Departure, in Navigation, the distance of a ship or place to the east or west of any meridian, expressed in nautical miles, whilst the difference of longitude is the same distance reckoned in degrees and minutes upon small circles of the earth, termed parallels of latitude; and as these small circles continually decrease as they approach the poles, and as every circle, large or small, contains 360°, each containing 60', the length of the degrees and minutes of longitude must vary with the latitude.