🔍 Key Concepts

• Mercator sailing uses difference of longitude (DLo) and meridional parts (MP) to find distance and course between two positions. • You must convert longitudes properly to get the difference of longitude east–west across Greenwich. • For long ocean passages, carefully compute the difference in latitude (DLat) and look up or compute the corresponding meridional parts for each latitude.

💭 Think About

• First, decide how to handle longitudes that are west on one side of Greenwich and west/east on the other—what is the correct algebraic way to find the total DLo? • After finding DLat, what table values (or formulas) do you need from Mercator sailing to get the change in meridional parts, and how does that relate to the course angle? • Once you have the course angle and DLo, how do you relate them to the distance using Mercator sailing relationships?

✅ Before You Answer

• Be sure you have the correct sign and magnitude for DLo, remembering one position is in the western hemisphere and the other is near 5° 45.0' W (close to Greenwich). • Confirm that the difference in latitude (DLat) is reasonable for going from 32° 05.0' N to 35° 57.0' N (is it only a few degrees, or many?). • After computing, check whether the final distance is reasonable for a North Atlantic leg from off Savannah/Charleston area (around 81° W) to near the Strait of Gibraltar (around 5–6° W).