We use the Equator to divide the Earth into two hemispheres. The hemisphere which is north of the Equator is called the Northern Hemisphere. The Arctic region, North America, Europe, Asia and most of Africa are in the Northern Hemisphere. The hemisphere which is south of the Equator is the Southern Hemisphere. Antarctica, Australia and most of South America are in the Southern Hemisphere. Planets and many moons are spheres. Astronomers divide their surfaces into northern and southern hemispheres to help them with their studies.

PS. Sometimes people also divide up the Earth in other ways. For example, Americans often say they are in the Western Hemisphere, but this has no scientific meaning.