🔍 Key Concepts

• Mercator sailing uses difference of longitude (DLo) and meridional parts to relate course, distance, and change in latitude/longitude. • On an oblique course (not due east/west), the departure is found from the distance and course, then related to DLo using mean latitude on a Mercator chart. • Check how course angle from due east/west affects the size of departure for a given distance.

💭 Think About

• What trigonometric function relates course angle and distance to find departure on a Mercator sailing? Think about the component of the track that is east-west. • Once you have the departure, how do you convert that to a difference of longitude at the mean latitude for this track? • Given your starting longitude is west, and you’re steaming generally southeast (118°T), will your longitude become more east or more west? How does that affect the sign of your DLo?

✅ Before You Answer

• Confirm you are using nautical miles for distance and that your trigonometric function uses the correct angle from north/south. • Verify you use the mean latitude (or appropriate meridional parts) when converting between departure and DLo on a Mercator chart. • After finding DLo, carefully apply it (add or subtract) to 74°01.0'W and see which choice is consistent with the direction of travel (toward or away from Greenwich).