🔍 Key Concepts

• Mercator sailing uses difference of longitude and meridional parts to find course and distance on a Mercator chart. • Compute the difference of latitude (ΔLat) and difference of longitude (ΔLong), being careful with E/W signs and crossing a meridian like 180°. • Use the relationship between departure, meridional parts, and course to find the true course, then use basic trig to find distance.

💭 Think About

• Is the course generally trending southeast, southwest, northeast, or northwest based on the starting and ending lat/long? Eliminate any answer choices with an obviously wrong quadrant. • Once you find ΔLat (in minutes) and ΔLong (in minutes of arc), think about how Mercator sailing adjusts for the change of scale with latitude. Which trig function relates the difference in meridional parts to departure for finding course? • Compare the approximate great-circle direction from Southern California (around 118°W) to Hawaii (around 157°W). Does that rough mental picture support an easterly or westerly course, and is the distance more likely a bit over or under 2,300 miles?

✅ Before You Answer

• Double-check your ΔLat and ΔLong signs (are you going toward the equator or away from it, and increasing or decreasing west longitude?). • Make sure you used the correct meridional parts for both latitudes and took their difference. • Confirm that the course angle you compute matches the correct quadrant (N/S and E/W) before comparing it to the numeric course options.