Hemisphere (geography)

In geography, a 'hemisphere is one half of the surface of Earth, or other planet, moon, or star. One may refer to the northern and southern hemispheres, whose boundary is the equator of Earth or other body. References are also made to the eastern and western hemispheres of Earth; in this case the boundary between hemispheres is the great circle containing the arbitrarily-chosen 0 and 180-degree lines of longitude.