🔍 Key Concepts

• Great-circle tracks on a gnomonic chart (appear as straight lines) and how to transfer them to a Mercator for distance/steering • Vertex of a great circle in the Northern Hemisphere (what it represents and where it lies relative to departure/arrival longitudes) • Composite sailing and when it is used (ice limits / limiting latitude)

💭 Think About

• On a gnomonic chart, how do you find the approximate longitude where the track reaches its highest latitude? What does the straight-line great-circle track look like when you mark lat/long along it? • For measuring distance on a great-circle route, do you use degrees of latitude along the track, or is there another standard practice involving transferring to a Mercator or using the meridional parts? • Ask yourself whether this specific Delaware–Lisbon great-circle track actually approaches 44°N or higher. Does that make a composite track with a limiting latitude necessary?

✅ Before You Answer

• Check the plotted great-circle track between 38°40'N 75°00'W and Lisbon on the gnomonic chart and identify its highest latitude (vertex) and the longitude at that point. • Verify how distance on great-circle routes is normally obtained in practice with gnomonic + Mercator charts, not just in theory. • Confirm whether the maximum latitude of the great-circle between these points ever gets close to the reported iceberg limit of 44°N; this will tell you if composite sailing is required.