The half of the Earth comprising North America, Central America, and South America is the Western Hemisphere.
There are four hemispheres. Hemispheres are each shaded a dark gray.
The Equator, that imaginary horizontal line at 0є degrees latitude at the center of the earth, divides the earth into the Northern and Southern Hemispheres.
The vertical imaginary line called the Prime Meridian, at 0є degrees longitude, and its twin line of longitude, opposite the Prime Meridian at 180є longitude, divides the earth into the Eastern and Western Hemispheres.
Therefore (with some exceptions) all countries north of the Equator are in the Northern Hemisphere, while all countries south of the Equator are in the Southern Hemisphere. In addition, all countries west of the Prime Meridian are in the Western Hemisphere while those east of the Prime Meridian are in the Eastern Hemisphere.
Generally the Eastern Hemisphere includes most of Africa, about half of Antarctica, all of Asia and Australia / Oceania, and most of Europe.
The Western Hemisphere includes about half of Antarctica and all of North and South America, which includes the Caribbean and Central America as well as Greenland.
The Northern Hemisphere includes all of North America, the northern reaches of South America, about two-thirds of Africa, all of Asia excluding (parts of Indonesia) and all of Europe.
The Southern Hemisphere includes most of South America, one-third of Africa, all of Antarctica, a small sliver of Asia (parts of Indonesia) and all of Australia / Oceania.
There are exceptions as some continents (and their countries) based on the straight line divisions of the Equator and Prime Meridian are a part of two hemispheres. For a map and list of countries in both the Eastern and Western Hemispheres, go here, and for countries in both the Northern and Southern Hemispheres, try this page.
The geographical Western Hemisphere of Earth or western hemisphere is a geographical term for the half of the Earth that lies west of the Prime Meridian (which crosses Greenwich, London, United Kingdom), the other half being the eastern hemisphere. It is also used to specifically refer to the Americas (or the New World) and adjacent waters, while excluding other territories that lie west of the Prime Meridian (parts of Africa, Europe, Antarctica, and Asia); thus, it is sometimes referred to as the American hemisphere.
Any definition of eastern and western hemispheres, however, requires the selection of an arbitrary meridian and a corresponding meridian on the other side of the Earth. The Prime Meridian at 0° longitude is typically used, which runs through Greenwich; this is used to define the International Date Line (or End Meridian) on the other side of the Earth at 180° longitude. In this sense, the western hemisphere includes not only the Americas, but the western portions of Europe and Africa, the easternmost tip of Russia, numerous territories in Oceania, and a portion of Antarctica while excluding some of the Aleutian Islands to the southwest of the Alaskan mainland. Sometimes, the meridians of 20° W and the diametrically opposed 160° E are used, which excludes the European and African mainlands but also excludes a small portion of northeast Greenland and includes more of eastern Russia and Oceania (e.g., New Zealand).
The two major regions of Antarctica are named after their positions mainly within a single hemisphere; West Antarctica is named for the Western Hemisphere.
The population of the Western Hemisphere exceeds 1 billion; however, the population of the Americas does not.
The Origin of Western Hemisphere.