🔍 Key Concepts

• Parallel sailing uses the departure to find distance when latitude is constant • Departure formula: departure = (difference of longitude in minutes of arc) × cosine of latitude • Convert degrees and minutes of longitude to total minutes of arc before applying cosine

💭 Think About

• Are both positions on the same parallel of latitude, and what does that allow you to assume about the track? • How do you convert the total difference in longitude between east and west longitudes into minutes of arc? • Once you have the departure, how do you relate it directly to distance in nautical miles on a Mercator chart at this latitude?

✅ Before You Answer

• Be sure you combine east and west longitudes correctly to get the full difference in longitude • Use the mean latitude (or in this case the common latitude) in your cosine, not the longitude • Check that your final distance is in nautical miles, not minutes of longitude or some other unit