HEM'ISPHERE, noun [Gr.] A half sphere; one half of a sphere or globe, when divided by a plane passing through its center. In astronomy, one half the mundane sphere. The equator divides the sphere into two equal parts. That on the north is called the northern hemisphere; the other, the southern. So the horizon divides the sphere into the upper and lower hemispheres. hemisphere is also used for a map or projection of half the terrestrial or celestial sphere, and is then often called planisphere. 1. A map or projection of half the terrestrial globe.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI