Seismic waves are fundamentally of two types, compressional, longitudinal waves or shear, transverse waves. Through the body of the Earth these are called P-waves (for primary because they are fastest) and S-waves (for secondary since they are slower).
The imaginary vertical lines that run from the North pole to the South pole on a map are called longitudinal lines. The equator is equidistant from the North and South poles. Then, the equator separates the northern and southern hemispheres.
