Multiply the 360 degrees of the Earth by the 60 minutes on a clock face. Because the Earth is circular, it is 360 degrees around in longitude; on a clock, you would have to travel 60 minutes around to make a full revolution. The multiplication yields 21,600 "minutes." Each minute corresponds to one nautical mile, so at the equator, the Earth is 21,600 nautical miles around. However, since the lines of longitude get smaller as your move away from the Earth's equator, nautical distances are measured by latitudinal minutes, which remain constant.
Find the length of a latitudinal minute, as represented on the nautical chart, or map. Each minute of latitude is 1 nautical mile. If two points are 10 latitudinal minutes apart on the map, they are 10 nautical miles apart. This is how nautical distances are calculated.
Adjust the arms of the V-shaped compass to match the length of the latitudinal minute on the map. Now you can use the set width of the compass to measure nautical distances on the map.
