🔍 Key Concepts

• Great circle vertex: point of greatest latitude where the course is 090°/270° true • Relationship between vertex latitude and departure longitude on a great circle • Formula: cos(DLo) = tan(lat1) / tan(lat_vertex) for same-name latitudes

💭 Think About

• First, decide whether the vertex latitude will be north or south, given both positions are in the same hemisphere and at similar latitudes • Think about whether the vertex longitude should be east or west of both departure and arrival longitudes, based on the initial course of 121° T • Use the relationship between the known latitude and the vertex latitude to solve for the change in longitude from the departure point

✅ Before You Answer

• Confirm you are using same-name latitudes (both in the Southern Hemisphere) so the standard vertex formulas apply • Be sure your calculator is in degrees mode and that you convert minutes to decimal degrees before using trig functions • After finding the change in longitude, add or subtract it correctly from the departure longitude, keeping track of E/W and making sure the result lies logically between Africa and Western Australia on a world map