The Equator is an imaginary horizontal line at the center of the earth that divides the earth into the Northern and Southern Hemispheres. The vertical imaginary line is called the Prime Meridian and it divides the earth into the Western and Eastern Hemispheres. Northern (North America, Europe, Asia, parts of Africa) Southern (most of South America, parts of Africa, and Australia) Western (North and South America) Eastern (Asia, Australia, and most of Europe and Africa)
The equator divides the earth into the northern and the southern hemispheres, and the prime meridian divides the earth into the eastern and western hemispheres. Disregarding the earth's variance from being a perfect sphere, any great circle route will define a plane that divides the earth into two hemispheres. The earth is close to being a sphere, but is closer to being an oblate spheroid.