🔍 Key Concepts

• Mercator sailing combines plane sailing (for course) and meridional parts (for distance on a Mercator chart). • Relationship between difference of longitude (DLo), meridional parts (MP), and course: tan(C) = DLo / (difference in meridional parts). • How to convert latitude and longitude differences into nautical miles using meridional parts, not just simple latitude difference.

💭 Think About

• First, decide which point is the departure and which is the destination, then find the difference of latitude and difference of longitude (east or west). What signs should each have? • Think about how to obtain meridional parts for each latitude (e.g., from tables) and then find the difference in meridional parts. How does that relate to the course angle in Mercator sailing? • Once you know the course angle from the meridional triangle, how do you get the total distance from the difference of latitude and the course, or from DLo and course?

✅ Before You Answer

• Be sure you are using difference in meridional parts (ΔMP), not just ΔLat, when applying the tangent relationship for Mercator sailing. • Double-check that your longitude difference is correctly measured as the smaller angle (0–180°) and in the correct east/west sense. • After computing course and distance, compare your result to the options: check that both true course (to the nearest degree) and distance (to the nearest 0.1 NM) are consistent with Mercator—not great-circle—sailing.