The Eastern Hemisphere is a geographical term for the half of the Earth that is east of the Prime Meridian (which crosses Greenwich, England, United Kingdom) and west of 180° longitude. It is also used to refer to Europe, Asia, Africa, and Australasia, vis-à-vis the Western Hemisphere, which includes the Americas. In addition, it may be used in a cultural or geopolitical sense as a synonym for 'Old World'.
The line demarcating the eastern and western hemispheres is an arbitrary convention, unlike the equator (an imaginary line encircling the Earth, equidistant from its poles) which divides the northern and southern hemispheres. The Prime Meridian at 0° longitude and the Antimeridian, at 180° longitude are the conventionally accepted boundaries, since they divide eastern longitudes from western longitudes. This convention was established in 1884 at the International Meridian Conference held in Washington, D.C. where the Standard Time concepts of Canadian railroad engineer Sir Sandford Fleming were adopted. Using this demarcation puts portions of western Europe, Africa and eastern Russia in the western hemisphere, thereby diluting its usefulness for cartography, as well as for geopolitical constructs, since all of Eurasia and Africa are typically included in the eastern hemisphere. Consequently, the meridians of 20°W and the diametrically opposed 160°E are often used, which includes all of the European and African mainlands, but also includes a small portion of northeast Greenland (typically reckoned as part of North America) and excludes more of eastern Russia and Oceania (e.g., New Zealand). Prior to the global adoption of Standard Time, numerous Prime Meridians were decreed by various countries where time was defined by local noon (thereby, local meridian).
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world or the Blue Planet.
Earth formed approximately 4.54 billion years ago, and life appeared on its surface within its first billion years. Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).
Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.