ἡμισφαίριον

Hēmisphaírion is built from Greek hēmi- (half) and sphaira (sphere, ball). The compound appeared in Greek astronomical writing by the fourth century BCE. Aristotle used it. The word initially described the visible dome of the sky — the half-sphere above the horizon. When Greek thinkers became confident that the earth was spherical (Eratosthenes calculated its circumference around 240 BCE), hemisphere naturally extended to describe halves of the earth itself.

The division of the earth into hemispheres required reference lines. The equator — from Latin aequare (to make equal) — divided north from south. The prime meridian, fixed at different locations throughout history before settling on Greenwich in 1884, divided east from west. These divisions were human choices, not natural features. The equator has a physical basis (the widest circumference perpendicular to the axis of rotation), but east and west are entirely conventional. London is in the Western Hemisphere because London decided where the line was.

The age of European exploration made 'hemisphere' a politically loaded word. The 'New World' was the 'Western Hemisphere.' This phrasing implied that the hemisphere containing the Americas was a discovery — new to the people who had not been living there for fifteen thousand years. The hemisphere was new only to the people who named it. The Aztecs, the Inca, and the hundreds of nations of North America had no need for the word because they were already in the place it described.

The word remains standard in geography, meteorology, and neuroscience. The brain's left and right hemispheres — named by analogy with the globe — process different functions. Meteorology tracks weather systems by hemisphere because the Coriolis effect makes storms rotate in opposite directions north and south of the equator. The Greek half-sphere became the way humans divide everything too large to see whole.