Finding a street address on earth is not a difficult task, however finding a specific location in the middle of the ocean can present the mariner with a few problems. In order to locate a specific location anywhere on earth, a grid system has been developed to give each area on the earth a specific address. This system deals with “parallels of latitude” and “meridians of longitude.”
In order to begin to understand how to navigate, the navigator must understand some basic terminology and how it relates to the grid system.
These basics are covered in the online Nautical Know How Basic Boating Safety Course at http://boatsafe.com/navigation .
A “great circle” is a circle formed on a sphere, such as the earth, by the intersection of a plane passing through the center of the sphere. An arc of a great circle is the shortest distance between two points, hence a great circle route is the shortest route between two points on the earth. You might think of splitting an orange down the middle and going through the center. The plane exposed would represent a great circle.
A “small circle” is a circle formed on a sphere, such as the earth, where the intersection of a plane does not pass through the center of the sphere. Cut that same orange in any other manner that does not pass through the center and you have created a small circle.
Parallels of Latitude are small circles that are measured from the equator (the only latitude that is a great circle) beginning at O° latitude at the equator to 90° North and South at the Poles. These parallels are equal distance apart and run horizontally across a chart, like the rungs on a ladder. You can use the “ladder” analogy to remember that “laddertude” lines are drawn across the chart, not up and down.
Because latitude lines are equal distance apart they can be used to measure distance on the surface of the earth. The only parallel of latitude that is a “great circle” is the equator itself as shown in the graphic. You will find latitude measurements on the side of your nautical chart.
Meridians of Longitude are “great circles” that pass through the north and south geographic poles. The meridian of longitude that cuts through the earth at Greenwich, England is labeled as O° longitude and is known as the prime meridian. Longitude is then measured from O° at Greenwich to 180° West and 180° East of Greenwich. The 180°th of longitude is the International Dateline.