The United States is located in both the Northern and Western hemispheres. The Equator divides the Earth into Northern and Southern hemispheres while the Prime Meridian divides the Earth into Eastern and Western hemispheres.
The Northern hemisphere also includes the rest of North America, northern portions of South America, all of Europe and parts of Africa and Asia. Antarctica, Australia, most of South America and parts of Africa and Asia are located in the Southern hemisphere. All of North America and South America are included in the Western hemisphere along with the Caribbean and half of Antarctica. Countries in the Eastern hemisphere include the other half of Antarctica, all of Asia and Australia and most of Europe and Africa.
Since the Prime Meridian and the Equator are used to divide the Earth into hemispheres, their measurements are zero degrees longitude and zero degrees latitude. All coordinates are based on these two numbers. The Equator was determined because of the Earth's rotation, but the Prime Meridian is an arbitrary line used for navigation, particularly in the shipping industry. In 1884, the Greenwich Meridian was chosen as the main meridian of the world after a vote held at the International Meridian Conference. This allowed the creation of a uniform method of measurement and classification.