Hemisphere, a half sphere. Any great circle—a circle whose plane passes through a sphere's center—divides a globe into two equal halves, making possible an infinite number of hemispheres. In relation to the earth, however, only four hemispheres are normally considered. The Northern and Southern hemispheres are separated by the Equator, 0° latitude. The Western and Eastern hemispheres, in the strictest sense, are separated by the Prime Meridian, 0° longitude, and the meridian of 180° longitude; in common usage, however, Western Hemisphere applies only to the Americas.