🔍 Key Concepts

• Difference of latitude (DLat) and difference of longitude (DLo) between two positions • How to find meridional parts (M) for each latitude and then compute the difference of meridional parts (dMP) for Mercator sailing • Relationship in Mercator sailing: course angle from tan(C) = DLo / dMP (using proper signs and units)

💭 Think About

• First, decide which way latitude and longitude are changing: is the vessel going generally NE, NW, SE, or SW? Which quadrant should the course angle fall into? • Compute DLat and DLo in minutes of arc; then think about whether to use plain difference of latitude or difference of meridional parts in the Mercator formula. • After you compute the course angle from the arctangent, how do you convert that acute angle into a true course in three digits, based on the correct quadrant?

✅ Before You Answer

• Confirm that DLo is converted to minutes of arc on the correct parallel or in the proper way for Mercator sailing (usually as an angle, not distance). • Check that you used meridional parts for the correct latitudes (starting and destination) and took the correct difference (dMP = MP2 − MP1, with sign). • Before picking an answer, verify that your final course lies in the correct quadrant (e.g., between 090°–180°, 180°–270°, etc.) that matches the direction of travel from the starting to ending coordinates.