Rhumb line vs great circle
A rhumb line, or mercator track, crosses all meridians at the same angle. The ship can steer one compass course the whole way. A great circle is shorter on long ocean passages but the course keeps changing. Coastal and short passages are usually planned as rhumb lines. Ocean passages often use a great circle with rhumb-line legs between waypoints.
How this calculator works
The tool uses mercator sailing on a spherical earth. Course is taken from the difference of longitude and the difference of meridional parts. Distance is returned in nautical miles. Positions near the poles or exactly east-west at high latitude should be checked on the chart. For the shortest long-ocean track use the Great Circle calculator already on this site.
Who should use it
Useful for officers laying a course on a mercator chart. Always plot the result and check dangers, TSS and land before using the course at sea.