The Earth’s surface is dominated by vast oceans, covering approximately 70% of the globe. To simplify the study of these interconnected water masses, geographers use lines of longitude and latitude to divide the planet into halves, known as hemispheres.
How Geographical Hemispheres Are Defined
The division of the Earth into Eastern and Western Hemispheres uses imaginary lines of longitude. The starting point is the Prime Meridian, designated as 0° longitude, which runs through Greenwich, London.
The Eastern Hemisphere is the half of the Earth located east of the Prime Meridian, extending up to the Antimeridian (180° meridian). This Antimeridian, which is close to the International Date Line, marks the boundary of the Eastern Hemisphere. This hemisphere encompasses a significant portion of the world’s landmass, including most of Europe, all of Asia, Africa, and Australia.
Identifying the Predominantly Eastern Ocean
The ocean that lies overwhelmingly within the geographical bounds of the Eastern Hemisphere is the Indian Ocean. It is the third-largest of the world’s five oceanic divisions, covering about 70.5 million square kilometers. Its borders are defined by Asia to the north, Africa to the west, and Australia to the east, placing its main bulk between the 20° East and 147° East meridians.
The Indian Ocean’s western border is separated from the Atlantic Ocean by the 20° East meridian, running south from Cape Agulhas in South Africa. Its eastern limit is the 147° East meridian, separating it from the Pacific Ocean near Tasmania.
No major ocean is 100% contained within the Eastern Hemisphere due to the arbitrary placement of the Prime Meridian. However, the Indian Ocean is the only one whose entire surface area is almost exclusively confined to the space between 0° and 180° East longitude.
Why Other Major Oceans Span Both Hemispheres
The other major oceans, the Atlantic and the Pacific, span both the Eastern and Western Hemispheres. The Atlantic Ocean, the second-largest, is fundamentally bisected by the Prime Meridian (0° longitude). Its central location between the Americas to the west and Europe and Africa to the east means it has massive portions lying on both sides of the dividing line.
The Pacific Ocean, the world’s largest, extends from the Americas to Asia, covering nearly one-third of the Earth’s surface. This ocean is cut in half by the 180° Antimeridian, which serves as the boundary between the Eastern and Western Hemispheres. Because the Pacific wraps around the globe, it occupies significant territory in both longitudinal halves of the planet.
The Arctic Ocean touches North America (Western Hemisphere) and Eurasia (Eastern Hemisphere). The Southern Ocean surrounds Antarctica and connects to the three main oceans. Thus, only the Indian Ocean maintains a geographical position predominantly bounded by the longitudinal lines that define the Eastern Hemisphere.