Navigation and Seamanship

1.2.6. Parallels and meridians

                                                                            

               Certain great and small circles have applications in navigation. A parallel is a small circle on the earth’s surface whose plane of inter section is parallel to the plane of the equator. The equator itself is a special parallel in that it is a great circle.

               Any circular line which runs around the Earth at its fattest point is called a Great Circle. This circle has to be centred over the centre of the Earth. Note the following:

• All Lines of Longitude ‘run through both Poles’ and each is therefore half of a Great Circle
• For Lines of Latitude only the Equator is a Great Circle
• Other Great Circles can be positioned at any angle over the Earth – the only limit on how it can be placed is that they must have the centre of the Earth as their centre

             Why the fuss about Great Circles? The answer is that for any two points on the surface of the Earth, the shortest distance between those points is always along a Great Circle. When you look at a map a Great Circle is usually not a straight line. This is because of distortions that are created when the surface of the Earth is projected onto a 'flat piece of paper'.