What splits the globe into northern and southern hemispheres?

The equator is a line that splits the globe into northern and southern hemispheres. This line is determined by finding the widest part of the earth, which is a spheroid (like a sphere but squished at the poles). The equator also is the line around which the magnetic field is reversing. The vectors of the field are going away from the north pole and coming in towards the south, and below the equator, the field is moving in towards the earth.