east–west? • After computing D.M.P. for the two latitudes, form the ratio D.Long / D.M.P. and estimate the angle whose tangent is that ratio. Which of the multiple-choice courses best matches that angle and the correct quadrant (SE, SW, etc.)?

✅ Before You Answer

• Confirm which vessel is farther south and which is farther west before deciding the direction of travel. • Check the signs: D.Lat (N or S?) and D.Long (E or W?) must match the direction from your vessel to the distress vessel. • Make sure you use meridional parts for both latitudes and take their difference, not just the raw difference of latitude, when applying Mercator sailing.